<table><caption>**Table SI 3-3 :** Mean concentration (mg kg⁻¹) of trace metals in the four BCR fractions for original (t=d0) and mature (t=d180) composts. The recovery (%) represents the ratio of the sum of the four fractions to the total concentration from alkali digestion results. Numbers in brackets correspond to the standard deviation. Different letters indicate statistically different groups of F1, F2, F3 and F4 between initial and mature composts at the 0.05 probability level (p-value<0.05). The absence of letters indicates no significative difference between groups</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Days</th><th colspan="2">F1: Exch.*, acid and water soluble</th><th colspan="2">F2: Iron and manganese oxyhydroxides</th><th colspan="2">F3: Organic matter and sulfides</th><th colspan="2">F4: Residual</th><th colspan="2">Sum of 4 fractions</th><th rowspan="2">Recovery</th></tr><tr><th>mg kg<sup>-1</sup></th><th>%</th><th>mg kg<sup>-1</sup></th><th>%</th><th>mg kg<sup>-1</sup></th><th>%</th><th>mg kg<sup>-1</sup></th><th>%</th><th>mg kg<sup>-1</sup></th><th>%</th></tr></thead><tbody><tr><td rowspan="2">Ni</td><td>0</td><td>6.6 (2.2) a</td><td>2.3</td><td>11 (4.0) a</td><td>3.9</td><td>71 (9.7)</td><td>25</td><td>198 (60)</td><td>68</td><td>287 (71)</td><td>82</td></tr><tr><td>180</td><td>1.7 (0.2) b</td><td>0.5</td><td>1.8 (0.1) b</td><td>0.6</td><td>70 (9.6)</td><td>23</td><td>235 (48)</td><td>76</td><td>310 (57)</td><td>102</td></tr><tr><td rowspan="2">Cr</td><td>0</td><td>1.7 (0.6) a</td><td>0.2</td><td>3.4 (2.1) a</td><td>0.4</td><td>95 (35)</td><td>12</td><td>686 (174)</td><td>87</td><td>786 (196)</td><td>78</td></tr><tr><td>180</td><td>1.2 (0.0) b</td><td>0.1</td><td>1.2 (0.0) b</td><td>0.1</td><td>96 (15)</td><td>10</td><td>867 (135)</td><td>90</td><td>966 (141)</td><td>106</td></tr><tr><td rowspan="2">Cu</td><td>0</td><td>1.9 (1.0) a</td><td>3.0</td><td>0.2 (0.2) a</td><td>0.4</td><td>47 (19) a</td><td>78</td><td>11 (4.2)</td><td>18</td><td>60 (24)</td><td>80</td></tr><tr><td>180</td><td>0.5 (0.0) b</td><td>1.1</td><td>0.1 (0.0) b</td><td>0.1</td><td>21 (1.9) b</td><td>42</td><td>28 (1.3)</td><td>57</td><td>50 (2.6)</td><td>85</td></tr><tr><td rowspan="2">Zn</td><td>0</td><td>52 (1.0) a</td><td>26</td><td>79 (17) a</td><td>39</td><td>49 (14) a</td><td>25</td><td>19 (4.8 a)</td><td>10</td><td>200 (24)</td><td>126</td></tr><tr><td>180</td><td>14 (2.7) b</td><td>7.6</td><td>47 (1.8) b</td><td>25</td><td>83 (14) b</td><td>44</td><td>44 (7.2) b</td><td>23</td><td>188 (15)</td><td>116</td></tr></tbody></table>

<figure><img src="image_53.png" /><figcaption>**Figure SI 3-4 :** XANES spectra (a) and zoomed in the pre-peak area (b) registered at Cr K-edge in the original (t=d0) and mature (t=d180) composts (A, B, C) (c) procedure used to extract Cr K-edge pre-peaks of XANES spectra.</figcaption></figure>

<footnote>* Exch.: exchangeable</footnote>
### III. Labilité du chrome et nature de la matière organique

#### III.1 Introduction

La méthode Rock-Eval permettant de mesurer la qualité, la quantité et la maturité thermique de la matière organique dans les roches a été initialement développée pour l'exploitation du pétrole (Peters 1986). Le principe repose sur la mesure continue des composés carbonés issus d'un échantillon, simultanément à sa dégradation thermique progressive en étant soumis à une température croissante programmée. La dégradation thermique des composés carbonés est effectuée par deux étapes de chauffe successive : une pyrolyse en absence d'oxygène qui permet d'estimer les composés labiles (PC), suivie d'une combustion (des résidus de pyrolyse), en présence d'oxygène qui permet d'oxyder les matières dites récalcitrantes (RC). Ce type de mesure a été appliqué sur différents types de sols et de sédiments pour mesurer la matière organique du sol (Disnar et al. 2003; Marchand et al. 2008; Sebag et al. 2016). Des études ont également été menées sur le processus de compostage qui engendre une évolution rapide de la matière organique (Albrecht, Sebag, et al. 2015; Albrecht, Verrecchia, et al. 2015). Ces dernières ont mis en évidence des matières organiques labiles (PC) et récalcitrantes (RC) selon la méthode Rock-Eval.

Dans cette partie, nous avons cherché à comprendre les résultats surprenants d'évolution des réservoirs extractibles EDTA du chrome. Il a notamment été question d'explorer les liens entre la nature de la matière organique et ces réservoirs extractibles du chrome. En effet, les concentrations de chrome extraites à l'EDTA après 1 h et 24 h montraient une augmentation significative pendant les 60 premiers jours de compostage, avant de diminuer légèrement tout en restant plus élevées que lors de la mise en place des andains (Table 3-4). Cette période correspond au stade thermophile du compostage qui est marquée par la biodégradation de la matière organique labile (Hassen et al. 2001).

Nous avons donc émis l'hypothèse que la nature de la matière organique influence la labilité du chrome durant le compostage. En effet, le chrome rapidement extrait pourrait être préférentiellement associé à des composés de faibles poids moléculaires rapidement dégradés lors du compostage, notamment lors de la phase thermophile.
### III.2. Résultats et discussions

Pour tous les andains étudiés, le compostage a conduit à une diminution de la matière organique labile, alors que la variation des concentrations de matière organique récalcitrante (RC) est fonction de l'andain considéré, et ne présentait pas de tendance généralisable (Figure 3-9A).

En conséquence, la fraction de carbone labile (% PC) contenu dans le carbone organique total (COT) a diminué avec le processus de compostage (baisse moyenne d'environ 10 %) pour les trois andains, alors que la fraction de carbone récalcitrant (% RC) contenu dans le COT a augmenté proportionnellement avec le temps (Figure 3-9B). Ces résultats sont en accord avec les études antérieures qui ont décrit une dégradation microbienne préférentielle des éléments facilement dégradables par les microorganismes, particulièrement durant la phase thermophile du compostage (Albrecht, Sebag, et al. 2015; Remy Albrecht et al. 2007; Caricasole et al. 2011). Cette dégradation des composés labiles précède une seconde phase, la phase mésophile, durant laquelle l'activité biologique diminue en raison de la réduction des nutriments, dont la quantité et la qualité deviennent un facteur limitant. Cette seconde phase va alors engendrer la formation de composés récalcitrants (RC) tels que des substances humiques et donc une augmentation de l'aromaticité de la matière organique.

<figure><img src="image_54.png" /><figcaption>**Figure 3-9** : Variations de la teneur en matière organique (MO %) labile (PC) et récalcitrante (RC) sur les 3 andains A, B et C sur les composts initiaux (t0) et matures (t180) (A) et variations du ratio de la matière organique sur le carbone organique total (MO/COT %) (B)</figcaption></figure>

Les réservoirs extractibles EDTA de chrome estimés précédemment sont rappelés dans la **Figure 3-10**.
Une augmentation significative du réservoir rapidement extractible du chrome ($Q_1$) a pu être mise en évidence, sans pour autant que le réservoir extractible total ($Q_1 + Q_2$) n'augmente de manière généralisée durant le compostage.
<figure><img src="image_55.png" /><figcaption>**Figure 3-10** : Réservoirs labiles extraits rapidement Q₁, extraits moins rapidement Q₂, et extractibles total (Q₁+Q₂) (mg.kg⁻¹) pour les trois andains A, B et C sur les composts initiaux (t0) et matures (t180)</figcaption></figure>

Durant le compostage, la matière organique labile (PC) tend à diminuer en raison notamment de sa biodégradation (Figure 3-9), alors que le chrome rapidement extrait (Q₁) tend à augmenter (Figure 3-10). Les résultats de la comparaison des évolutions des fractions labiles du chrome et de la matière organique indiquent que le réservoir rapidement extractible (Q₁) de chrome diminue avec l'augmentation de la concentration en matière organique labile (PC) (Figure 3-11A), mais ne présente pas de lien avec la matière organique récalcitrante (RC) (Figure 3-11B). Ce lien suggère que le phénomène de libération du chrome pourrait persister jusqu'à ce que l'ensemble de la matière organique labile soit dégradée et/ou transformée. Récemment, de Souza et al. (2019) ont montré que la transformation de la matière organique labile en début de compostage conduisait à une solubilisation de structures organiques contenant des ETM. Néanmoins, il est également possible que le chrome rapidement extractible (Q₁) libéré par la biodégradation de la matière organique labile, soit au moins partiellement stabilisé par la matière organique récalcitrante durant la maturation du compost. Le réservoir moins rapidement extractible (Q₂) de chrome ne présente pas de lien spécifique avec les différentes composantes de la matière organique (Figure 3-11). En revanche, le réservoir extractible total (Q₁ + Q₂) de chrome diminue significativement avec la concentration en matière organique récalcitrante (RC) (Figure 3-11F). La fraction en matière organique récalcitrante (RC en %) ne présente pas de variation spécifique au cours du compostage (Figure 3-9), tout comme le réservoir extractible total (Q₁ + Q₂) de chrome (Figure 3-10). Ces résultats suggèrent ainsi un lien plus important du réservoir extractible totale (Q₁ + Q₂) de chrome avec la concentration en matière organique récalcitrante (RC en %) qu'une réponse au compostage lui-même.
Chapitre 3

ÉVOLUTION DE LA MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC AU COURS DU PROCESSUS DE COMPOSTAGE

<figure><img src="image_56.png" /><figcaption>**Figure 3-11** : Relations linéaires entre les réservoirs Q₁, Q₂, Q₁+Q₂ de chrome (Cr) et les concentrations en matière organique labile (PC %) ou récalcitrante (RC %)</figcaption></figure>
### III.3. Conclusion

Ces résultats montrent que le réservoir de chrome rapidement extractible diminue quand la fraction de matière organique labile augmente. La labilité du chrome est liée à la nature de la matière organique et donc à l'évolution de la matière organique au cours du processus de compostage. Ainsi, le type de compostage et son intensité ont une influence sur la qualité de la matière organique et donc sur la mobilité du chrome.

L'augmentation du chrome rapidement extractible durant le processus de compostage souligne que cet élément peut être lessivé hors du tas de compost. Ainsi, les concentrations en chrome des lixiviats issus des tas de composts sur les plateformes de compostage en plein air devraient être suivies, particulièrement lors des saisons des pluies
IV. Conclusion et perspectives du chapitre

Ce chapitre a mis en évidence les caractéristiques d'un compost constitué d'un mélange de boues de station d'épuration et de déchets verts produits en Nouvelle-Calédonie. L'effet du compostage sur le nickel et le chrome géogènes et le cuivre et le zinc anthropiques, a été étudié en appréhendant plus particulièrement la labilité de ces ETM.

Le nickel et le chrome contenus dans le compost sont apparus comme très peu mobiles selon les extractions séquentielles BCR (F1+F2), i.e. respectivement 6 % et <1 % de la concentration totale dans le compost mature, alors que le cuivre et le zinc, sont relativement mobiles, i.e. environ 30 % de la concentration totale pour les deux ETM. Selon le code d'évaluation des risques basé sur la fraction la plus soluble (F1 échangeable) des ETM (Avadí et al. 2022), les très faibles fractions F1 en chrome (0,1%) et nickel (0,5%) suggèrent une absence de risque (<1%) et les faibles fractions en cuivre (1,1%) et en zinc (7,6%), un faible risque (1 à 10%). Cependant, le réservoir extractible total ($Q_1 + Q_2$) en nickel (30 mg.kg$^{-1}$) représente la moitié de la concentration totale autorisée par la réglementation française NFU 44-095 (60 mg.kg$^{-1}$). Ces données montrent un risque environnemental potentiel lié à la libération sur le long terme du nickel du compost. De plus, les quantités totales en nickel et en chrome qui seraient apportées aux sols amendés lors d'apports répétitifs de compost conduiraient à une accumulation de ces éléments dans les sols puisque les éléments métalliques ne sont pas biodégradables (Tang et al. 2019).

Par ailleurs, nous avons pu montrer que la mobilité du nickel, du cuivre et du zinc tend à diminuer lors du compostage. En revanche, les extractions cinétiques à l'EDTA, nous ont permis de mettre en évidence une réelle spécificité du comportement du chrome. En effet, ces extractions permettent de discriminer des réservoirs rapidement extractibles sur le court terme $Q_1$, et des réservoirs moins rapidement extractibles sur le long terme $Q_2$. Les résultats obtenus pour le chrome indiquent que le réservoir $Q_1$ augmente entre le compost initial et mature, alors que le réservoir extractible total ($Q_1 + Q_2$) reste inchangé. Nous avons donc cherché à évaluer la présence de Cr(VI) dans le compost et à étudier son évolution durant le compostage. Les résultats mesurés par rayonnement synchrotron ne détectent pas de Cr(VI) dans les composts initiaux et matures.

Bien que la fraction extractible total ($Q_1 + Q_2$ %) de chrome soit faible (< 1 %), son comportement au cours du processus de compostage demeure intrigant, notamment en regard de l'absence de chrome hexavalent. Aussi, nous avons exploré la possibilité d'un lien entre chrome labile et labilité de la matière organique. De premiers résultats obtenus par la méthode Rock-Eval, suggèrent que c'est la biodégradation de la matière organique labile qui conduirait à la solubilisation du chrome et donc à l'augmentation du réservoir $Q_1$ de chrome durant le processus de compostage. Ces résultats méritent
néanmoins d'être approfondis pour mieux comprendre et quantifier les liens entre nature, voire structure, de la matière organique et labilité du chrome mais également des autres ETM au cours du compostage. En parallèle, il serait intéressant d'étudier les concentrations en ETM contenues dans les lixiviats récupérés pendant le processus de compostage, comme le suggèrent plusieurs auteurs (Chu et al. 2019; Molaey et al. 2021), afin de déterminer les formes solubles des ETM réellement libérées par les andains.

Les concentrations en nickel (300 mg.kg⁻¹) et en chrome (900 mg.kg⁻¹) particulièrement élevées dans les composts de Nouvelle-Calédonie semblent avoir pour origine les particules de sols dérivées des substrats ultrabasiques associées aux déchets verts. L'analyse des phases porteuses minérales contenues dans le compost par rayonnement synchrotron pourrait valider cette hypothèse, comme d'autres travaux menés pour caractériser la spéciation des ETM dans les composts et les déchets organiques (Legros et al. 2017; Wu et al. 2013). Le contrôle de l'origine des matières premières utilisées pour le compostage pourrait limiter la contamination en nickel et en chrome dans les déchets verts. Cependant, les gisements de déchets verts sur substrats ultrabasiques représentent un volume conséquent avec plus de 25 000 t récoltés en Province Sud qu'il est nécessaire de valoriser (Province Sud 2018). Par ailleurs, bien que les fractions extractibles EDTA en nickel et en chrome demeurent relativement faibles par rapport aux concentrations totales, il n'en demeure pas moins que le réservoir extractible total (Q₁+Q₂ mg.kg⁻¹) de ces ETM peut être conséquent, particulièrement pour le nickel avec 30 mg.kg⁻¹. Dans tous les cas, l'apport régulier de compost avec de telles concentrations est susceptible d'engendrer des phénomènes d'accumulation et d'enrichissement en nickel et en chrome dans les sols, notamment si ces derniers présentent un fond pédo-géochimique naturel faiblement concentré en ces ETM.

Le mélange de différentes sources de déchets verts afin de diluer les concentrations en ETM pourrait représenter une première solution. Cependant, se poserait alors la question du transport des sources des zones les moins contaminées vers les sources les plus contaminées qui correspondent aux substrats ultrabasiques, soit un tiers de la Nouvelle-Calédonie (Latham et al. 1978). L'impact écologique de telles pratiques à une échelle insulaire est non-négligeable, sachant que la Nouvelle-Calédonie se situe dans le top 10 mondial des pays les plus émetteurs de carbone par habitant. Et, il n'est d'ailleurs pas certain que la quantité de déchets verts non contaminés soit suffisante pour diluer les déchets verts potentiellement problématiques. Un « nettoyage » en amont de déchets verts pourrait représenter une solution alternative pour diminuer la concentration en contaminants dans le compost final. Néanmoins, au regard des quantités produites annuellement, les verrous techniques et économiques d'une telle pratique devraient être identifiées et leurs impacts écologiques évalués.
A court terme et malgré ces fortes concentrations en nickel et en chrome, il est nécessaire de trouver une voie de valorisation pour ces composts produits localement. Aussi, des filières agricoles présentant de plus faibles risques sanitaires de transfert des ETM vers les chaînes alimentaires sont à prioriser. En effet, les systèmes en arboriculture sont connus pour faiblement accumuler les ETM comparé à des systèmes en maraîchage (Houot et al. 2009; ADEME 2017; Gori et al. 2019). Deux filières potentielles ont ainsi été ciblées dans cette étude : la sylviculture sur Ferralsols développés sur substrats ultrabasiques et l'arboriculture fruitière en raison de transferts généralement limités vers les organes consommés. L'apport de compost engendre de réels bénéfices vis-à-vis de la productivité de ces systèmes (Oyetunji et al. 2022; Zhou et al. 2023). Cependant, il demeure primordial d'évaluer le devenir des ETM dans le système sol-eau-plante suite à des apports répétés de compost, notamment dans une optique d'évaluation des risques vis-à-vis de leur disponibilité environnementale, à plus long terme. Aussi, dans le chapitre suivant, nous évaluerons l'impact de l'épandage d'un tel compost sur deux sols agricoles contrastés et représentatifs des filières cibles, i.e. un Ferralsol sous occupation sylvicole présentant de fortes concentrations en nickel et en chrome et un Fluvisol sous occupation agrumicole, présentant de très faibles concentrations en ces éléments.
EVOLUTION DE LA MOBILITÉ DU NI, CR, CU & ZN AU COURS DU PROCESSUS DE COMPOSTAGE

<figure><img src="image_57.png" /></figure>

CARACTÉRISTIQUES

<table><thead><tr><th></th><th>t<sub>0</sub></th></tr></thead><tbody><tr><td>Ni<br>(mg kg<sup>-1</sup>)</td><td>349<br>±61</td></tr><tr><td>Cr<br>(mg kg<sup>-1</sup>)</td><td>1002<br>±249</td></tr><tr><td>pH</td><td>7.1<br>±0.1</td></tr><tr><td>C/N</td><td>13<br>±0.7</td></tr></tbody></table>

<figure><img src="image_58.png" /></figure>

RÉSULTATS

* Ni et Cr géogènes → phases porteuses minérales (chromite, goethite)

<figure><img src="image_59.png" /></figure>

Vérification par MEB

<figure><img src="image_60.png" /></figure>

Dans compost mature :

* Spéciation selon BCR fractions mobiles (F1+F2) Cu et Zn (~30%) > Ni (6%) > Cr (1%)

* Extraction cinétique EDTA réservoir extractible total : Zn (79%) >> Cu (41%) > Ni (8%) > Cr (1%)

Avec le processus de compostage (t0 à t+180j) :

* ↕ Ni, Cu & Zn mobilité → Capacité de la MO à créer des complexes organo-métalliques

Vérification par rayonnement Synchrotron

<figure><img src="image_61.png" /></figure>

* ↕ Cr réservoir rapidement extrait Q₁ (x2)

Présence Cr(VI) ?

Vérification par Rock-Eval

<figure><img src="image_62.png" /></figure>

Biodégradation de la MO labile

<figure><img src="image_63.png" /></figure>

Ni réservoir extractible total (Q₁+Q₂) : 30 mg.kg⁻¹ vs. 60 mg.kg⁻¹ en concentration totale autorisée dans les composts de boues de STEP (NFU 44-095)

→ risque d'être libéré dans l'environnement

Ouverture : Quel est le comportement des ETM suite à l'épandage d'un compost riche en Ni et du Cr sur des sols agricoles ?
## VI. Références

ADEME (2017) Phytodisponibilté des ETM Pour les plantes potagères et extrapolations dans la quantification de l'exposition des consommateurs

AFNOR (2002) NFU44-095

Albrecht R, Sebag D, Verrecchia E (2015a) Organic matter decomposition: bridging the gap between Rock-Eval pyrolysis and chemical characterization (CPMAS13C NMR). Biogeochemistry 122:101-111. https://doi.org/10.1007/s10533-014-0033-8

Albrecht R, Terrom G, Perissol C (2007) Co-compostage de boues de station d'épuration et de déchets verts : Nouvelle méthodologie du suivi des transformations de la matière organique. Dr en Biosci l'Environnement Doctorat:189

Albrecht R, Verrecchia E, Pfeifer HR (2015b) The use of solid-phase fluorescence spectroscopy in the characterisation of organic matter transformations. Talanta 134:453-459. https://doi.org/10.1016/j.talanta.2014.11.056

Amir S, Hafidi M, Merlina G, Revel JC (2005) Sequential extraction of heavy metals during composting of sewage sludge. Chemosphere 59:801-810. https://doi.org/10.1016/j.chemosphere.2004.11.016

Avadí A, Benoit P, Bravin MN, et al (2022) Trace contaminants in the environmental assessment of organic waste recycling in agriculture: Gaps between methods and knowledge. Adv Agron 174:53-188. https://doi.org/10.1016/bs.agron.2022.03.002

Awasthi MK, Duan Y, Awasthi SK, et al (2020) Emerging applications of biochar: Improving pig manure composting and attenuation of heavy metal mobility in mature compost. J Hazard Mater 389: https://doi.org/10.1016/j.jhazmat.2020.122116

Barbier G, Gueyte R, L'Huillier L (2014) Détermination des teneurs en éléments traces métalliques dans les déchets verts

Becquer T, Quantin C, Boudot JP (2010) Toxic levels of metals in Ferralsols under natural vegetation and crops in New Caledonia. Eur J Soil Sci 61:994-1004. https://doi.org/10.1111/j.1365-2389.2010.01294.x

Becquer T, Quantin C, Rotte-Capet S, et al (2006) Sources of trace metals in Ferralsols in New Caledonia. Eur J Soil Sci 200-213. https://doi.org/10.1111/j.1365-2389.2005.00730.x

Becquer T, Quantin C, Sicot M, Boudot JP (2003) Chromium availability in ultramafic soils from New Caledonia. Sci Total Environ 301:251-261. https://doi.org/10.1016/S0048-9697(02)00298-X

Bermond A, Varrault G, Sappin-Didier V, Mench M (2005) A kinetic approach to predict soil trace metal bioavailability: preliminary results. Plant Soil 275:21-29. https://doi.org/10.1007/s11104-004-7599-1

Bolan NS, Baskaran S, Thiagarajan S (1996) An evaluation of the methods of measurement of dissolved organic carbon in soils, manures, sludges, and stream water. Commun Soil Sci Plant Anal 27:2723-2737. https://doi.org/10.1080/00103629609369735

Bolaños-benítez V, Hullebusch ED Van, Birck J, et al (2020) Chromium mobility in ultramafic areas affected by mining activities in Barro Alto massif, Brazil: An isotopic study. Chem Geol 120000. https://doi.org/10.1016/j.chemgeo.2020.120000

Borch T, Kretzschmar R, Skappler A, et al (2010) Biogeochemical redox processes and their impact on contaminant dynamics. Environ Sci Technol 44:15-23. https://doi.org/10.1021/es9026248
Brunori C, Cremisini C, D'Annibale L, et al (2005) A kinetic study of trace element leachability from abandoned-mine-polluted soil treated with SS-MSW compost and red mud. Comparison with results from sequential extraction. *Anal Bioanal Chem* 381:1347-1354.
https://doi.org/10.1007/s00216-005-3124-5

Caricasole P, Provenzano MR, Hatcher PG, Senesi N (2011) Evolution of organic matter during composting of different organic wastes assessed by CPMAS 13C NMR spectroscopy. *Waste Manag* 31:411-415. https://doi.org/10.1016/j.wasman.2010.09.020

Chen YX, Huang XD, Han ZY, et al (2010) Effects of bamboo charcoal and bamboo vinegar on nitrogen conservation and heavy metals immobility during pig manure composting. *Chemosphere* 78:1177-1181. https://doi.org/10.1016/j.chemosphere.2009.12.029

Cheng J, Ding C, Li X, et al (2015) Heavy metals in navel orange orchards of Xinfeng County and their transfer from soils to navel oranges. *Ecotoxicol Environ Saf* 122:153-158.
https://doi.org/10.1016/j.ecoenv.2015.07.022

Chrysochoou M, Theologou E, Bompoti N, et al (2016) Occurrence, Origin and Transformation Processes of Geogenic Chromium in Soils and Sediments. *Curr Pollut Reports* 2:224-235.
https://doi.org/10.1007/s40726-016-0044-2

Chu Z, Fan X, Wang W, Huang W chiao (2019) Quantitative evaluation of heavy metals' pollution hazards and estimation of heavy metals' environmental costs in leachate during food waste composting. *Waste Manag* 84:119-128. https://doi.org/10.1016/j.wasman.2018.11.031

Ciba J, Zo M, Kluczka J, et al (2003) Comparison of methods for leaching heavy metals from composts. *Waste Manag* 23:897-905. https://doi.org/10.1016/S0956-053X(03)00128-4

de Souza C da CB, Amaral Sobrinho NMB do, Lima ESA, et al (2019) Relation between changes in organic matter structure of poultry litter and heavy metals solubility during composting. *J Environ Manage* 247:291-298. https://doi.org/10.1016/j.jenvman.2019.06.072

Disnar JR, Guillet B, Keravis D, et al (2003) Soil organic matter (SOM) characterization by Rock-Eval pyrolysis: Scope and limitations. *Org Geochem* 34:327-343. https://doi.org/10.1016/S0146-6380(02)00239-5

Døelsch E, Deroche B, Van de Kerchove V (2006) Impact of sewage sludge spreading on heavy metal speciation in tropical soils (Réunion, Indian Ocean). *Chemosphere* 65:286-293.
https://doi.org/10.1016/j.chemosphere.2006.02.046

Dublet G, Juillot F, Morin G, et al (2012a) Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation. *Geochim Cosmochim Acta* 95:119-133.
https://doi.org/10.1016/j.gca.2012.07.030

Dublet G, Juillot F, Morin G, et al (2012b) Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation. *Geochim Cosmochim Acta* 95:119-133.
https://doi.org/10.1016/j.GCA.2012.07.030

El Fels L (2014) Suivi physico-chimique, microbiologique et écotoxicologique du compostage de boues de step mélangées à des déchets de palmier : validation de nouveaux indices de maturité. *Institut National Polytechnique de Toulouse*

El Fels L, Zamama M, El Asli A, Hafidi M (2014) Assessment of biotransformation of organic matter during co-composting of sewage sludge-lignocelulosic waste by chemical, FTIR analyses, and phytotoxicity tests. *Int Biodeterior Biodegrad* 87:128-137.
https://doi.org/10.1016/j.ibiod.2013.09.024
Fandeur D, Juillot F, Fritsch E, et al (2010) Crystal-chemistry of Ni and Cr within phyllosilicates and Fe/mn oxyhydroxydes along a lateritic regolith in NC. In: EM61 - Mineralogy of mine wastes and contaminated soils. pp 337-

Fandeur D, Juillot F, Morin G, et al (2009a) XANES evidence for oxidation of Cr(III) to Cr(VI) by Mn-oxides in a lateritic regolith developed on serpentinized ultramafic rocks of New Caledonia. Environ Sci Technol 43:7384-7390. https://doi.org/10.1021/es900498r

Fandeur D, Juillot F, Morin G, et al (2009b) Synchrotron-based speciation of chromium in an oxisol from new caledonia: Importance of secondary fe-oxyhydroxides. Am Mineral 94:710-719. https://doi.org/10.2138/am.2009.3073

Fang M, Wong JWC (1999) Effects of lime amendment on availability of heavy metals and maturation in sewage sludge composting. Environ Pollut 106:83-89. https://doi.org/10.1016/S0269-7491(99)00056-1

Fangueiro D, Bermond A, Santos E, et al (2005) Kinetic approach to heavy metal mobilization assessment in sediments: Choose of kinetic equations and models to achieve maximum information. Talanta 66:844-857. https://doi.org/10.1016/j.talanta.2004.12.036

Farrell M, Jones DL (2009) Heavy metal contamination of a mixed waste compost: Metal speciation and fate. Bioresour Technol 100:4423-4432. https://doi.org/10.1016/j.biortech.2009.04.023

Francou C (2003) Stabilisation de la matière organique au cours du compostage de déchets urbains : influence de la nature des déchets et du procédé de compostage - recherche d'indicateurs pertinents. INAPG (AgroParisTech)

Garcia C, Moreno JL, Hernfindez T, et al (1995) Effect of composting on sewage sludges contaminated with heavy metals. Bioresour Technol 53:13-19. https://doi.org/10.1016/0960-8524(95)00040-L

Gori A, Ferrini F, Fini A (2019) Reprint of: Growing healthy food under heavy metal pollution load: Overview and major challenges of tree based edible landscapes. Urban For Urban Green 45:126292. https://doi.org/10.1016/j.ufug.2019.02.009

Greenway GM, Song QJ (2002) Heavy metal speciation in the composting process. J Environ Monit 4:300-305. https://doi.org/10.1039/b110608m

Gustafsson JP, Persson I, Oromieh AG, et al (2014) Chromium(III) complexation to natural organic matter: Mechanisms and modeling. Environ Sci Technol 48:1753-1761. https://doi.org/10.1021/es404557e

Hanc A, Szakova J, Svehla P (2012) Effect of composting on the mobility of arsenic, chromium and nickel contained in kitchen and garden waste. Bioresour Technol 126:444-452. https://doi.org/10.1016/j.biortech.2011.11.053

Hassen A, Belguith K, Jedidi N, et al (2001) Microbial characterization during composting of municipal solid waste. Bioresour Technol 80:217-225. https://doi.org/10.1016/S0960-8524(01)00065-7

Hazarika J, Ghosh U, Kalamdhad AS, et al (2017) Transformation of elemental toxic metals into immobile fractions in paper mill sludge through rotary drum composting. Ecol Eng 101:185-192. https://doi.org/10.1016/j.ecoleng.2017.02.005

He M miao, Tian G ming, Liang X qiang (2009) Phytotoxicity and speciation of copper, zinc and lead during the aerobic composting of sewage sludge. J Hazard Mater 163:671-677. https://doi.org/10.1016/j.jhazmat.2008.07.013

Houles A, Vincent B, David M, et al (2018) Ectomycorrhizal Communities Associated with the Legume Acacia spirorbis Growing on Contrasted Edaphic Constraints in New Caledonia. Microb Ecol
76:964–975. https://doi.org/10.1007/s00248-018-1193-1

Houot S, Cambier P, Benoit P, et al (2009) Effet d'apports de composts sur la disponibilité de micropolluants métalliques et organiques dans un sol cultivé. Etude Gest des Sols 16:255-274

Houot S, Pons M-N, Pradel M, Tibi AT (2016) Recyclage de déchets organiques en agriculture - Effets agronomiques et environnementaux de leur épandage, Quae

Idrees M, Batool S, Kalsoom T, et al (2018) Animal manure-derived biochars produced via fast pyrolysis for the removal of divalent copper from aqueous media. J Environ Manage 213:109–118.
https://doi.org/10.1016/j.jenvman.2018.02.003

Iwegbue CMA, Emuh FN, Isirimah NO, Egun AC (2007) Fractionation, characterization and speciation of heavy metals in composts and compost-amended soils. African J Biotechnol 6:067–078.
https://doi.org/10.1002/chin.200748228

Jakubus M, Czekala J (2010) Chromium and nickel speciation during composting process of different biosolids. Fresenius Environ Bull 19:289–299

Kamaludeen SPB, Megharaj M, Juhasz AL, et al (2003) Chromium-microorganism interactions in soils: remediation implications. Rev Environ Contam Toxicol 178:93–164. https://doi.org/10.1007/0-387-21728-2_4

Kierczak J, Neel C, Aleksander-Kwaterczak U, et al (2008) Solid speciation and mobility of potentially toxic elements from natural and contaminated soils: A combined approach. Chemosphere
73:776–784. https://doi.org/10.1016/j.chemosphere.2008.06.015

Kierczak J, Pietranik A, Pędziwiatr A (2021) Ultramafic geoecosystems as a natural source of Ni, Cr, and Co to the environment: A review. Sci Total Environ 755:.
https://doi.org/10.1016/j.scitotenv.2020.142620

Kumar A, Joseph S, Tsechansky L, et al (2018) Biochar aging in contaminated soil promotes Zn immobilization due to changes in biochar surface structural and chemical properties. Sci Total Environ 626:953–961. https://doi.org/10.1016/j.scitotenv.2018.01.157

Labanowski J, Monna F, Bermond A, et al (2008) Kinetic extractions to assess mobilization of Zn, Pb, Cu, and Cd in a metal-contaminated soil: EDTA vs. citrate. Environ Pollut 152:693–701.
https://doi.org/10.1016/j.envpol.2007.06.054i

Latham M, Quantin P, Aubert G (1978) Étude des sols de la Nouvelle-Calédonie Nouvel essai sur la classification, la caractérisation, la pédogénèse et les aptitudes des sols de Nouvelle-Calédonie. Paris

Legros S, Levard C, Marcato-Romain C-E, et al (2017) Anaerobic Digestion Alters Copper and Zinc Speciation. Environ Sci Technol 51:10326–10334. https://doi.org/10.1021/acs.est.7b01662

Li C, Li H, Yao T, et al (2021) Effects of swine manure composting by microbial inoculation: Heavy metal fractions, humic substances, and bacterial community metabolism. J Hazard Mater 415:.
https://doi.org/10.1016/j.jhazmat.2021.125559

Li J, Song N (2020) Graphene oxide-induced variations in the processing performance, microbial community dynamics and heavy metal speciation during pig manure composting. Process Saf Environ Prot 136:214–222. https://doi.org/10.1016/j.psep.2020.01.028

Liu Y, Ma L, Li Y, Zheng L (2007) Evolution of heavy metal speciation during the aerobic composting process of sewage sludge. Chemosphere 67:1025–1032.
https://doi.org/10.1016/j.chemosphere.2006.10.056
Löv Å, Sjöstedt C, Larsbo M, et al (2017) Solubility and transport of Cr(III) in a historically contaminated soil – Evidence of a rapidly reacting dimeric Cr(III) organic matter complex. Chemosphere 189:709–716. https://doi.org/10.1016/j.chemosphere.2017.09.088

Lv B, Xing M, Yang J (2016) Speciation and transformation of heavy metals during vermicomposting of animal manure. Bioresour Technol 209:397–401. https://doi.org/10.1016/j.biortech.2016.03.015

Marchand C, Lallier-Vergès E, Disnar JR, Kéravis D (2008) Organic carbon sources and transformations in mangrove sediments: A Rock-Eval pyrolysis approach. Org Geochem 39:408–421. https://doi.org/10.1016/j.orggeochem.2008.01.018

Maturi KC, Banerjee A, Kalamdhad AS (2021) Assessing mobility and chemical speciation of heavy metals during rotary drum composting of Ageratum conyzoides. Environ Technol Innov 24:. https://doi.org/10.1016/j.eti.2021.101871

Medina J, Monreal C, Chabot D, et al (2017) Microscopic and spectroscopic characterization of humic substances from a compost amended copper contaminated soil: main features and their potential effects on Cu immobilization. Environ Sci Pollut Res 24:14104–14116. https://doi.org/10.1007/s11356-017-8981-x

Merrot P, Juillot F, Flipo L, et al (2022) Bioavailability of chromium, nickel, iron and manganese in relation to their speciation in coastal sediments downstream of ultramafic catchments: A case study in New Caledonia. Chemosphere 302:. https://doi.org/10.1016/j.chemosphere.2022.134643

Molaey R, Yesil H, Calli B, Tugtas AE (2021) Enhanced heavy metal leaching from sewage sludge through anaerobic fermentation and air-assisted ultrasonication. Chemosphere 279:130548. https://doi.org/10.1016/j.chemosphere.2021.130548

Nomeda S, Valdas P, Chen SY, Lin JG (2008) Variations of metal distribution in sewage sludge composting. Waste Manag 28:1637–1644. https://doi.org/10.1016/j.wasman.2007.06.022

Oyetunji O, Bolan N, Hancock G (2022) A comprehensive review on enhancing nutrient use efficiency and productivity of broadacre (arable) crops with the combined utilization of compost and fertilizers. J Environ Manage 317:115395. https://doi.org/10.1016/j.jenvman.2022.115395

Paillat J-M, Hassouna M, Robin P (2005) Abattements d'azote lors du compostage de fumier de vaches laitières : exemple de cinq élevages des Côtes d'Armor. Rennes

Paredes C, Bernal MP, Cegarra J, et al (1996) Nitrogen transformation during the composting of different organic wastes. Prog Nitrogen Cycl Stud 121–125. https://doi.org/10.1007/978-94-011-5450-5_19

Pasquet C, Monna F, van Oort F, et al (2018) Mobility of Ni, Co, and Mn in ultramafic mining soils of New Caledonia, assessed by kinetic EDTA extractions. Environ Monit Assess 190:. https://doi.org/10.1007/s10661-018-7029-0

Payet N, Van de Kerchove V, Doelsch E, Moussard G (2008) Composts de déchets verts urbains réunionnais : origine des concentrations élevées en éléments traces métalliques (ETM)

Peters KE (1986) Guidelines for Evaluating Petroleum Source Rock Using Programmed Pyrolysis. Am Assoc Pet Geol Bull 70:318–329

Pribyl DW (2010) A critical review of the conventional SOC to SOM conversion factor. Geoderma 156:75–83. https://doi.org/10.1016/j.geoderma.2010.02.003

Province Sud (2018) Schéma Provincial de Prévention et de Gestion des Déchets 2018 – 2022
Quantin C, Becquer T, Rouiller JH, Berthelin J (2002) Redistribution of metals in a New Caledonia Ferralsol after microbial weathering. Soil Sci Soc Am J 66:1797-1804.
https://doi.org/10.2136/sssaj2002.1797

Quevauviller P (2002) Operationally-defined extraction procedures for soil and sediment analysis. Part 3: New CRMs for trace-element extractable contents. TrAC - Trends Anal Chem 21:774-785.
https://doi.org/10.1016/S0165-9936(02)01105-6

Quevauviller P, Rauret G, Lopez-Sanchez J-F, et al (1997) Certification of trace metal extractable contents in a sediment reference material (CRM 601) following a three-step sequential extraction procedure. Sci Total Environ 205:223-234. https://doi.org/10.1016/S0048-9697(97)00205-2

Quevauviller P, Rauret G, Ure A, et al (1995) Preparation of Candidate Certified Reference Materials for the Quality Control of EDTA-and Acetic Acid-Extractable Trace Metal Determinations in Sewage Sludge-Amended Soil and Terra Rossa Soil Background of the Project. Mikrochim Acta 120:289-300. https://doi.org/10.1007/BF01244439

Rehana MR, Joseph B, Gladis R (2020) Heavy Metal Stabilization in Sewage Sludge Composting Process. Curr J Appl Sci Technol 38-48. https://doi.org/10.9734/cjast/2020/v39i1930789

Roba C, Roșu C, Piștea I, et al (2016) Heavy metal content in vegetables and fruits cultivated in Baia Mare mining area (Romania) and health risk assessment. Environ Sci Pollut Res 23:6062-6073.
https://doi.org/10.1007/s11356-015-4799-6

Sahuquillo A, Rigol A, Rauret G (2003) Overview of the use of leaching/extraction tests for risk assessment of trace metals in contaminated soils and sediments. TrAC - Trends Anal. Chem. 22:152-159

Santos S, Costa CAE, Duarte AC, et al (2010) Influence of different organic amendments on the potential availability of metals from soil: A study on metal fractionation and extraction kinetics by EDTA. Chemosphere 78:389-396. https://doi.org/10.1016/j.chemosphere.2009.11.008

Sebag D, Verrecchia EP, Cécillon L, et al (2016) Dynamics of soil organic matter based on new Rock-Eval indices. Geoderma 284:185-203. https://doi.org/10.1016/j.geoderma.2016.08.025

Singh J, Kalamdhad AS (2013) Assessment of bioavailability and leachability of heavy metals during rotary drum composting of green waste (Water hyacinth). Ecol Eng 52:59-69.
https://doi.org/10.1016/j.ecoleng.2012.12.090

Smith SR (2009) A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge. Environ Int.
https://doi.org/10.1016/j.envint.2008.06.009

Sun SS, Ao M, Geng KR, et al (2022) Enrichment and speciation of chromium during basalt weathering: Insights from variably weathered profiles in the Leizhou Peninsula, South China. Sci Total Environ 822:153304. https://doi.org/10.1016/j.scitotenv.2022.153304

Szulczewski MD, Helmke PA, Bleam WF (2001) XANES spectroscopy studies of Cr(VI) reduction by thiols in organosulfur compounds and humic substances. Environ Sci Technol 35:1134-1141.
https://doi.org/10.1021/es001301b

Tang J, Zhang J, Ren L, et al (2019) Diagnosis of soil contamination using microbiological indices: A review on heavy metal pollution. J Environ Manage 242:121-130.
https://doi.org/10.1016/j.jenvman.2019.04.061

Teitler Y, Cathelineau M, Ulrich M, et al (2019) Petrology and geochemistry of scandium in New Caledonian Ni-Co laterites. J Geochemical Explor 196:131-155.
https://doi.org/10.1016/j.gexplo.2018.10.009

Tella M, Doelsch E, Letourmy P, et al (2013) Investigation of potentially toxic heavy metals in different organic wastes used to fertilize market garden crops. Waste Manag 33:184-192.
https://doi.org/10.1016/j.wasman.2012.07.021

Ulrich M, Cathelineau M, Muñoz M, et al (2019) The relative distribution of critical (Sc, REE) and transition metals (Ni, Co, Cr, Mn, V) in some Ni-laterite deposits of New Caledonia. J Geochemical Explor 197:93-113. https://doi.org/10.1016/j.gexplo.2018.11.017

Ure A, Davidson C (2008) Chemical speciation in the environment, Second Edt. John Wiley & Sons

Vincent B, Jourand P, Juillot F, et al (2018) Biological in situ nitrogen fixation by an Acacia species reaches optimal rates on extremely contrasted soils. Eur J Soil Biol 86:52-62.
https://doi.org/10.1016/j.ejsobi.2018.03.003

Walter I, Martínez F, Cala V (2006) Heavy metal speciation and phytotoxic effects of three representative sewage sludges for agricultural uses. Environ Pollut 139:507-514.
https://doi.org/10.1016/j.envpol.2005.05.020

Wang YL, Tsou MC, Liao HT, et al (2020) Influence of soil properties on the bioaccessibility of Cr and Ni in geologic serpentine and anthropogenically contaminated non-serpentine soils in Taiwan. Sci Total Environ 714:136761. https://doi.org/10.1016/j.scitotenv.2020.136761

Wells MA, Ramanaidou ER, Zakaria Quadir M, et al (2022) Morphology, composition and dissolution of chromite in the Goro lateritic nickel deposit, New Caledonia: Insight into ophiolite and laterite genesis. Ore Geol Rev 143:104752. https://doi.org/10.1016/j.oregeorev.2022.104752

Wu J, Zhang H, Shao LM, He PJ (2013) Nondestructive characterization of the contaminated biodegradable fraction of municipal solid waste using synchrotron radiation-induced micro-X-ray fluorescence. Bioresour Technol 132:239-243. https://doi.org/10.1016/j.biortech.2013.01.066

Xu S, Chen M, Feng T, et al (2021) Use ggbreak to Effectively Utilize Plotting Space to Deal With Large Datasets and Outliers. Front Genet 12:. https://doi.org/10.3389/fgene.2021.774846

Xu X, Xia L, Huang Q, et al (2012) Biosorption of cadmium by a metal-resistant filamentous fungus isolated from chicken manure compost. Environ Technol (United Kingdom) 33:1661-1670.
https://doi.org/10.1080/09593330.2011.641591

Zhan J, Han Y, Xu S, et al (2022) Succession and change of potential pathogens in the co-composting of rural sewage sludge and food waste. Waste Manag 149:248-258.
https://doi.org/10.1016/j.wasman.2022.06.028

Zheng G Di, Gao D, Chen T Bin, Luo W (2007) Stabilization of nickel and chromium in sewage sludge during aerobic composting. J Hazard Mater 142:216-221.
https://doi.org/10.1016/j.jhazmat.2006.08.003

Zhou S, Jiang Z, Shen J, et al (2023) Biochar-amended compost as a promising soil amendment for enhancing plant productivity: A meta-analysis study. Sci Total Environ 879:163067.
https://doi.org/10.1016/j.scitotenv.2023.163067

Zorpas AA, Inglezakis VJ, Loizidou M (2008) Heavy metals fractionation before, during and after composting of sewage sludge with natural zeolite. Waste Manag 28:2054-2060.
https://doi.org/10.1016/j.wasman.2007.09.006
Chapitre 4 MOBILITE DU NICKEL, CHROME,
CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES
AVEC ET SANS AMENDEMENT
Le chapitre précédent a montré que le processus de compostage influençait les fractions mobiles du nickel, du chrome, du cuivre et du zinc du compost. En effet, nos résultats ont mis en évidence une stabilisation des fractions mobiles du nickel, du cuivre et du zinc à maturité du compost, qui serait liée à la capacité de la matière organique à créer des complexes organo-métalliques stables. Le réservoir extractible EDTA total ($Q_1+Q_2$) des ETM dans le compost mature suit l'ordre décroissant : Zn >> Cu > Ni > Cr. Bien que les pourcentages extraits par rapport aux concentrations totales soient faibles pour le nickel et le chrome, les concentrations extraites (mg.kg⁻¹) correspondantes sont élevées. Pour le nickel, le réservoir extractible EDTA total, i.e. 30 mg.kg⁻¹ représente la moitié des concentrations (pseudo)totales autorisées par la réglementation NFU 44-095 dans les composts de boues de STEP. Pour le chrome, le réservoir extractible EDTA total est plus faible, i.e. 7 mg.kg⁻¹, mais le réservoir rapidement extractible EDTA ($Q_1$) est multipliée par deux au cours du compostage. De tels résultats suggèrent un risque environnemental et sanitaire non-négligeable pour ces deux éléments, lié à leur potentialité de mobilisation. Ainsi, des questionnements demeurent quant au devenir de ces ETM après épandage sur les sols agricoles du territoire dont les caractéristiques physico-chimiques peuvent être particulièrement contrastées.

Deux filières agricoles de Nouvelle-Calédonie pourraient être identifiées comme des voies judicieuses de valorisation d'un compost présentant de telles concentrations en nickel et en chrome. La première est la filière sylvicole dont la moitié des parcelles sont présentes sur Ferralsols dérivés de substrats ultrabasiques (pers.com SudForêt). La sylviculture représente une voie judicieuse de recyclage car il n'existe pas de risque de contamination des ressources alimentaires. Cependant, les sols sur lesquels sont implantés les arbres sont particulièrement drainants en raison de leur texture grossière, et des épisodes fréquents d'engorgement pourraient favoriser la lixiviation des ETM consécutivement à la réduction des phases porteuses du nickel et du chrome, i.e. les oxydes de fer et de manganèse (Quantin et al. 2002; Fandeur et al. 2009a; Borch et al. 2010). L'apport de matière organique *via* l'épandage du compost pourra également favoriser le transfert d'ETM en créant des complexes organo-métalliques solubles (Hernandez-Soriano and Jimenez-Lopez 2012). La seconde filière de valorisation du compost est l'arboriculture fruitière, notamment l'agrumiculture, qui représente la deuxième production de fruits du territoire (DAVAR 2010). Dans cette filière, la culture d'oranges représente les plus grandes surfaces de vergers, dont les trois quarts sont localisées dans le bassin versant de la Foa présentant des sols présentant de faibles concentrations en nickel et en chrome. L'accumulation des ETM en arboriculture serait moindre comparé aux céréales, légumes feuilles ou légumes racines (Houot et al. 2009; Cheng et al. 2015; Roba et al. 2016; ADEME 2017; Gori et al. 2019). Ainsi, bien que le compost soit susceptible d'engendrer une accumulation d'ETM sur ce type de sol contenant naturellement des
concentrations faibles, le risque de transfert de ces ETM dans le système compost-sol-plante devrait
être faible.

La littérature indique également que l'apport de compost peut modifier la mobilité des ETM du sol (Chapitre 1, 3.3.). Des résultats contrastés et variables sont répertoriés selon la nature du compost, le type de sol et les ETM considérés. De plus, des études antérieures ont montré que les caractéristiques du sol amendé jouent un rôle plus important sur le devenir à long terme des ETM du sol que la nature du compost (Smith 2009; Paradelo et al. 2018). La fraction mobile des ETM dans les sols amendés dépend des propriétés physico-chimiques des sols, le pH ou la teneur en matière organique, mais également de la fréquence et de la répétition des apports sur le long terme (Smith 2009; Houot et al. 2014). Cependant, à notre connaissance, il n'existe aucune étude sur l'impact de l'épandage d'un compost présentant de telles concentrations en nickel et en chrome géogènes, sur le devenir de ces deux ETM dans le sol après épandage. En modifiant les paramètres physico-chimiques des sols par épandage de compost, la question est de savoir si les fractions mobiles du nickel, chrome, cuivre et zinc de ces sols va varier ? Et est-ce que ces variations sont fonction du type de sol considéré ?

L'objectif du chapitre 4 est d'étudier le comportement des ETM dans deux types de sols agricoles représentatifs des deux filières agricoles favorables à une valorisation de ce type de compost en Nouvelle-Calédonie :

(I) Un Ferralsol dérivé de substrats ultrabasiques qui présente des concentrations élevées en nickel et en chrome, i.e. jusqu'à 7 000 mg.kg⁻¹ et 30 000 mg.kg⁻¹ respectivement (Vincent et al. 2018), sol largement utilisé en sylviculture sur le territoire, et particulièrement en Province Sud ;

(II) Un Fluvisol qui présente de faibles concentrations en nickel et en chrome, i.e. environ 20 mg.kg⁻¹ et 50 mg.kg⁻¹ respectivement, sol du bassin versant de la Foa qui représente le plus grand bassin de productions d'agrumes du territoire.

L'impact de l'épandage sur la mobilisation des ETM a été évaluée à travers la mise en œuvre d'extractions cinétiques EDTA sur le sol témoin et le sol amendé dont les résultats sont comparés à ceux du compost. Ces dernières permettent de discriminer deux réservoirs extractibles d'ETM, i.e. un réservoir rapidement extractible ($Q_1$) et un réservoir moins rapidement extractible ($Q_2$) (Fangueiro et al. 2005) ; et ainsi d'appréhender théoriquement la mobilité des ETM sur le court ($Q_1$) et le long terme ($Q_1+Q_2$) en réponse à l'épandage.

Les résultats obtenus sont présentés en trois parties. La première partie concerne les Ferralsols utilisés en sylviculture et elle est présentée sous forme d'article "Ni and Cr remobilization from ultramafic soils
*after compost input: Evidence from EDTA kinetic extractions in New Caledonia*" soumis dans le journal
Environnemental Chemistry Letters. La deuxième partie concerne les Fluvisols utilisés en agrumiculture
et elle est présentée sous forme d'ébauche d'un article intitulé "*Effect of geogenic Cr- and Ni-rich
compost inputs on trace metal mobility in a low trace metal soil : a preliminary assessment*". La
troisième partie, rédigée en français, concerne les résultats d'une analyse statistique destinée à mettre
en évidence l'influence des paramètres physico-chimiques des deux types de sols sélectionnés sur les
réservoirs extractibles EDTA des ETM étudiés
# I. Ni and Cr remobilization from ultramafic soils after compost input: Evidence from EDTA kinetic extractions in New Caledonia

Authors: Perrine Klein<sup>1,2,a</sup>, Peggy Gunkel-Grillon<sup>2</sup>, Farid Juillot<sup>3,4</sup>, Hélène Kaplan<sup>1</sup>, Anthony Pain<sup>1</sup>, Frédéric Feder<sup>5,6</sup>, Audrey Léopold<sup>1</sup>

## Abstract (less than 250 words)

Compost inputs are known to improve soil fertility and represent a valuable way to valorize organic wastes. However, compost application could increase metal mobility in metal-concentrated soils, generating environmental and health risks. The aim of this study was to assess the influence of a single compost input on the evolution of Ni and Cr mobility, in ultramafic Ferralsols from New Caledonia. The behavior of Cu and Zn, usually found in compost, was also assessed for comparison purposes on metal mobility origin. The approach relied on EDTA kinetic extractions, which measures the evolution of metal release by discriminating a rapidly and a slowly extractable pool. The results indicated an increase in the extractable pools for all the metals in amended soils compared to their control. More interestingly, they showed the enhanced Ni and Cr mobility observed after compost input came from ultramafic soils, while Cu and Zn mainly originated from compost. Although quite low when expressed as a percentage of total concentration, <10% and <1%, respectively, the amounts of Ni and Cr remobilized corresponded to significant extractable concentrations, <500 and <100 mg kg<sup>-1</sup> respectively, confirming the environmental and health risks. Before improving ultramafic Ferralsols with compost input, it is important to monitor metal mobility in not only different agricultural settings in New-Caledonia, but also in other ultramafic geological settings worldwide.

Keywords (6 keywords): Compost; Ultramafic Ferralsols; EDTA kinetic extraction; Extractable pools; Ni and Cr remobilization.

<footnote n="1"><sup>1</sup> IAC Institut Agronomique néo-Calédonien, Équipe SolVeg, 98848 Noumea, New Caledonia, France.</footnote>

<footnote n="2"><sup>2</sup> UNC Université de la Nouvelle-Calédonie, Institut des Sciences Exactes et Appliquées, BP R4, 98851 Noumea Cedex, New Caledonia, France.</footnote>

<footnote n="3"><sup>3</sup> IRD Institut de Recherche pour le Développement, ERL 206 IMPMC, 98848 Noumea, New Caledonia, France.</footnote>

<footnote n="4"><sup>4</sup> Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université, UMR 7590 CNRS, MNHN, IRD, 75252 Paris Cedex 5, France.</footnote>

<footnote n="5"><sup>5</sup> Cirad, UPR Recyclage et Risque, 34398 Montpellier, France.</footnote>

<footnote n="6"><sup>6</sup> Recyclage et risque, Univ Montpellier, Cirad, 34398 Montpellier, France.</footnote>

<footnote><sup>a</sup> corresponding author: klein.perrine@outlook.fr, +687 72 92 11</footnote>
**I.1. Introduction**

Soils derived from ultramafic rocks, widespread in tropical regions, contain high nickel (Ni) and chromium (Cr) contents (Kierczak et al. 2021). For cultivation purposes, these soils require nutrient inputs to overcome their low fertility. Spreading compost, which is a stabilized organic matter (Sharma et al. 2019) appears to be a good solution, but with the risk of increasing the mobility of Ni and Cr in amended soils (Amir and Pineau 2003; Cerdeira-Pérez et al. 2019). Organic matter decomposition yields dissolved organic ligands that can bind these metals and further increase their concentration in the soil solution (Gustafsson et al. 2014). Moreover, organic matter input can modify the bio-physico-chemical parameters of the soil such as pH or microbial activity, which in turn, can enhance (bio)dissolution of Ni- and Cr-bearing mineral species, such as manganese and iron oxy-hydroxides or clay minerals (Quantin et al. 2002; Borch et al. 2010). Such an increase in Ni and Cr mobility might be of environmental and health concern for local communities regarding their potential toxicity and their high concentrations in ultramafic soils (Kierczak et al. 2021).

In New Caledonia, Ferralsols from ultramafic rocks, covering one third of the island, contain up to 7,000 mg kg⁻¹ of Ni and 30,000 mg kg⁻¹ of Cr (Vincent et al. 2018). Mobilizable fractions of Ni and Cr are relatively low in these soils, <10% and <1% of total concentration respectively (Pasquet et al. 2018). However, Ni has been shown to accumulate in plants, notably crops, suggesting significant bioavailability (Kierczak et al. 2021). Additionally, despite much lower Cr availability due to the greater stability of Cr-bearing minerals (Sun et al. 2022), significant Cr(VI) concentrations have been reported in the soil solution of mining sediments (Gunkel-Grillon et al. 2014) or ultramafic soils with use of a mineral phosphorus fertilization (Becquer et al. 2010). Composts produced of sewage sludges, with green wastes mixture coming from these ultramafic Ferralsols, could also contain naturally high Ni and Cr contents. Composts are known to contain anthropogenic copper (Cu) and zinc (Zn) (Tella et al. 2013), but there is a lack of knowledge concerning the evolution of geogenic Ni and Cr and anthropogenic Cu and Zn mobility upon compost-amended Ferralsols.

In this study, we want to determine if compost input, rich in Ni and Cr, on ultramafic Ferralsols rich in Ni and Cr, induces the presence of high contents of mobile Ni and Cr in amended soils, which would generate environmental and health risks. We also want to know if mobile metals are geogenic metals released from the soils or if they are from compost. To answer these questions, kinetic extractions with EDTA were applied on compost, on both control soils and amended soils. The study of the leaching behavior of metals, using kinetic extraction techniques, makes it possible to estimate a total pool of extractable metals which are mobile, and also to discriminate the rapidly and slowly extractable pools (Fangueiro et al. 2005). These two pools yield relevant information of the short and long-term dynamics
of metals that translates to potential short and long-term risks of release in the environment. To our best knowledge, this study is the first to assess the influence of compost on the origin of Ni, Cr, Cu and Zn release in a Ni- and Cr-rich ultramafic Ferralsol.

## 1.2. Experimental

Study sites are in the South of the main Island of New Caledonia (Figure SI 4-2). The information related to sample description, chemical and physical analysis and control quality with statistical analysis can be found in Text SI 4-1.

Ni, Cr, Cu and Zn kinetic extractions (10, 20, 40, 60, 120, 240, 420, 840, and 1440 min) were carried out in duplicate on compost, control soils without any compost inputs, on amended soils with a single application of compost, using a batch mode with a 0.05M Na₂-EDTA solution (Quevauviller et al. 1996), before ICP-OES analyses. Kinetic EDTA extraction curves can be modeled as the sum of two first-order reactions according to equation (1) (Fangueiro et al. 2005):

$$Q_i(t) = Q_1(1 - e^{-k_1 t}) + Q_2(1 - e^{-k_2 t}) + \varepsilon_i \quad (1)$$

where $Q_i(t)$ (mg kg⁻¹) is the amount of metal extracted per weight unit at a time t (min). $Q_1$ and $Q_2$ are respectively the rapidly and slowly extractable pools, the sum ($Q_1 + Q_2$) is named the total extractable pool. $\varepsilon_i$ is the standard error. $k_1$, $k_2$ (min⁻¹) are the kinetic constant rates associated with $Q_1$ and $Q_2$ respectively. $Q_1$, $Q_2$, $k_1$ and $k_2$ were retrieved from equation (1) after fitting the experimental kinetic data through a non-linear regression analysis using CurveExpert Professional (2.7).

The non-extractable pool, $Q_3$, was calculated according to equation (2):

$$Q_3 = Q_{tot} - (Q_1 + Q_2) \quad (2)$$

where $Q_{tot}$ is the total concentration of the selected metal.
**I.3. Results and discussion**

**I.3.1. Physical and chemical properties of the studied compost and soils**

Compost, with 33% organic matter, 1.15% P, and 1.08% K and C/N ratio equal to 8.8, showed all characteristics needed to improve soil quality for agricultural purposes (Table SI 4-5). Total Cu and Zn contents, i.e., 92 and 296 mg kg⁻¹ respectively, were below threshold values of NFU 44-095 French regulation, i.e., 300 and 600 mg kg⁻¹ respectively, and of AS4454 Australian volunteer regulation, i.e., 150 and 300 mg kg⁻¹ respectively. They were also below those reported for a similar tropical compost from La Réunion, i.e., 294 and 553 mg kg⁻¹ respectively (Legros et al. 2017). In contrast, total Ni and Cr contents, i.e., 465 and 1 409 mg kg⁻¹ respectively accounted for up to ten times thresholds limits authorized by French regulation, i.e., 60 and 120 mg kg⁻¹ respectively and Australian regulation, i.e., 60 and 100 mg kg⁻¹ respectively. These contents were much higher than those reported in a tropical compost composed of mixed municipal and green wastes from La Réunion, i.e., 61 and 55 mg kg⁻¹ respectively (Doelsch et al. 2010), suggesting that New Caledonia is an interesting model to study Ni and Cr evolution in compost-amended soils.

Non-amended soils showed a low fertility (low organic matter, dissolved organic carbon, P, K contents, and low cation exchange capacity) and were slightly acidic (pH<6.2) (Table SI 4-5; Figure SI 4-6) confirming the need for organic inputs to improve their agricultural productivity. Their total Fe, Mn, Ni, and Cr contents were very high, i.e., up to 540 000 mg kg⁻¹, 6 600 mg kg⁻¹, 5 000 mg kg⁻¹ and 23 000 mg kg⁻¹ respectively, while their Cu and Zn contents were low, i.e., below 38 and 437 mg kg⁻¹ respectively.

Amended soils showed increased pH, dissolved organic carbon, organic matter, cation exchange capacity, P, and K contents and a decrease by half of their C/N ratio compared to their control. All these changes are considered to improve the quality of these ultramafic soils, as described by Sharma et al. (2019). On the contrary, organic inputs did not significantly modify the total metal contents, i.e., Fe, Mn, Ni, Cr, Cu, and Zn, of amended soils compared to control soils (Table SI 4-5; Figure SI 4-6).

**I.3.2. Metal mobility assessment**

**I.3.2.1. Compost**

In compost, total extractable pools ($Q_1+Q_2$ in mg kg⁻¹) of metals ranked as Cr<Cu~Ni<<Zn, with respectively mean values of 6.76±3.02, 28.70±2.97, 37.37±1.31 and 166.00±44.81 mg kg⁻¹ (Table SI 4-7). Comparison with regulations suggested that a risk could have arisen from Ni and Zn, whose total extractable pool represented more than half the guidelines values.

When expressed as total extractable fractions (% $Q_1+Q_2$), metals ranked as Cr<Ni<Cu>Zn, with respective mean values of 0.47±0.19%, 8.07±0.68%, 31.16±0.45%, and 55.73±3.37% (Table SI 4-7). Cu
and Zn were thus significantly more mobile than Ni and Cr which showed a very low mobility. These results are in agreement with previous studies that reported Cu and Zn as the most mobile elements in compost, while Ni and Cr were reported to be less mobile, as indicated by their main occurrence in residual fraction of sequential extractions (Maturi et al. 2021; Xu et al. 2022).

### 1.3.2.2. Control soils

In control soils, total extractable pools ($Q_1+Q_2$ in mg kg⁻¹) of metals ranked as Cu<Zn<Cr<Ni, with respective mean values of 1.05±0.08, 1.83±0.46, 12.59±1.57 and 21.76±20.11 mg kg⁻¹ (Table SI 4-7). These low concentrations suggest a weak environmental risk linked to metal mobility, under natural conditions.

When expressed as total extractable fractions (% $Q_1+Q_2$), metals ranked as Cr<Zn~Ni<Cu, with respective mean values of 0.06±0.01%, 0.60±0.25%, 0.63±0.71%, and 3.17±0.39% (Table SI 4-7). Thus, under natural conditions, extractable fractions of these metals are low as reported by Pasquet et al. (2018).

The mean value of the $Q_1/Q_2$ ratio was <0.83 for all four metals. This ratio indicates that the largest fraction was released more slowly. Therefore, the potential environmental and health risks associated with Ni, Cr, Cu and Zn would likely be a long-term risk in non-amended soils.

#### 1.3.2.3. Amended soils

In amended soils, total extractable pools ($Q_1+Q_2$ in mg kg⁻¹) of metals ranked as Cu<Zn<Cr<Ni, with respective mean values of 4.86±2.41, 16.63±9.30, 68.80±33.72 and 212.65±252.93 mg kg⁻¹ (Table SI 4-7). These values were between 4 and 100 times higher than those measured in control soils, which suggests that an environmental risk might have occurred from metals in soil, after compost application.

Moreover, all kinetic curves computed according to equation (1) for amended soils were significantly higher than those computed for their control (p-value<0.05) (Figure 4-1; Figure SI 4-8). In amended soils, total extractable fractions (% $Q_1+Q_2$) of the metals ranked as Cr<Ni<Zn<Cu, with respective mean values of 0.33±0.16%, 3.79±4.16%, 4.56±2.59%, and 12.55±4.41% (Table SI 4-7). As already reported for compost, Cu and Zn were more mobile than Cr and Ni. These results are in agreement with previous studies that reported strong Cu binding by organic matter and increased Zn extractability in amended soils (Paradelo et al. 2018).

Significantly larger extractable pools of metals measured in Soil₁ that received 25 t ha⁻¹ of compost compared to Soil₂ that received 6 t ha⁻¹ (Figure 4-1; Table SI 4-7; Figure SI 4-8), i.e., between 1.2 and 6 times higher in amended-Soil₁, confirming this effect of compost adding on metals dynamics.
Finally, total extractable pools of metals were systematically smaller in Soil₃ that was sampled 11 months after compost application compared to Soil₁ and Soil₂ that were sampled only 1 month after compost application (Figure 4-1; Table SI 4-7; Figure SI 4-8). This latter result suggests that the enhanced mobility of metals induced by compost application on ultramafic soils might attenuate upon long-term in-situ compost mineralization.

<figure><img src="image_64.png" /></figure>

**Figure 4-1:** Extractable Ni (A), Cr (B), Cu (C) and Zn (D) (mg kg⁻¹) obtained by kinetic EDTA extractions for compost (red square), control (yellow circle) and amended (orange triangle) soil₁. The blue dashed line is the theoretical extractable metal concentration calculated for amended Soil₁ and considering extractable metals provided by the compost and by the control soil. For Ni and Cr, extractable contents in amended soil are higher than the blue dashed line indicating that they are remobilized from mineral-bearing phases of the soil. For Cu and Zn, extractable contents in amended soil are superimposable to the blue dashed line indicating that they are mainly provided by compost having the higher extractable metal content
I.3.3. Origin of mobile metals in amended soils

The total extractable pools, theoretically arising from compost and from control soils, were calculated and compared to values measured in amended soils. Ratio "measured/theoretical in amended soil" provide information on the influence of compost on metal mobility (**Table SI 4-9**). More the ratio is >>1, more metals are released in higher quantity in amended soils than theoretically, coming from a metal remobilization either of the soil or of the compost.

For Cu and Zn, extractable pools ($Q_1$ and $Q_2$ in mg kg$^{-1}$) were higher in compost than in control or amended soils (Table SI 4-7, Figure 4-1). Ratios of measured to theoretical in amended soils ranged from 1.2 to 2.5 and from 0.8 to 1.8 (Table SI 4-9), highlighting that the increase of Cu and Zn extractable pools was a result of compost input. Moreover, direct relationships between these pools and compost input were also supported by the highly significant and positive correlations between extractable pools and dissolved organic carbon or cation exchange capacity, whose increases were directly related to compost inputs (Table SI 4-10).

For Ni and Cr, extractable pools ($Q_1$ and $Q_2$ in mg kg$^{-1}$) were higher in amended soils than in compost or control soils (Table SI 4-7, Figure 4-1). This suggests that the increase of Ni and Cr extractable pools observed in amended soils did not come from the extractable pools of the compost. Ratios of measured to theoretical in amended soils, obtained for Ni and Cr, ranged from 2.6 to 17.0 and from 2.3 to 9.0 (Table SI 4-9). These values emphasized an indirect impact of compost input leading to a massive remobilization of Ni and Cr, likely arising from soil. Biological activity following compost input could reduce manganese and iron-oxides in these soils, and therefore increase Ni and Cr extractability (Quantin et al. 2002; Borch et al. 2010). This indirect impact was also suggested by either no or low correlations between extractable Ni and Cr pools and soil characteristics, i.e., pH, C/N ratio, dissolved organic carbon and cation exchange capacity (Table SI 4-10). Nevertheless, better correlations between Cr-$Q_2$ pool and these soil parameters could suggest complexation of Cr(III) by organic matter even if metal speciation was not assessed here (Gustafsson et al. 2014; Löv et al. 2017).
I.4. Conclusion

This study demonstrates, for the first time, that a single compost input on ultramafic Ferralsols, significantly increases extractable pools, by a mean of 100-fold for Ni, 6-fold for Cr, 4-fold for Cu and 16-fold for Zn. The primary source of mobile Cu and Zn in amended soils was from compost, while the presence of mobile Ni and Cr was attributed to a massive remobilization of Ni and Cr from the mineral-bearing phases of ultramafic Ferralsols. Although extractable fractions of Ni and Cr remained low in amended soils, <10% and <1% respectively, their absolute concentrations were especially high, <500 and <100 mg kg⁻¹ respectively, indicating an environmental and health risk. This risk has been confirmed after a single application of compost, so it must be seriously considered as the effect of repeated applications is unknown. Before improving the quality of ultramafic Ferralsols with compost input, it is not only necessary to monitor metal mobility in different agricultural settings in New-Caledonia, but also in other ultramafic geological settings worldwide.
**I.5. Supplementary material**

***Text SI 4-1: Experimental***

**2.1. Sample description**

New Caledonia is a French archipelago located in the South-West Pacific (Figure SI4.2). Study sites are in the South of the main Island, characterized by a tropical climate and a high cumulative precipitation (more than 4000 mm in 2021).

In three ultramafic soils from forestry parcels, i.e., Soil₁, Soil₂ and Soil₃, both control soils without any compost inputs, and amended soils with a single application of compost, were sampled in the 0-20 cm horizon at the base of a tree on the same parcel's block. One sample represented nine composite samples for Soil₁ and Soil₂, and six composite samples for Soil₃. For all parcels, applied compost is a mixture of sewage sludges and green wastes. The amount of dry compost applied was 25 t ha⁻¹ for Soil₁ and 6 t ha⁻¹ for Soil₂ and Soil₃ regarding the planting density (Table SI4.3). The use of the three soils taken from different plots and having received different doses of compost aimed to ensure the reproducibility of the results observed. The sampling time after compost application was within one month for Soil₁ and Soil₂ and 11 months for Soil₃.

**2.2. Chemical and physical analysis**

Soils and composts were air-dried at room temperature, sieved at 2 mm, before being analyzed for: percentage of humidity (AFNOR, 2000), pH (NF ISO 10390), and crushed at 0.1 mm for cation exchange capacity (NFX31-130), total carbon and nitrogen by dry combustion with a CHN analyzer (IRMS Sercon Integra 2), dissolved organic carbon (Ciavatta et al. 1989) was measured with a TOC-L-CSH (Shimadzu N°H54205000462) by the ISO-9001 certified LAMA laboratory (Imago IRD, Noumea, New Caledonia). Percentage organic matter was calculated by multiplying the total carbon content by the van Bemmelen factor, 1.724 (Pribyl 2010), assuming that total carbon content was organic carbon. In addition, total concentrations of phosphorus (P), potassium (K), magnesium (Mg), manganese (Mn), iron (Fe), nickel (Ni), chromium (Cr), copper (Cu), and zinc (Zn) were determined by inductively coupled plasma optical emission spectrometer (ICP-OES VARIAN® 730ES) after alkali fusion (EN ISO 14869-2).

**2.3. Control quality and statistical analysis**

Quality control was performed for each element analysis by calculating the z-score statistic indicator of the calibration standards set, intern soil reference samples, and certified reference material (BCR n°483 sewage amended soil (European Commission)). The average blanks were negligible and blank concentrations were subtracted from sample concentrations. All data were presented after blank
correction. All relevant material was conveniently rinsed with distilled water, left in a 10% HNO₃ bath for 24h, rinsed again with distilled water and with ultrapure water, and dried at room temperature.

Analyses on certified reference material and an intern reference sample were used to control the stability and accuracy of ICP-OES measurements throughout the analytical session.

Statistical analyses were performed with R software (R-i386 4.1.2) with rstatix and FSA packages for non-parametric test Wilcoxon test and correlation matrix was performed with the Spearman method.

The statistical parameters used to access the validity of the fitting process are the standard error (εᵢ) and correlation coefficient (R²) which are indicators of good curve fitting (Table S14.4). All curves exhibit R²≥0.98, except Cr fitting for the control of Soil₁.
<figure><img src="image_65.png" /><figcaption>Figure SI 4-2: Map of the sampling sites</figcaption></figure>
<table><caption>Table SI 4-3: Dose explanation of compost input</caption><thead><tr><th>Soil</th><th>Compost input (kg raw material)</th><th>Dry matter (%)</th><th>Dry compost input (kg)</th><th>Soil dry equivalent (kg)*</th><th>Soil + compost amount (kg)</th><th>% by mass of soil</th><th>% by mass of compost</th></tr></thead><tbody><tr><td><i>Soil<sub>1</sub></i></td><td>48</td><td>64</td><td>31</td><td>163</td><td>193</td><td>84</td><td>16</td></tr><tr><td><i>Soil<sub>2</sub></i></td><td>12</td><td>53</td><td>6</td><td>163</td><td>169</td><td>96</td><td>4</td></tr><tr><td><i>Soil<sub>3</sub></i></td><td>12</td><td>64</td><td>8</td><td>166</td><td>174</td><td>96</td><td>4</td></tr></tbody></table>

The input of compost represented an amount supplied by tree. In these forestry soils, the planting density was 833 trees for 1 ha of land.

<footnote>*For calculating the amount of dry equivalent, the volume of soil was estimated at 0.125 m³ for *Soil₁* and *Soil₂*, and 0.128 m³ for *Soil₃*. The bulk density corresponded to 1.3 g cm⁻³.</footnote>
Table SI 4-4: Kinetic constant rates

$k_1$ and $k_2$ (min$^{-1}$), the standard error ($\epsilon_1$) and correlation coefficient ($R^2$) of the EDTA kinetic extractions models

<table><thead><tr><th>Trace metal element</th><th>Matrices</th><th>Fertilization</th><th>k<sub>1</sub> min<sup>-1</sup></th><th>k<sub>2</sub> min<sup>-1</sup></th><th>ε mg kg<sup>-1</sup></th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td rowspan="8">Ni</td><td rowspan="2">Compost (±sd)</td><td>Control</td><td>0.090 (0.030)</td><td>0.0042 (0.0008)</td><td>1.47 (0.38)</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.065 (0.067)</td><td>0.0014 (0.0004)</td><td>2.45 (12.30)</td><td>0.99</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>0.103</td><td>0.0007</td><td>0.21</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.175</td><td>0.0026</td><td>5.44</td><td>0.99</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>0.114</td><td>0.0048</td><td>0.56</td><td>1.00</td></tr><tr><td>Amended soil</td><td>0.061</td><td>0.0024</td><td>1.94</td><td>0.99</td></tr><tr><td rowspan="8">Cr</td><td rowspan="2">Compost (±sd)</td><td>Control</td><td>0.056 (0.007)</td><td>0.0040 (0.0001)</td><td>0.25 (0.01)</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.000 (0.002)</td><td>0.0003 (0.0024)</td><td>2.07 (3.90)</td><td>0.71</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>0.098</td><td>0.0005</td><td>0.19</td><td>1.00</td></tr><tr><td>Amended soil</td><td>0.020</td><td>0.0005</td><td>3.74</td><td>0.98</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>0.211</td><td>0.0019</td><td>0.39</td><td>1.00</td></tr><tr><td>Amended soil</td><td>0.081</td><td>0.0026</td><td>2.02</td><td>0.98</td></tr><tr><td rowspan="8">Cu</td><td rowspan="2">Compost (±sd)</td><td>Control</td><td>0.057 (0.016)</td><td>0.0022 (0.0006)</td><td>0.90 (0.30)</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.234 (0.070)</td><td>0.0048 (0.0011)</td><td>0.07 (0.25)</td><td>0.99</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>0.141</td><td>0.0019</td><td>0.05</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.068</td><td>0.0028</td><td>0.20</td><td>0.99</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>0.319</td><td>0.0065</td><td>0.04</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.074</td><td>0.0028</td><td>0.12</td><td>0.99</td></tr><tr><td rowspan="8">Zn</td><td rowspan="2">Compost (±sd)</td><td>Control</td><td>2.367 (0.090)</td><td>0.0090 (0.0006)</td><td>6.35 (2.74)</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.084 (0.132)</td><td>0.0011 (0.0008)</td><td>0.14 (0.73)</td><td>0.98</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>0.084</td><td>0.0007</td><td>0.04</td><td>1.00</td></tr><tr><td>Amended soil</td><td>0.162</td><td>0.0034</td><td>0.54</td><td>1.00</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>0.066</td><td>0.0050</td><td>0.10</td><td>0.99</td></tr><tr><td>Amended soil</td><td>0.191</td><td>0.0088</td><td>0.57</td><td>0.98</td></tr></tbody></table>
**Table SI 4-5: Physico-chemical parameters of compost and soils**

Physico-chemical parameters of compost and Soil₁, Soil₂ and Soil₃ (control and amended). Weight %Clay, Silt and Sand fractions, pH, C/N ratio, dissolved organic carbon (DOC, mg kg⁻¹), organic matter (OM, g 100 g⁻¹), cationic exchange capacity (CEC, cmol(+) kg⁻¹), and concentrations (mg kg⁻¹) of phosphorus (P), potassium (K), magnesium (Mg), iron (Fe), manganese (Mn), nickel (Ni), chromium (Cr), copper (Cu) and zinc (Zn). Numbers in brackets correspond to the standard deviation (n=2)

<table><thead><tr><th>Parameters</th><th>Units</th><th>Compost<br>(±sd)</th><th></th><th>Soil<sub>1</sub></th><th>Soil<sub>2</sub></th><th>Soil<sub>3</sub></th></tr></thead><tbody><tr><td>Clay</td><td></td><td>-</td><td>Control</td><td>2.6</td><td>0.1</td><td>1.7</td></tr><tr><td>Silt</td><td>%</td><td>-</td><td>Control</td><td>9.9</td><td>11</td><td>10</td></tr><tr><td>Sand</td><td></td><td>-</td><td>Control</td><td>86</td><td>87</td><td>86</td></tr><tr><td rowspan="2">pH</td><td></td><td>6.0<br>(0.1)</td><td>Control</td><td>5.1</td><td>5.0</td><td>5.4</td></tr><tr><td></td><td></td><td>Amended</td><td>6.2</td><td>5.9</td><td>6.0</td></tr><tr><td rowspan="2">C/N</td><td></td><td>8.8<br>(0.8)</td><td>Control</td><td>37</td><td>27</td><td>24</td></tr><tr><td></td><td></td><td>Amended</td><td>14</td><td>15</td><td>16</td></tr><tr><td rowspan="2">DOC</td><td rowspan="2">mg kg<sup>-1</sup></td><td>2 348<br>(378)</td><td>Control</td><td>42</td><td>41</td><td>36</td></tr><tr><td></td><td>Amended</td><td>245</td><td>73</td><td>62</td></tr><tr><td rowspan="2">OM</td><td rowspan="2">g 100g<sup>-1</sup></td><td>33<br>(8.1)</td><td>Control</td><td>2.1</td><td>2.5</td><td>1.9</td></tr><tr><td></td><td>Amended</td><td>6.1</td><td>3.7</td><td>3.0</td></tr><tr><td rowspan="2">CEC</td><td rowspan="2">cmol(+) kg<sup>-1</sup></td><td>65<br>(0.2)</td><td>Control</td><td>2.3</td><td>2.4</td><td>2.0</td></tr><tr><td></td><td>Amended</td><td>15</td><td>7.7</td><td>6.1</td></tr><tr><td rowspan="2">P</td><td></td><td>11 513<br>(2 139)</td><td>Control</td><td>124</td><td>107</td><td>79</td></tr><tr><td></td><td></td><td>Amended</td><td>1 035</td><td>777</td><td>363</td></tr><tr><td rowspan="2">K</td><td></td><td>10 849<br>(1 472)</td><td>Control</td><td>81</td><td>46</td><td>25</td></tr><tr><td></td><td></td><td>Amended</td><td>959</td><td>535</td><td>353</td></tr><tr><td rowspan="2">Mg</td><td></td><td>19 941<br>(1 314)</td><td>Control</td><td>919</td><td>4 004</td><td>4 057</td></tr><tr><td></td><td></td><td>Amended</td><td>3 575</td><td>4 919</td><td>6 136</td></tr><tr><td rowspan="2">Fe</td><td></td><td>39 389<br>(4 005)</td><td>Control</td><td>536 119</td><td>477 262</td><td>452 140</td></tr><tr><td></td><td></td><td>Amended</td><td>448 594</td><td>456 828</td><td>424 219</td></tr><tr><td rowspan="2">Mn</td><td rowspan="2">mg kg<sup>-1</sup></td><td>698<br>(99)</td><td>Control</td><td>4 545</td><td>4 678</td><td>4 849</td></tr><tr><td></td><td>Amended</td><td>7 917</td><td>6 358</td><td>4 986</td></tr><tr><td rowspan="2">Ni</td><td></td><td>465<br>(55)</td><td>Control</td><td>3 082</td><td>4 396</td><td>4 749</td></tr><tr><td></td><td></td><td>Amended</td><td>5 888</td><td>4 596</td><td>5 238</td></tr><tr><td rowspan="2">Cr</td><td></td><td>1 409<br>(74)</td><td>Control</td><td>19 373</td><td>22 748</td><td>19 688</td></tr><tr><td></td><td></td><td>Amended</td><td>19 487</td><td>22 903</td><td>18 983</td></tr><tr><td rowspan="2">Cu</td><td></td><td>92<br>(8.2)</td><td>Control</td><td>34</td><td>38</td><td>28</td></tr><tr><td></td><td></td><td>Amended</td><td>43</td><td>44</td><td>27</td></tr><tr><td rowspan="2">Zn</td><td></td><td>296<br>(62)</td><td>Control</td><td>268</td><td>341</td><td>348</td></tr><tr><td></td><td></td><td>Amended</td><td>363</td><td>361</td><td>376</td></tr></tbody></table>
Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES

**Figure SI 4-6: Physio-chemical parameters of control and amended soils**

Mean values of pH, ratio C/N, dissolved organic carbon (DOC, mg kg⁻¹), organic matter (OM, g 100 g⁻¹), cationic exchange capacity (CEC, cmol(+)-kg⁻¹), and concentrations (mg kg⁻¹) of i.e., phosphorus (P), potassium (K), magnesium (Mg), iron (Fe), manganese (Mn), nickel (Ni), chromium (Cr), copper (Cu) and zinc (Zn). Non-significant differences between control and amended soils

<figure><img src="image_66.png" /></figure>
Table SI 4-7: kinetic constants of Ni, Cr, Cu and Zn in compost and soils

Extractable pools (mg kg⁻¹) and fractions (%) obtained by EDTA kinetic extractions for Ni, Cr, Cu and Zn in compost and soils (control and amended). Q₁, rapidly extractable; Q₂, slowly extractable; Q₃, non-extractable; Q₁+Q₂, total extractable. Numbers in brackets correspond to the standard deviation (n=2)

<table><thead><tr><th>Trace metal element</th><th>Matrices</th><th>Fertilization</th><th>Q<sub>1</sub><br>mg kg<sup>-1</sup></th><th>%Q<sub>1</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub><br>mg kg<sup>-1</sup></th><th>%(Q<sub>1</sub>+Q<sub>2</sub>)</th><th>Q<sub>1</sub>/Q<sub>2</sub></th><th>Q<sub>3</sub><br>mg kg<sup>-1</sup></th></tr></thead><tbody><tr><td rowspan="8">Ni</td><td>Compost<br>(±sd)</td><td></td><td>14.37<br>(0.82)</td><td>3.10<br>(0.19)</td><td>37.37<br>(1.31)</td><td>8.07<br>(0.68)</td><td>0.62<br>(0.02)</td><td>428<br>(54)</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>19.45</td><td>0.05</td><td>44.57</td><td>1.45</td><td>0.77</td><td>3 038</td></tr><tr><td>Amended soil</td><td>81.28</td><td>1.38</td><td>503.73</td><td>8.56</td><td>0.19</td><td>5 384</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>2.05</td><td>0.05</td><td>6.57</td><td>0.15</td><td>0.45</td><td>4 390</td></tr><tr><td>Amended soil</td><td>23.25</td><td>0.51</td><td>87.66</td><td>1.93</td><td>0.36</td><td>4 449</td></tr><tr><td rowspan="2">Soil<sub>3</sub></td><td>Control</td><td>3.52</td><td>0.07</td><td>14.13</td><td>0.30</td><td>0.33</td><td>4 735</td></tr><tr><td>Amended soil</td><td>17.22</td><td>0.33</td><td>46.56</td><td>0.89</td><td>0.59</td><td>5 191</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td>Control</td><td>8.34<br>(9.65)</td><td>0.06<br>(0.02)</td><td>21.76<br>(20.11)</td><td>0.63<br>(0.71)</td><td>0.52<br>(0.23)</td><td>4 054<br>(897)</td></tr><tr><td>Amended soil</td><td>40.59<br>(35.37)</td><td>0.74<br>(0.56)</td><td>212.65<br>(252.92)</td><td>3.79<br>(4.16)</td><td>0.38<br>(0.20)</td><td>5 008<br>(494)</td></tr><tr><td rowspan="8">Cr</td><td>Compost<br>(±sd)</td><td></td><td>3.27<br>(0.74)</td><td>0.23<br>(0.04)</td><td>6.76<br>(3.02)</td><td>0.47<br>(0.19)</td><td>1.10<br>(0.51)</td><td>1 400<br>(71)</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>1.80</td><td>0.01</td><td>11.80</td><td>0.06</td><td>0.18</td><td>19 373</td></tr><tr><td>Amended soil</td><td>11.19</td><td>0.06</td><td>94.24</td><td>0.48</td><td>0.13</td><td>19 392</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>1.80</td><td>0.01</td><td>11.58</td><td>0.05</td><td>0.18</td><td>22 737</td></tr><tr><td>Amended soil</td><td>13.68</td><td>0.06</td><td>81.59</td><td>0.36</td><td>0.20</td><td>22 821</td></tr><tr><td rowspan="2">Soil<sub>3</sub></td><td>Control</td><td>2.91</td><td>0.01</td><td>14.40</td><td>0.07</td><td>0.25</td><td>19 674</td></tr><tr><td>Amended soil</td><td>7.67</td><td>0.04</td><td>30.55</td><td>0.16</td><td>0.34</td><td>18 952</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td>Control</td><td>2.17<br>(0.64)</td><td>0.01<br>(0.00)</td><td>12.59<br>(1.57)</td><td>0.06<br>(0.01)</td><td>0.21<br>(0.04)</td><td>20 594<br>(1 864)</td></tr><tr><td>Amended soil</td><td>10.85<br>(3.02)</td><td>0.05<br>(0.01)</td><td>68.80<br>(33.72)</td><td>0.33<br>(0.16)</td><td>0.22<br>(0.10)</td><td>20 389<br>(2 118)</td></tr><tr><td rowspan="8">Cu</td><td>Compost<br>(±sd)</td><td></td><td>15.59<br>(3.12)</td><td>16.86<br>(1.90)</td><td>28.70<br>(2.97)</td><td>31.16<br>(0.45)</td><td>1.19<br>(0.25)</td><td>63<br>(5.2)</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>0.44</td><td>1.46</td><td>1.14</td><td>3.35</td><td>0.62</td><td>33</td></tr><tr><td>Amended soil</td><td>3.13</td><td>7.33</td><td>7.53</td><td>17.60</td><td>0.71</td><td>35</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>0.56</td><td>1.46</td><td>1.04</td><td>2.72</td><td>1.16</td><td>37</td></tr><tr><td>Amended soil</td><td>1.84</td><td>4.14</td><td>4.22</td><td>9.50</td><td>0.77</td><td>40</td></tr><tr><td rowspan="2">Soil<sub>3</sub></td><td>Control</td><td>0.40</td><td>1.42</td><td>0.98</td><td>3.43</td><td>0.71</td><td>27</td></tr><tr><td>Amended soil</td><td>1.49</td><td>5.53</td><td>2.83</td><td>10.54</td><td>1.10</td><td>24</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td>Control</td><td>0.47<br>(0.08)</td><td>1.45<br>(0.02)</td><td>1.05<br>(0.08)</td><td>3.17<br>(0.39)</td><td>0.83<br>(0.29)</td><td>33<br>(4.9)</td></tr><tr><td>Amended soil</td><td>2.15<br>(0.87)</td><td>5.67<br>(1.60)</td><td>4.86<br>(2.41)</td><td>12.55<br>(4.41)</td><td>0.86<br>(0.21)</td><td>33<br>(8.3)</td></tr><tr><td rowspan="8">Zn</td><td>Compost<br>(±sd)</td><td></td><td>85.86<br>(22.84)</td><td>28.84<br>(1.63)</td><td>166.00<br>(44.81)</td><td>55.73<br>(3.37)</td><td>1.07<br>(0.01)</td><td>130<br>(18)</td></tr><tr><td rowspan="2">Soil<sub>1</sub></td><td>Control</td><td>0.96</td><td>0.17</td><td>2.36</td><td>0.88</td><td>0.69</td><td>265</td></tr><tr><td>Amended soil</td><td>12.20</td><td>3.36</td><td>26.43</td><td>7.27</td><td>0.86</td><td>337</td></tr><tr><td rowspan="2">Soil<sub>2</sub></td><td>Control</td><td>0.59</td><td>0.17</td><td>1.57</td><td>0.46</td><td>0.60</td><td>339</td></tr><tr><td>Amended soil</td><td>7.74</td><td>2.15</td><td>15.51</td><td>4.30</td><td>1.00</td><td>345</td></tr><tr><td rowspan="2">Soil<sub>3</sub></td><td>Control</td><td>0.76</td><td>0.22</td><td>1.56</td><td>0.45</td><td>0.94</td><td>347</td></tr><tr><td>Amended soil</td><td>3.39</td><td>0.90</td><td>7.94</td><td>2.11</td><td>0.74</td><td>368</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td>Control</td><td>0.77<br>(0.19)</td><td>0.19<br>(0.03)</td><td>1.83<br>(0.46)</td><td>0.60<br>(0.25)</td><td>0.74<br>(0.18)</td><td>317<br>(45)</td></tr><tr><td>Amended soil</td><td>7.77<br>(4.41)</td><td>2.13<br>(1.23)</td><td>16.63<br>(9.30)</td><td>4.56<br>(2.59)</td><td>0.87<br>(0.13)</td><td>350<br>(16)</td></tr></tbody></table>
**Figure SI 4-8: kinetic curves of Ni, Cr, Cu and Zn from EDTA extractions**

Extractable Ni, Cr, Cu and Zn (mg kg⁻¹) obtained by kinetic EDTA extractions for compost (red square), control (yellow circle) and amended (orange triangle) soil₁. The blue dash line is the theoretical extractable metal calculated for amended soil₁ and considering extractable metals provided by the compost. Y scale depended on metals, until 100, 50, 40 and 200 mg kg⁻¹, respectively

Soil₂

<figure><img src="image_67.png" /></figure>

Soil₃

<figure><img src="image_68.png" /></figure>
**Table SI 4-9: mass balance**

Mass balance calculated for Soil1, Soil2 and Soil3 in order to assess the rapidly extractable Q₁ (mg.kg⁻¹), slowly extractable Q₂ (mg.kg⁻¹) and total extractable Q₁ + Q₂ (mg.kg⁻¹) pools of Ni, Cr, Cu and Zn arising from the compost.

* represents the concentration calculated by considering the percent volume of soil and compost in the amended soil

<table><thead><tr><th></th><th></th><th>Soil1</th><th>Soil2</th><th>Soil3</th></tr></thead><tbody><tr><td colspan="2">Dry matter of compost (%)</td><td>64</td><td>53</td><td>64</td></tr><tr><td colspan="2">% volume of soil in amended soil</td><td>84</td><td>96</td><td>96</td></tr><tr><td colspan="2">% volume of compost in amended soil</td><td>16</td><td>4</td><td>4</td></tr><tr><td rowspan="4">Ni-Q<sub>1</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>2.2</td><td>0.6</td><td>3.9</td></tr><tr><td>Control*</td><td>16</td><td>1.9</td><td>57</td></tr><tr><td>Amended soil measured</td><td>81</td><td>23</td><td>17</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>4.5</strong></td><td><strong>9.3</strong></td><td><strong>4.3</strong></td></tr><tr><td rowspan="4">Ni-Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>3.6</td><td>0.9</td><td>1.0</td></tr><tr><td>Control*</td><td>21</td><td>4.4</td><td>10</td></tr><tr><td>Amended soil measured</td><td>422</td><td>64</td><td>29</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>17</strong></td><td><strong>12</strong></td><td><strong>2.6</strong></td></tr><tr><td rowspan="4">Ni-Q<sub>1</sub>+Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>5.7</td><td>1.4</td><td>1.6</td></tr><tr><td>Control*</td><td>38</td><td>6.4</td><td>13</td></tr><tr><td>Amended soil measured</td><td>504</td><td>88</td><td>47</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>12</strong></td><td><strong>11</strong></td><td><strong>3.1</strong></td></tr><tr><td rowspan="4">Cr-Q<sub>1</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>0.6</td><td>0.1</td><td>0.2</td></tr><tr><td>Control*</td><td>1.5</td><td>1.7</td><td>0.7</td></tr><tr><td>Amended soil measured</td><td>11</td><td>14</td><td>3.0</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>5.2</strong></td><td><strong>7.6</strong></td><td><strong>3.4</strong></td></tr><tr><td rowspan="4">Cr-Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>0.8</td><td>0.1</td><td>0.2</td></tr><tr><td>Control*</td><td>8.4</td><td>11</td><td>9.4</td></tr><tr><td>Amended soil measured</td><td>83</td><td>68</td><td>23</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>9.0</strong></td><td><strong>6.1</strong></td><td><strong>2.4</strong></td></tr><tr><td rowspan="4">Cr-Q<sub>1</sub>+Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>1.4</td><td>0.2</td><td>0.4</td></tr><tr><td>Control*</td><td>9.9</td><td>12</td><td>13</td></tr><tr><td>Amended soil measured</td><td>94</td><td>82</td><td>31</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>8.3</strong></td><td><strong>7.0</strong></td><td><strong>2.3</strong></td></tr><tr><td rowspan="4">Cu-Q<sub>1</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>2.2</td><td>0.7</td><td>0.6</td></tr><tr><td>Control*</td><td>0.4</td><td>0.5</td><td>0.4</td></tr><tr><td>Amended soil measured</td><td>3.1</td><td>1.8</td><td>1.5</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>1.2</strong></td><td><strong>1.5</strong></td><td><strong>1.5</strong></td></tr><tr><td rowspan="4">Cu-Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>2.1</td><td>0.5</td><td>0.6</td></tr><tr><td>Control*</td><td>0.6</td><td>0.5</td><td>0.6</td></tr><tr><td>Amended soil measured</td><td>4.4</td><td>2.4</td><td>1.3</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>1.6</strong></td><td><strong>2.5</strong></td><td><strong>1.2</strong></td></tr><tr><td rowspan="4">Cu-Q<sub>1</sub>+Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>4.3</td><td>1.2</td><td>1.2</td></tr><tr><td>Control*</td><td>0.9</td><td>1.0</td><td>1.0</td></tr><tr><td>Amended soil measured</td><td>7.5</td><td>4.2</td><td>2.8</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>1.4</strong></td><td><strong>2.0</strong></td><td><strong>1.3</strong></td></tr><tr><td rowspan="4">Zn-Q<sub>1</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>11</td><td>3.8</td><td>3.1</td></tr><tr><td>Control*</td><td>0.8</td><td>0.6</td><td>0.8</td></tr><tr><td>Amended soil measured</td><td>12</td><td>7.7</td><td>3.4</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>1.0</strong></td><td><strong>0.8</strong></td><td><strong>1.8</strong></td></tr><tr><td rowspan="4">Zn-Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>10</td><td>3.6</td><td>2.8</td></tr><tr><td>Control*</td><td>1.2</td><td>1.0</td><td>0.8</td></tr><tr><td>Amended soil measured</td><td>14</td><td>7.8</td><td>4.5</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>1.3</strong></td><td><strong>1.7</strong></td><td><strong>1.3</strong></td></tr><tr><td rowspan="4">Zn-Q<sub>1</sub>+Q<sub>2</sub><br>(mg kg<sup>-1</sup>)</td><td>Compost*</td><td>21</td><td>7.4</td><td>5.9</td></tr><tr><td>Control*</td><td>2.0</td><td>1.5</td><td>1.5</td></tr><tr><td>Amended soil measured</td><td>26</td><td>16</td><td>7.9</td></tr><tr><td>Ratio measured/theoretical in amended soil</td><td><strong>1.1</strong></td><td><strong>1.8</strong></td><td><strong>1.1</strong></td></tr></tbody></table>
**Table SI 4-10: correlation matric**

**Correlation matrix** (Spearman test) coefficients of Q₁, rapidly extractable; Q₂, slowly extractable; Q₁+Q₂, total extractable concentrations of Ni, Cr, Cu and Zn with physio-chemical parameters, i.e., pH, C/N, dissolved organic carbon (DOC) and cationic exchange capacity (CEC). *, **, ***, ns: statistically significant at the probability level 0.05, 0.01 and 0.001 and non-significant respectively

<table><thead><tr><th rowspan="2"></th><th colspan="3">Ni</th><th colspan="3">Cr</th><th colspan="3">Cu</th><th colspan="3">Zn</th></tr><tr><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th></tr></thead><tbody><tr><td>pH</td><td>0.80<br/>*</td><td>0.54<br/>ns</td><td>0.62<br/>ns</td><td>0.28<br/>ns</td><td>0.76<br/>*</td><td>0.74<br/>*</td><td>0.74<br/>*</td><td>0.81<br/>*</td><td>0.78<br/>*</td><td>0.73<br/>*</td><td>0.80<br/>*</td><td>0.77<br/>*</td></tr><tr><td>C/N</td><td>-0.52<br/>ns</td><td>-0.50<br/>ns</td><td>-0.52<br/>ns</td><td>-0.56<br/>ns</td><td>-0.82<br/>*</td><td>-0.82<br/>*</td><td>-0.67<br/>ns</td><td>-0.75<br/>*</td><td>-0.71<br/>*</td><td>-0.67<br/>ns</td><td>-0.76<br/>*</td><td>-0.71<br/>*</td></tr><tr><td>DOC</td><td>0.67<br/>ns</td><td>0.53<br/>ns</td><td>0.58<br/>ns</td><td>0.04<br/>ns</td><td>0.81<br/>**</td><td>0.75<br/>*</td><td>0.97<br/>***</td><td>0.98<br/>***</td><td>0.99<br/>***</td><td>0.97<br/>***</td><td>0.98<br/>***</td><td>0.98<br/>***</td></tr><tr><td>CEC</td><td>0.66<br/>ns</td><td>0.55<br/>ns</td><td>0.59<br/>ns</td><td>0.18<br/>ns</td><td>0.87<br/>*</td><td>0.83<br/>*</td><td>0.97<br/>***</td><td>0.99<br/>***</td><td>0.99<br/>***</td><td>0.98<br/>***</td><td>0.99<br/>***</td><td>0.99<br/>***</td></tr></tbody></table>
Acknowledgments

The authors thank the 'Caledonienne des Eaux' and 'Sud Forêt'. Thanks are extended to our colleagues from the LAMA US Imago IRD Noumea and the technical staff of Cirad Montpellier for their analytical support.

Statements and Declarations

Funding

Perrine Klein's PhD thesis is financed by a doctoral contract financed half by UNC and half by IAC through the 'Ecole Doctorale du Pacifique'. The study was part of the ValoPRO-NC project, financed by the French Environment and Energy Management Agency (ADEME's) 2017 Graine call for projects), and coordinated by Audrey Léopold. Financial support was also provided by COGES South Province (New Caledonia) and ADEME (New Caledonia).

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Ethics approval

The authors attest that this paper has not been published elsewhere and has not been submitted simultaneously for publication elsewhere. The results presented in this study are true and without fabrication, falsification, or data manipulation.

Consent to participate and to publish

All authors involved in this study gave their informed consent to the publication of this study.

Availability of data and material

Not applicable.

Code availability

Not applicable.

Author Contributions

Perrine Klein, Audrey Léopold and Peggy Gunkel-Grillon contributed to the study conception and design. Material preparation, data collection and analysis were performed by Perrine Klein, Audrey Leopold, Hélène Kaplan and Anthony Pain. The first draft of the manuscript was written by Perrine Klein and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
## II. Effect of geogenic Cr- and Ni-rich compost inputs on trace metal mobility in a low trace metal soil: a preliminary assessment

### II.1. Introduction

Municipal organic waste represents worldwide up to 2 billion tons per year and is going to increase because of the demographic growth (Kaza et al. 2018). It is judicious to recycle them to ensure sustainable development within a circular economy way. Organic wastes, i.e., sewage sludge, green waste, compost, are known to be a source of organic matter and nutrients with fertilizing properties (high N, P, K contents) and can thus be an alternative to mineral fertilizer (Wassenaar et al. 2014). However, organic wastes can contain biologic, organic, and inorganic contaminants, and represent the major source of trace metals, i.e., nickel (Ni), chromium (Cr), copper (Cu) or zinc (Zn), in agricultural soils, leading to environmental and human health concerns (Belon et al. 2012; Houot et al. 2014; Avadí et al. 2022). Among these trace metals, Cu and Zn frequently occur in organic composts where they are considered as common anthropogenic trace metals (Belon et al. 2012; Tella et al. 2013; Legros et al. 2017; Laurent et al. 2020). By contrast, Cr and Ni occur less often in organic composts. However, in New Caledonia, composts of mixed sewage sludges and green wastes can show exceptionally high Ni and Cr contents, i.e., up to 300 and 900 mg.kg⁻¹ respectively (Klein et al., 2023). This is due to the occurrence of ultramafic soil particles in green wastes that yield significant amounts of geogenic Ni and Cr. Although these two trace metals are known to show quite a low mobility in these soils, i.e., <10 and <1% respectively (Becquer et al. 2003; Garnier et al. 2006, 2009; Pasquet et al. 2018; Klein et al., 2023), high organic matter loading with compost could influence their behavior. Indeed, compost application in soils can increase microbiological activity, which can in turn enhance (bio)dissolution of Ni- and Cr-bearing mineral species, such as Mn- and Fe-oxides as well as clay minerals (Quantin et al. 2002; Amir and Pineau 2003; Fischer et al. 2007; Borch et al. 2010; Cerdeira-Pérez et al. 2019).

As a consequence, one judicious way to valorize such Ni- and Cr-rich composts is to select an agricultural sector known for its low trace metals transfer into the soil-plant system in order to minimize the risk of food contamination. Arboriculture is reported in the literature to not or weakly accumulate trace metals in plant organs (Houot et al. 2009; Cheng et al. 2015; Roba et al. 2016; Gori et al. 2019). Moreover, orange is one of the main (20 %) fruit production in New Caledonia and orange orchards are mostly located in soils of the watershed of la Foa which present a low trace metal background. Some studies showed an increase of the labile fractions of trace metals shortly after compost application in soils, i.e., Cu and Zn (Legros et al. 2013; Laurent et al. 2020), or Ni and Zn (Dœlsch et al. 2006a), or Ni and Cr (Amir and Pineau 2003; Cerdeira-Pérez et al. 2019; Klein et al., 2023), due to the degradation process of organic materials. Indeed, mineralization of organic matter from compost could lead to a release of soluble forms because their dependency with dissolved organic carbon which increase
overtime (Clemente et al. 2006; Rodríguez-Espinosa et al. 2023). By contrast, other studies highlighted that compost application was able to stabilize trace metals in soils by creating metals organo-complexes with organic matter (Zhang et al. 2006; Hargreaves et al. 2008). These contrasting results depend on the compost composition and its degree of maturity, the type of the soil receiving compost inputs, the trace metals considered and the local climatic conditions (Hargreaves et al. 2008; Smith 2009; Legros et al. 2013; Rodríguez-Espinosa et al. 2023). Thus, there is a lack of knowledge on the evolution of Ni and Cr mobility after application of a Ni- and Cr-rich compost on soils containing initially low amounts of these trace metals under tropical climate.

In this study, we hypothesized that application of a Ni- and Cr-rich compost on a Ni- and Cr-poor agricultural soil could lead both to a Ni and Cr contamination related to the increase of the total concentrations of these two trace metals and to an increase of their mobile fractions in the amended soils, especially with the mineralization of compost in soils and after repeated applications. Nevertheless, the environmental risk related to trace metal mobilization could be linked to the origin of trace metal, *i.e.*, a risk at short term for anthropogenic Cu and Zn, and a risk at longer term for geogenic Ni and Cr. To test these hypotheses, the influence of compost application was determined on Ni, Cr, Cu and Zn labile fractions using EDTA-kinetic extractions, which allowed to discriminate a rapidly and a slowly extractable pool of the selected trace metals (Bermond et al. 2005; Fangueiro et al. 2005; Labanowski et al. 2008). Since these two pools represent the short- and long-term dynamics of trace metals, their quantification should help to better inform on the short- and long-term risk of trace metals release in the environment upon compost application on such type of soil.

## II.2. Materials and methods

### II.2.1. Samples description

New Caledonia is a French archipelago in the South-West Pacific, which is characterized by a subtropical climate. The study site is located in an orange orchard in the agronomic research station of the "Institut Agronomique néo-Caledonien" (IAC) in Pocquereux (21°43'895'S; 165°53'922'E; La Foa, New Caledonia). The soil in the parcel is a clayey loam Fluvisol with 26% of clay, 29% of silt, and 42% of sand (Table 4-1). Sampling was performed in the 0-20cm horizon from block one of the orchards on the mulch modality without fertilization (PO) and receiving compost (PD2). One soil sampling corresponded to one composite sample of 12 samplings, *i.e.*, one per tree. Three sampling campaigns were realized on the same plots after compost application at different times, *i.e.*, 3 months (t<sub>3-1</sub>), 11 months (t<sub>11-1</sub>) and 3 months after a second compost input (t<sub>3-2</sub>). The amount of dry weight compost applied by tree was 8.63 kg at t<sub>3-1</sub> and 6.52 kg at t<sub>3-2</sub>.
The compost applied was a mixture of sewage sludges and green wastes produced by the New Caledonian water company and its characteristics are described in **Table 4-1**.

### II.2.2. Chemical and physical analysis

Soils and composts were air-dried at room temperature, sieved at 2 mm before being analyzed for their granulometry (NF X31 107), humidity (AFNOR, 2000), and pH (NF ISO 10390). Other analyses such as cation exchange capacity (CEC) (NFX31-130), total carbon and nitrogen quantification by dry combustion with a CHN analyzer (IRMS Sercon Integra 2) were performed on mechanically ground soil samples that were further sieved to 0.1 mm. In addition, total concentrations of phosphorus (P), potassium (K), magnesium (Mg), manganese (Mn), iron (Fe), nickel (Ni), chromium (Cr), copper (Cu), and zinc (Zn) were also determined on these sieved samples by inductively coupled plasma optical emission spectrometer (ICP-OES VARIAN® 730ES) after alkali fusion (EN ISO 14869-2). Finally, dissolved organic carbon (DOC) concentration was quantified in soil extracts with a TOC-L-CSH (Shimadzu N°H54205000462) (Ciavatta et al. 1989). All these analyses were performed at the ISO-9001 certified LAMA laboratory (Imago IRD, Noumea, New Caledonia).

Since Cr(VI) is known as the soluble form of Cr (Sun et al. 2022), the amount of extractable Cr(VI) was assessed in order to know if the enhanced Cr mobility observed after compost application could be related to the occurrence of this Cr species. Total extractable Cr(VI) was thus evaluated by reacting soil samples with a 0.1M KH₂PO₄ extraction on a 1 : 25 ratio (solid : liquid) under constant agitation during 1h at 20°C (Becquer et al. 2003; Garnier et al. 2006). After the extraction step, Cr(VI) concentration in the extract was determined by measuring the absorbance at 540 nm 20 minutes after reaction with a diphenylcarbazide solution prepared according to Bartlett and James (1979).

### II.2.3. Kinetic extraction to assess trace metal mobility

Ni, Cr, Cu and Zn kinetic extractions with a 0.05M Na₂-EDTA solution (10, 20, 40, 60, 120, 240, 420, 840, and 1440 min) were performed in a batch mode (Quevauviller et al. 1996), before ICP-OES analyses. These analyses were done in duplicate. The kinetic EDTA extraction curves resulting from these extractions can usually be modeled as the sum of two first-order exponential reactions according to equation (1) (Fangueiro et al. 2005):

$$Q_i(t) = Q_1(1 - e^{-k_1 t}) + Q_2(1 - e^{-k_2 t}) + \varepsilon_i \quad (1)$$

where $Q_i(t)$ (mg kg⁻¹) is the amount of trace metal extracted per weight unit at a time t (min). $Q_1$ and $Q_2$ are respectively the rapidly and slowly extractable pools, the sum ($Q_1 + Q_2$) is named the total extractable pool. $\varepsilon_i$ is the standard error. $k_1, k_2$ (min⁻¹) are the kinetic constant rates associated with $Q_1$ and $Q_2$ respectively.
The non-extractable pool, Q₃ was calculated according to equation (2):

$$Q_3 = Q_{tot} - (Q_1 + Q_2) \qquad (2)$$

where Qtot is the total concentration of the selected trace metal.

In this study, all these parameters were retrieved from Equation (1) after fitting the experimental kinetic data through a non-linear regression analysis using CurveExpert Professional (2.7). The statistical parameters used to access the validity of the fitting process are the standard error (ε) and correlation coefficient (R²) which are indicators of good curve fitting (Table 4-2). All fittings exhibited R² ≥ 0.99.

### II.2.4. Quality control and statistical analysis

Quality control was performed for each element analysis by calculating the z-score statistic indicator of the calibration standards set, intern soil reference samples, and certified reference material (BCR n°483 sewage amended soil (European Commission)). The average blanks were negligible and blank concentrations were subtracted from sample concentrations. All data were presented after blank correction. All relevant material was conveniently rinsed with distilled water, left in a 10% HNO₃ bath for 24h, rinsed again with distilled water and with ultrapure water, and dried at room temperature. Analyses on certified reference material and an intern reference sample were used to control the stability and accuracy of ICP-OES measurements throughout the analytical session.

Statistical analyses were performed with R software (R-i386 4.1.2) with rstatix and FSA packages for parametric t-test and non-parametric test Wilcoxon test and correlation matrix was performed with the Spearman method.

## II.3. Results and discussion

### II.3.1. Physical and chemical properties of the studied compost and soils

The compost made of mixed sewage sludges and green wastes showed physical-chemical properties able to improve the quality of agricultural soils, i.e., 38 ± 0.8% of organic matter, 11 674 ± 1 912 mg kg⁻¹ of P, 10 152 ± 486 mg kg⁻¹ of K and a C/N ratio equal to 9.0 ± 1.1 (Table 4-1), as previously described in Klein et al., 2023. The concentrations of Cu and Zn, i.e., 88 ± 14 and 282 ± 183 mg kg⁻¹ respectively, were below the threshold value of the European (EEC No. 2092/91/EEC, 1991: 25 and 70 mg kg⁻¹ respectively) or the French regulation (NFU 44-051, NFU 44-095: 60 and 120 mg kg⁻¹). They were similar to those reported for composts in the literature, i.e., 200 ± 20 and 490 ± 60 mg kg⁻¹ respectively (Payet and van de Kerchove 2007; Tella et al. 2013; Legros et al. 2017). By contrast, the concentrations of Ni and Cr, i.e., 457 ± 67 and 1294 ± 89 mg kg⁻¹ respectively, were exceptionally high with values up to ten
times the threshold limits authorized by the French regulation, i.e., 60 and 120 mg kg⁻¹ respectively, and the Australian regulation, i.e., 60 and 100 mg kg⁻¹ respectively. They were also much higher than those reported for the same type of composts in La Réunion, i.e., 100 ± 20 and 140 ± 30 mg kg⁻¹ respectively (Doelsch et al. 2010; Tella et al. 2013).

The non-amended soils (P0) show low contents in organic matter, i.e., 2.5%, and alkaline pH, i.e., around 7.1 ± 0.2. Total concentrations of trace metals were low, i.e., Ni (18 ± 6 mg kg⁻¹), Cr (42 ± 8 mg kg⁻¹), Cu (28 ± 3 mg kg⁻¹) and Zn (116 ± 7 mg kg⁻¹). Similar physical-chemical characteristics were reported for this type of soil in the literature (Latham et al. 1978; Houles et al. 2018; Vincent et al. 2018).

After the compost input, the amended soils (PD2) showed a significant increase, i.e., 1.5-fold, in organic matter content. Compost input thus improved soil quality, as reported by Hargreaves et al. (2008). While Ni and Cr concentrations significantly increased by ca. 2.3-fold in the amended soils, Cu and Zn concentrations did not change significantly with compost application (Table 4-1). Mass balance calculation indicated that these increased concentrations in the amended soils could be attributed to the amount of compost added (Table SI 4-11). Other studies reported similar results with no changes in the total concentration of these two trace metals after compost input (Zhang et al. 2006; Dœlsch et al. 2006a; Laurent et al. 2020). This result could be related to the low rate of compost application or the lower concentration of trace metals in compost compared to their concentration in the non-amended soil. After the second compost input, Ni and Cr concentrations increased by ca. 3-fold and ca. 4-fold, respectively. Indeed, Ni and Cr concentrations in composts were 31-times higher compared to concentrations measured in non-amended soils. Since these trace metals were known to be non-biodegradable, they would have accumulated upon compost application. Such an accumulation of trace metals in soils was usually reported as a common undesirable effect of compost application and its magnitude was considered to mainly depend on the compost application rate and frequency (Balabane and Van Oort 2002; Achiba et al. 2010; Farrell et al. 2010; Santos et al. 2010). Nevertheless, such increases of both Ni and Cr are uncommon in the literature and are related to the quality of compost produced in New Caledonia. Consequently, few is known about the fate of these trace metals in soils after compost inputs.
Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES

<table><caption>**Table 4-1:** Physical and chemical parameters measured on control soil (PO) and amended (PD2) soils at the three sampling campaigns t3-1, t11-1, t3-2 and averaged values between the first (t3-1 and t11-1) and second (t3-2) compost applied. C/N ratio, dissolved organic carbon (DOC, mg kg⁻¹), organic matter content (OM, g 100g⁻¹), cation capacity exchange (CEC, cmol⁺ kg⁻¹), total concentrations (mg·kg⁻¹) of phosphorus (P), potassium (K), magnesium (Mg), manganese (Mn), iron (Fe), nickel (Ni), chromium (Cr), copper (Cu), and zinc (Zn). Letters represent statistically significant differences (p-value<0.05 with a t-test) between control soils (PO) and compost amended soils (PD2). When no letter is given the difference for the corresponding parameter was not significant. Numbers in brackets correspond to the standard deviation</caption><thead><tr><th></th><th></th><th>Clay</th><th>Silt</th><th>Sand</th><th>pH</th><th>C/N</th><th>DOC</th><th>OM</th><th>CEC</th><th>P</th><th>K</th><th>Mg</th><th>Fe</th><th>Mn</th><th>Ni</th><th>Cr</th><th>Cu</th><th>Zn</th></tr><tr><th></th><th></th><th colspan="2">%</th><th></th><th></th><th colspan="2">mg kg<sup>-1</sup></th><th colspan="2">g 100g<sup>-1</sup></th><th colspan="2">cmol<sub>(+)</sub> kg<sup>-1</sup></th><th colspan="2">mg kg<sup>-1</sup></th><th colspan="2"></th><th colspan="2"></th><th colspan="2"></th></tr></thead><tbody><tr><td rowspan="2">t<sub>3-1</sub></td><td>PO</td><td>26</td><td>29</td><td>42</td><td>7.1</td><td>10.0</td><td>204</td><td>2.5</td><td>22</td><td>883</td><td>6930</td><td>5572</td><td>52257</td><td>2752</td><td>12</td><td>46</td><td>28</td><td>124</td></tr><tr><td>PD2</td><td></td><td></td><td></td><td>7.1</td><td>9.4</td><td>376</td><td>4.1</td><td>23</td><td>1359</td><td>6969</td><td>5570</td><td>49839</td><td>3185</td><td>44</td><td>102</td><td>30</td><td>125</td></tr><tr><td rowspan="2">t<sub>11-1</sub></td><td>PO</td><td></td><td></td><td></td><td>6.9</td><td>11.6</td><td>194</td><td>2.7</td><td>21</td><td>846</td><td>7007</td><td>5176</td><td>49453</td><td>2676</td><td>23</td><td>46</td><td>26</td><td>113</td></tr><tr><td>PD2</td><td></td><td></td><td></td><td>6.8</td><td>10.3</td><td>225</td><td>3.8</td><td>22</td><td>1135</td><td>6908</td><td>4764</td><td>40552</td><td>2642</td><td>32</td><td>57</td><td>28</td><td>96</td></tr><tr><td rowspan="2">t<sub>3-2</sub></td><td>PO</td><td></td><td></td><td></td><td>7.3</td><td>10.6</td><td>186</td><td>2.5</td><td>22</td><td>857</td><td>8236</td><td>5537</td><td>49765</td><td>2749</td><td>20</td><td>33</td><td>31</td><td>111</td></tr><tr><td>PD2</td><td></td><td></td><td></td><td>7.0</td><td>9.3</td><td>275</td><td>4.2</td><td>24</td><td>1545</td><td>8435</td><td>5799</td><td>46820</td><td>2840</td><td>51</td><td>125</td><td>35</td><td>126</td></tr><tr><td rowspan="2">Mean</td><td>PO</td><td></td><td></td><td></td><td>7.1<br>(0.2)</td><td>10.7<br>(0.8)</td><td>195<br>(9)</td><td>2.6 <b>a</b></td><td>22<br>(1)</td><td>862<br>(19)</td><td>7391<br>(734)</td><td>5428<br>(219)</td><td>50 492<br>(1537)</td><td>2726<br>(43)</td><td>18 <b>a</b></td><td>42<br>(6)</td><td>28<br>(8)</td><td>116<br>(7)</td></tr><tr><td>PD2</td><td></td><td></td><td></td><td>7.0<br>(0.2)</td><td>9.7<br>(0.6)</td><td>292<br>(77)</td><td>4.0 <b>b</b></td><td>23<br>(1)</td><td>1346<br>(205)</td><td>7437<br>(865)</td><td>5378<br>(544)</td><td>45 737<br>(4737)</td><td>2889<br>(275)</td><td>42 <b>b</b></td><td>95<br>(10)</td><td>31<br>(35)</td><td>116<br>(17)</td></tr><tr><td>t<sub>3-1</sub></td><td>Compost</td><td></td><td></td><td></td><td>6.0</td><td>9.8</td><td>1 894</td><td>37.6</td><td>64.8</td><td>10 322</td><td>10 495</td><td>13 742</td><td>36 080</td><td>789</td><td>410</td><td>1 231</td><td>77</td><td>223</td></tr><tr><td>t<sub>3-2</sub></td><td></td><td></td><td></td><td></td><td>5.9</td><td>8.2</td><td>2 616</td><td>38.8</td><td>64.9</td><td>13 025</td><td>9 808</td><td>19 012</td><td>42 221</td><td>629</td><td>504</td><td>1 357</td><td>98</td><td>340</td></tr><tr><td rowspan="2">Mean</td><td>Compost</td><td></td><td></td><td></td><td>5.9<br>(1.1)</td><td>9.0<br>(1.1)</td><td>2 255<br>(510)</td><td>38.2<br>(0.8)</td><td>65<br>(0.0)</td><td>11 674<br>(1912)</td><td>10 152<br>(486)</td><td>16 377<br>(3726)</td><td>39 150<br>(4343)</td><td>709<br>(114)</td><td>457<br>(67)</td><td>1294<br>(89)</td><td>88<br>(14)</td><td>282<br>(83)</td></tr></tbody></table>
## II.3.2. Trace metals mobility assessment

### II.3.2.1. In the compost

In the compost, the total extractable pools ($Q_1+Q_2$ mg kg$^{-1}$) of trace metal mean concentrations ranked as Cr (3.97 ± 0.93 mg kg$^{-1}$) < Cu (28.96 ± 2.21 mg kg$^{-1}$) < Ni (39.42 ± 1.59 mg kg$^{-1}$) < Zn (211.33 ± 19.30 mg kg$^{-1}$) (Table 4-2). A similar trace metal mobility order was already reported in other studies (Klein et al., 2023, Doelsch et al., 2006). It suggested that a potential risk could directly arise from Ni and Zn whose extractable concentrations in the compost were more than half the threshold of total concentrations authorized by the regulations.

When expressed as total extractable fraction ($Q_1+Q_2$ %), the ranking mean values changed for Cr (0.31 ± 0.05 %) < Ni (8.57 ± 1.38 %) < Cu (33.30 ± 2.97 %) < Zn (79.48 ± 30.21 %) (Table 4-2). Cu and Zn appeared thus as the most mobile trace metals compared to Ni and Cr, in agreement with results from previous studies (Klein et al., 2023; Maturi et al., 2021, Smith 2009; Xu et al., 2022).

The $Q_1/Q_2$ ratio of trace metal mean values ranked as Ni (0.78 ± 0.20) < Cu (0.92 ± 0.16) < Cr (1.71 ± 0.35) < Zn (1.88 ± 1.15). For Ni and Cu the ratio was below 1 which indicated that the amount of these two trace metals slowly released was higher than the rapidly released pool. On the opposite, the ratio of Cr and Zn above 1, indicated that these two trace metals rapidly released was higher than that slowly released. These latter results suggested that Cr and Zn could represent an environmental risk on the short term, while Ni and Cu would represent an environmental risk on the long term.

### II.3.2.2. In the control soils

In the non-amended soil (P0), the total extractable pools ($Q_1+Q_2$ mg kg$^{-1}$) of trace metal mean concentrations were very low and ranked as Cr (0.23 ± 0.10 mg kg$^{-1}$) < Ni (0.99 ± 0.07 mg kg$^{-1}$) << Cu (11.71 ± 0.66 mg kg$^{-1}$) < Zn (13.95 ± 1.12 mg kg$^{-1}$) (Table 4-2). When considering the total extractable fraction ($Q_1+Q_2$ %), the ranking of mean values changed for Cr (0.58 ± 0.32 %) < Ni (5.84 ± 2.25 %) < Zn (12.04 ± 1.21 %) < Cu (41.61 ± 4.75 %). Thus, Cu appeared as the most mobile trace metal in the control soil and even if the extractable concentrations were low, they represented almost half the total concentration. Regarding risk assessment code based on the percentage of total concentration of each trace metal occurred in the most available fraction (Avadí et al. 2022), the rapidly released fraction ($Q_1$ %) presented no risk for Cr (<1 %), a low risk for Ni (between 1 and 10 %) and a medium risk for Zn and Cu (between 11 and 30 %).

Mean values of the $Q_1/Q_2$ ratio were close to 1 for Ni and Zn, i.e., respectively 1.17 ± 0.23 and 1.11 ± 0.29, indicating similar amounts of rapidly and slowly released trace metal (Table 4-2). For Cu, the value
of the Q₁/Q₂ ratio was 2.00 ± 0.52, indicating a largest fraction of rapidly released trace metal. So, if any,
the potential environmental risk associated with Cu would arise on the short timescale.

For Cr, the results concerning the amount of extractable Q₁ and Q₂ were surprising. Indeed, Q₁/Q₂ ratio
was below 1, i.e., 0.03 ± 0.01, suggesting a much larger fraction of slowly released trace metal. Thus, a
potential risk occurred mainly at long term. This result must be considered with caution, because of
the low amount of extractable Cr in the non-amended soils which represented less than 0.3 mg kg⁻¹
(Figure 4-2B). Such low extracted concentrations could emphasize the analytical limits of the kinetic
extraction method when the amount of trace metal extracted was too weak.

II.3.3.3. In the amended soils

In amended-soils (PD2), total extractable pools ($Q_1+Q_2$ mg kg$^{-1}$) of trace metal mean concentrations ranked as Cr (1.04 ± 0.27 mg kg$^{-1}$) ~ Ni (4.82 ± 1.14 mg kg$^{-1}$) < Cu (14.80 ± 0.85 mg kg$^{-1}$) < Zn (24.44 ± 3.48 mg kg$^{-1}$) (Table 4-2). All kinetic curves computed according to equation (1) for these soils were higher than those computed for their corresponding control soils. Total extractable pools were higher than those measured in control soils, i.e., 5-times higher for Ni and Cr, 1.3-times for Cu and 1.7-times for Zn. This increase was statistically significant (p-value < 0.05) for all trace metals, except for Cr due to the heterogeneity of the kinetic constants computed for this trace metal. Thus, potential environmental risk arising from trace metal mobility in soil might have increased after compost application.

The total extractable fractions (% Q₁+Q₂) of trace metals mean values ranked as Cr (1.15 ± 0.26 %) < Ni
(11.37 ± 0.46 %) < Zn (21.17 ± 0.28 %) < Cu (48.00 ± 4.36 %) (Figure 4-2). This higher mobility for Cu
and Zn compared to Ni and Cr has already been reported in previous studies (Walter et al., 2001;
Døelsch et al. 2006; Santos et al. 2010; Klein et al., 2023; Laurent et al., 2020; Farrell et al., 2010). For
Cu, this could be explained by the well-known strong affinity towards organic matter (Amery et al. 2010;
Manceau and Matynia 2010) that can lead to dissolved organic carbon-Cu complexes. Although less
marked for Zn (Antoniadis and Alloway 2002), such complexation with dissolved organic carbon could
also explain the higher mobility for this trace metal compared to Cr and Ni.

Mean value of the Q₁/Q₂ ratio was below 1 for Ni and Cr, i.e., 0.80 ± 0.08 and 0.42 ± 0.02, respectively, while it was above 1 for Cu and Zn, i.e., 1.41 ± 0.20 and 1.30 ± 0.20, respectively. This result indicated opposite behavior between the two groups of trace metals, with a major release on the long timescale for Ni and Cr and on the short timescale for Cu and Zn. If any, the potential environmental risk due to compost application on the studied soil would thus arise on the long timescale for Ni and Cr and on the short timescale for Cu and Zn.
Mass balance calculation allowed to estimate the trace metal extractable pools ($Q_1+Q_2$ mg kg$^{-1}$) theoretically arising separately from the compost and from the soil through a "measured/theoretical" ratio (Table SI 4-11). This ratio was close to 1 for Cu and Zn, with values ranging respectively, from 1.10 to 1.26 and from 0.98 to 1.11. It was slightly larger for Ni, with values ranging from 1.73 to and 1.74. Finally, Cr showed much larger "measured/theoretical" ratios with values ranging from 1.71 to 5.63. These results suggested that, despite an increased mobility for all trace metals, the effect of compost application on Cu and Zn mobility in the amended soil corresponded to the summed mobility in the "control + compost" system. By contrast, compost application led to slightly higher Ni and strongly higher Cr mobility in the amended soil compared to the summed mobility in the "control + compost" system. In addition, the especially high "measured/theoretical" ratio calculated for the slowly extractable pool $Q_2$ of this latter trace metal suggested a possible influence of compost application on the long timescale. Similar effects have already been reported in a recent study on Ferralsols from New Caledonia (Klein et al., 2023). These observations have to be considered in light of the known geogenic origin of Ni and Cr in the compost, related to the presence of soil particles from ultramafic Ferralsols in the green wastes (Klein et al., 2023). In such a context, compost application is expected to increase microbial activity in the amended soil, which could in turn enhance (bio)dissolution of the Ni- and Cr- bearing mineral species, such as chromite, Mn-, Fe-oxides or clay minerals contained in the compost (Quantin et al. 2002; Amir and Pineau 2003; Fischer et al. 2007; Borch et al. 2010; Bolaños-Benítez et al. 2018; Cerdeira-Pérez et al. 2019). This remobilization of Ni and Cr from mineral bearing phases occurring in the compost suggested that application of Ni- and Cr-rich compost could represent a potential long-term environmental risk in amended soils. Moreover, a first assessment of Cr(VI) measurement was made in amended soils from $KH_2PO_4$ extraction DPC method. The results obtained indicated a 3-times increase of the amount of extractable Cr(VI) after the first compost application ($t_{3-1}$) and a 4-times increase after the second compost application ($t_{3-2}$) (Figure SI 4-12). Although quite weak regarding the concentrations involved, i.e., less than 0.3 mg kg$^{-1}$, these latter results suggested that the potential environmental risk associated with the increase of Cr(VI) mobility after Cr-rich compost application can not be ruled out and that it should especially be further evaluated. Cr speciation should be further investigated in amended soils, especially because microbial activity and organic matter matrix are known to favorize Cr(VI) reduction into Cr(III) (Chen et al. 2017; Bolaños-Benítez et al. 2018).

In addition to increasing the extractable EDTA pools for all trace metals, compost application also had some influence on their extraction rates. From a kinetic perspective, compost application increased both $k_1$ and $k_2$ extraction rates of Ni and Cr in amended soils compared to control soils. The effect was less marked and opposite on Cu and Zn, since compost application decreased $k_1$ extraction rates but
Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES

did not significantly change k₂ extraction rates. These latter results suggested that, in addition to
influencing the behavior of the studied trace metals by increasing their extractable pools, compost
application also influenced their behavior by modifying their extraction rates. For Ni and Cr, the effect
observed suggested an accelerated release, while for Cu and Zn it would rather indicate a slowed
release. Similar range values were reported in studies on kinetic extraction of trace metals in
agricultural or forest soils (Santos et al. 2010; Pasquet et al. 2018).

<figure><img src="image_69.png" /></figure>

**Figure 4-2:** Extractable Ni (A), Cr (B), Cu (C) and Zn (D) (mg.kg⁻¹) obtained by kinetic EDTA extractions for compost, control and amended soils at t₃⁻₁, t₁₁⁻₁ and t₃⁻₂. The blue dashed lines are the theoretical extractable metal concentration calculated for amended soils at t₃⁻₁ and t₃⁻₂, considering extractable trace metals provided by the compost. The Y scale on each panel depends on the trace metal concentration.
Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES

<table><caption>**Table 4-2:** Extractable pools (mg kg⁻¹) and fractions (%) obtained by EDTA kinetic extractions for Ni, Cr, Cu and Zn in the studied compost, control soils (P0) and amended soils (PD2) 3 months and 11 months after the first compost application (respectively t₃⁻₁ and t₁₁⁻₁) and 3 months after the second compost application (t₃⁻₂). Q₁, rapidly extractable; Q₂, slowly extractable; Q₃, non-extractable; Q₁+Q₂, total extractable, R² and standard error (ε) relate to the quality of the model computed on experimental data. Letters represent statistically significant differences (p-value<0.05) between control soils (P0) and amended soils (PD2). When no letter is given the difference for the corresponding parameters is not significant. Numbers in brackets correspond to the standard deviation. All results given for the compost corresponded to averaged values between the first compost applied (t₃⁻₁ and t₁₁⁻₁) and the second compost applied (t₃⁻₂)</caption><thead><tr><th>Trace metal element</th><th>Matrices</th><th>Modality</th><th>Q<sub>1</sub><br>mg kg<sup>-1</sup></th><th>%Q<sub>1</sub></th><th>Q<sub>2</sub><br>mg kg<sup>-1</sup></th><th>%Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub><br>mg kg<sup>-1</sup></th><th>%(Q<sub>1</sub>+Q<sub>2</sub>)</th><th>Q<sub>1</sub>/Q<sub>2</sub></th><th>Q<sub>3</sub><br>mg kg<sup>-1</sup></th><th>%Q<sub>3</sub></th><th>k<sub>1</sub><br>min<sup>-1</sup></th><th>k<sub>2</sub><br>min<sup>-1</sup></th><th>R<sup>2</sup></th><th>ε</th></tr></thead><tbody><tr><td rowspan="10">Ni</td><td rowspan="2">Compost<br>(±sd)</td><td></td><td>17.17<br>(3.14)</td><td>3.76<br>(1.13)</td><td>22.26<br>(1.55)</td><td>4.80<br>(0.25)</td><td>39.42<br>(1.59)</td><td>8.57<br>(1.38)</td><td>0.78<br>(0.20)</td><td>425<br>(58)</td><td>91<br>(1)</td><td>0.106<br>(0.008)</td><td>0.00353<br>(0.00164)</td><td>1.00</td><td>1.16<br>(0.06)</td></tr><tr><td>t<sub>3-1</sub></td><td>P0<br>PD2</td><td>0.48<br>2.46</td><td>2.23<br>5.62</td><td>0.54<br>2.73</td><td>4.38<br>6.23</td><td>1.02<br>5.19</td><td>8.32<br>11.85</td><td>0.90<br>0.90</td><td>11<br>39</td><td>92<br>88</td><td>0.106<br>0.078</td><td>0.00298<br>0.00250</td><td>1.00<br>0.99</td><td>0.03<br>0.24</td></tr><tr><td rowspan="2">t<sub>11-1</sub></td><td></td><td>P0<br>PD2</td><td>0.51<br>1.53</td><td>2.23<br>4.74</td><td>0.39<br>2.00</td><td>1.71<br>6.19</td><td>0.90<br>3.54</td><td>3.94<br>10.93</td><td>1.30<br>0.77</td><td>22<br>29</td><td>96<br>89</td><td>0.160<br>0.231</td><td>0.00400<br>0.00814</td><td>1.00<br>1.00</td><td>0.02<br>0.12</td></tr><tr><td>t<sub>3-2</sub></td><td>P0<br>PD2</td><td>0.58<br>2.44</td><td>2.97<br>4.83</td><td>0.45<br>3.28</td><td>2.28<br>6.49</td><td>1.03<br>5.73</td><td>5.26<br>11.32</td><td>1.30<br>0.74</td><td>19<br>45</td><td>95<br>89</td><td>0.068<br>0.173</td><td>0.00178<br>0.00593</td><td>0.98<br>0.99</td><td>0.07<br>0.21</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td></td><td>P0<br>PD2</td><td>0.53 <b>a</b><br>(0.05)</td><td>2.48 <b>a</b><br>(0.43)</td><td>0.46 <b>a</b><br>(0.07)</td><td>2.79 <b>a</b><br>(1.40)</td><td>0.99 <b>a</b><br>(0.07)</td><td>5.84 <b>a</b><br>(2.25)</td><td>1.17<br>(0.23)</td><td>17 <b>a</b><br>(5)</td><td>94 <b>a</b><br>(2)</td><td>0.112<br>(0.046)</td><td>0.00292 <b>a</b><br>(0.00111)</td><td></td><td></td></tr><tr><td></td><td></td><td>2.15 <b>b</b><br>(0.53)</td><td>5.06 <b>b</b><br>(0.48)</td><td>2.67 <b>b</b><br>(0.64)</td><td>6.30 <b>b</b><br>(0.16)</td><td>4.82 <b>b</b><br>(1.14)</td><td>11.37 <b>b</b><br>(0.46)</td><td>0.80<br>(0.08)</td><td>37 <b>b</b><br>(8)</td><td>89 <b>b</b><br>(1)</td><td>0.161<br>(0.008)</td><td>0.00552 <b>b</b><br>(0.00284)</td><td></td><td></td></tr><tr><td rowspan="10">Cr</td><td rowspan="2">Compost<br>(±sd)</td><td></td><td>2.47<br>(0.39)</td><td>0.19<br>(0.02)</td><td>0.09<br>(0.04)</td><td>0.11<br>(0.03)</td><td>3.97<br>(0.93)</td><td>0.31<br>(0.05)</td><td>1.71<br>(0.35)</td><td>1 290<br>(88)</td><td>99.7<br>(0.1)</td><td>0.086<br>(0.035)</td><td>0.00872<br>(0.00980)</td><td>1.00</td><td>0.19<br>(0.10)</td></tr><tr><td>t<sub>3-1</sub></td><td>P0<br>PD2</td><td>0.05<br>0.27</td><td>0.10<br>0.27</td><td>0.07<br>0.69</td><td>0.15<br>0.67</td><td>0.11<br>0.96</td><td>0.25<br>0.94</td><td>0.03<br>0.40</td><td>46<br>101</td><td>100<br>99</td><td>0.004<br>0.020</td><td>0,00121<br>0,00169</td><td>0.99<br>0.99</td><td>0.01<br>0.04</td></tr><tr><td rowspan="2">t<sub>11-1</sub></td><td></td><td>P0<br>PD2</td><td>0.09<br>0.25</td><td>0.20<br>0.44</td><td>0.18<br>0.57</td><td>0.39<br>1.00</td><td>0.27<br>0.82</td><td>0.59<br>1.44</td><td>0.04<br>0.04</td><td>46<br>56</td><td>99<br>99</td><td>0.004<br>0.020</td><td>0,00121<br>0,00169</td><td>0.99<br>1.00</td><td>0.01<br>0.02</td></tr><tr><td>t<sub>3-2</sub></td><td>P0<br>PD2</td><td>0.10<br>0.40</td><td>0.30<br>0.32</td><td>0.20<br>0.93</td><td>0.59<br>0.75</td><td>0.30<br>1.34</td><td>0.90<br>1.07</td><td>0.03<br>0.03</td><td>33<br>124</td><td>99<br>99</td><td>0.004<br>0.033</td><td>0,00121<br>0,00234</td><td>0.99<br>1.00</td><td>0.01<br>0.04</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td></td><td>P0<br>PD2</td><td>0.08 <b>a</b><br>(0.03)</td><td>0.20<br>(0.10)</td><td>0.15 <b>a</b><br>(0.07)</td><td>0.38<br>(0.22)</td><td>0.23 <b>a</b><br>(0.10)</td><td>0.58<br>(0.32)</td><td>0.03<br>(0.01)</td><td>41<br>(8)</td><td>100<br>(1)</td><td>0.003 <b>a</b><br>(0.000)</td><td>0.00091<br>(0.00000)</td><td></td><td></td></tr><tr><td></td><td></td><td>0.31 <b>b</b><br>(0.08)</td><td>0.34<br>(0.09)</td><td>0.73 <b>b</b><br>(0.19)</td><td>0.81<br>(0.17)</td><td>1.04 <b>b</b><br>(0.27)</td><td>1.15<br>(0.26)</td><td>0.35<br>(0.25)</td><td>94<br>(35)</td><td>99<br>(0)</td><td>0.024 <b>b</b><br>(0.007)</td><td>0.00191<br>(0.00037)</td><td></td><td></td></tr></tbody></table>
Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES

<table><thead><tr><th>Trace metal element</th><th>Matrices</th><th>Modality</th><th>Q<sub>1</sub><br>mg kg<sup>-1</sup></th><th>%Q<sub>1</sub></th><th>Q<sub>2</sub><br>mg kg<sup>-1</sup></th><th>%Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub><br>mg kg<sup>-1</sup></th><th>%(Q<sub>1</sub>+Q<sub>2</sub>)</th><th>Q<sub>1</sub>/Q<sub>2</sub></th><th>Q<sub>3</sub><br>mg kg<sup>-1</sup></th><th>%Q<sub>3</sub></th><th>k<sub>1</sub><br>min<sup>-1</sup></th><th>k<sub>2</sub><br>min<sup>-1</sup></th><th>R<sup>2</sup></th><th>ε</th></tr></thead><tbody><tr><td rowspan="12">Cu</td><td>Compost<br>(±sd)</td><td></td><td>13.76<br>(0.22)</td><td>15.94<br>(2.88)</td><td>15.20<br>(2.43)</td><td>17.36<br>(0.09)</td><td>28.96<br>(2.21)</td><td>33.30<br>(2.97)</td><td>0.92<br>(0.16)</td><td>59<br>(12)</td><td>67<br>(3)</td><td>0.058<br>(0.077)</td><td>0.03955<br>(0.05059)</td><td>0.99</td><td>1.17<br>(0.17)</td></tr><tr><td rowspan="2">t<sub>3-1</sub></td><td>P0</td><td>7.45</td><td>32.09</td><td>4.99</td><td>17.63</td><td>12.44</td><td>43.96</td><td>1.49</td><td>16</td><td>56</td><td>0.272</td><td>0.00416</td><td>1.00</td><td>0.35</td></tr><tr><td>PD2</td><td>8.52</td><td>28.31</td><td>6.81</td><td>22.64</td><td>15.33</td><td>50.94</td><td>1.25</td><td>15</td><td>49</td><td>0.192</td><td>0.00399</td><td>0.99</td><td>0.57</td></tr><tr><td rowspan="2">t<sub>11-1</sub></td><td>P0</td><td>8.29</td><td>32.09</td><td>3.26</td><td>12.63</td><td>11.56</td><td>44.72</td><td>2.54</td><td>14</td><td>55</td><td>0.188</td><td>0.00373</td><td>1.00</td><td>0.29</td></tr><tr><td>PD2</td><td>8.58</td><td>31.08</td><td>5.24</td><td>18.98</td><td>13.82</td><td>50.06</td><td>1.64</td><td>14</td><td>50</td><td>0.100</td><td>0.00559</td><td>1.00</td><td>0.33</td></tr><tr><td rowspan="2">t<sub>3-2</sub></td><td>P0</td><td>7.40</td><td>23.99</td><td>3.75</td><td>12.14</td><td>11.15</td><td>36.13</td><td>1.98</td><td>20</td><td>64</td><td>0.212</td><td>0.00551</td><td>1.00</td><td>0.39</td></tr><tr><td>PD2</td><td>8.77</td><td>24.72</td><td>6.48</td><td>18.27</td><td>15.24</td><td>42.99</td><td>1.35</td><td>20</td><td>57</td><td>0.145</td><td>0.00539</td><td>1.00</td><td>0.46</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td>P0</td><td>7.71<br>(0.50)</td><td>29.39<br>(4.68)</td><td>4.00 <b>a</b></td><td>14.14<br>(3.04)</td><td>11.71 <b>a</b></td><td>41.61<br>(4.75)</td><td>2.00<br>(0.52)</td><td>17<br>(3)</td><td>58<br>(5)</td><td>0.224<br>(0.044)</td><td>0.00447<br>(0.00093)</td><td></td><td></td></tr><tr><td>PD2</td><td>8.62<br>(0.13)</td><td>28.03<br>(3.19)</td><td>6.18 <b>b</b></td><td>19.96<br>(2.35)</td><td>14.80 <b>b</b></td><td>48.00<br>(4.36)</td><td>1.41<br>(0.20)</td><td>16<br>(3)</td><td>52<br>(4)</td><td>0.146<br>(0.058)</td><td>0.00499<br>(0.00087)</td><td></td><td></td></tr><tr><td rowspan="12">Zn</td><td>Compost<br>(±sd)</td><td></td><td>133.01<br>(43.84)</td><td>51.75<br>(30.78)</td><td>78.32<br>(24.54)</td><td>27.73<br>(0.56)</td><td>211.33<br>(19.30)</td><td>79.48<br>(30.21)</td><td>1.88<br>(1.15)</td><td>71<br>(101)</td><td>21<br>(30)</td><td>1.248<br>(1.493)</td><td>0.00559<br>(0.00551)</td><td>1.00</td><td>7.06<br>(1.67)</td></tr><tr><td rowspan="2">t<sub>3-1</sub></td><td>P0</td><td>6.35</td><td>5.94</td><td>7.82</td><td>6.33</td><td>14.17</td><td>11.46</td><td>0.81</td><td>109</td><td>89</td><td>2.787</td><td>0.00408</td><td>1.00</td><td>0.44</td></tr><tr><td>PD2</td><td>15.74</td><td>12.60</td><td>10.30</td><td>8.25</td><td>26.04</td><td>20.85</td><td>1.53</td><td>99</td><td>79</td><td>0.157</td><td>0.00281</td><td>1.00</td><td>0.63</td></tr><tr><td rowspan="2">t<sub>11-1</sub></td><td>P0</td><td>6.74</td><td>5.94</td><td>6.00</td><td>5.29</td><td>12.74</td><td>11.23</td><td>1.12</td><td>101</td><td>89</td><td>0.165</td><td>0.00482</td><td>1.00</td><td>0.43</td></tr><tr><td>PD2</td><td>10.94</td><td>11.41</td><td>9.56</td><td>9.97</td><td>20.50</td><td>21.38</td><td>1.14</td><td>75</td><td>79</td><td>0.200</td><td>0.00429</td><td>1.00</td><td>0.70</td></tr><tr><td rowspan="2">t<sub>3-2</sub></td><td>P0</td><td>8.70</td><td>7.82</td><td>6.25</td><td>5.62</td><td>14.96</td><td>13.44</td><td>1.39</td><td>96</td><td>87</td><td>0.152</td><td>0.00405</td><td>0.99</td><td>0.50</td></tr><tr><td>PD2</td><td>14.77</td><td>11.75</td><td>11.99</td><td>9.54</td><td>26.77</td><td>21.29</td><td>1.23</td><td>99</td><td>79</td><td>0.275</td><td>0.00740</td><td>1.00</td><td>0.56</td></tr><tr><td rowspan="2">Mean<br>(±sd)</td><td>P0</td><td>7.26 <b>a</b></td><td>6.57 <b>a</b></td><td>6.69 <b>a</b></td><td>5.74 <b>a</b></td><td>13.95 <b>a</b></td><td>12.04 <b>a</b></td><td>1.11<br>(0.29)</td><td>102<br>(7)</td><td>88 <b>a</b></td><td>1.035<br>(1.518)</td><td>0.00432<br>(0.00044)</td><td></td><td></td></tr><tr><td>PD2</td><td>13.82 <b>b</b></td><td>11.92 <b>b</b></td><td>10.62 <b>b</b></td><td>9.25 <b>b</b></td><td>24.44 <b>b</b></td><td>21.17 <b>b</b></td><td>1.30<br>(0.20)</td><td>91<br>(14)</td><td>79 <b>b</b></td><td>0.211<br>(0.060)</td><td>0.00483<br>(0.00234)</td><td></td><td></td></tr></tbody></table>
II.3.3. *Evolution of extractable trace metal between sampling times*

II.3.3.1. Assessing behavior of in-situ compost mineralization: comparison t<sub>3-1</sub>
and t<sub>11-1</sub>

The possible influence of *in-situ* compost mineralization on the total concentrations and total extractable EDTA pools (*Q*₁ + *Q*₂) was assessed by comparing the results obtained on amended soils 3 months and 11 months after the first compost application, *i.e.*, *t*<sub>3-1</sub> and *t*<sub>11-1</sub>, respectively. This comparison indicated a decrease of the total concentrations between 3 months (*t*<sub>3-1</sub>) and 11 months (*t*<sub>11-1</sub>) after compost application for all trace metals. The highest decrease, *i.e.*, from 102 to 57 mg kg<sup>-1</sup>, was observed for Cr. The three other trace metals showed a much lower decrease, *i.e.*, from 44 to 32 mg kg<sup>-1</sup> for Ni, from 125 to 96 mg kg<sup>-1</sup> for Zn and from 30 to 28 mg kg<sup>-1</sup> for Cu (**Table 4-1**). Such a decrease of total trace metals content over time was already reported by Walter et al. (2002) after biosolids application to agricultural soils in Spain. It could be due to a partial leaching downward the soil profile upon water percolation and/or to accumulate into the plant system growing on the studied soil. Another explanation could be related to soil sampling heterogeneity with more or less compost present in the sampling taken.

Total extractable pools ($Q_1 + Q_2$ mg kg$^{-1}$) of Zn, Cu and Ni decreased in same order of magnitude than their total concentrations (Table 4-2). These pools changed from 26.04 to 20.50 mg kg$^{-1}$ for Zn, from 15.33 to 13.82 mg kg$^{-1}$ for Cu and from 5.19 to 3.54 mg kg$^{-1}$ for Ni between 3 months ($t_{3-1}$) and 11 months ($t_{11-1}$) after compost application. Total extractable fractions ($Q_1 + Q_2$ %) of Cu, Ni and Zn vary similarly between $t_{3-1}$ and $t_{11-1}$, i.e., from 50.94 to 50.06 % for Cu and from 11.95 to 10.93 % for Ni, and from 20.85 to 21.38 % for Zn (Table 4-2). The trend was different for Cr that showed a quite similar total extractable pools with 0.96 mg kg$^{-1}$ after 3 months of compost application ($t_{3-1}$) and 0.82 mg kg$^{-1}$ after 11 months of compost application ($t_{11-1}$), while the total concentration has significantly decreased.
As a consequence, Cr showed a more marked increase of its total extractable fraction ($Q_1 + Q_2$ %) from 0.94 % after 3 months of compost application ($t_{3-1}$) to 1.44 % after 11 months of compost application ($t_{11-1}$).

Our results indicated thus a concomitant decrease of the total concentrations, total extractable EDTA
pools ($Q_1 + Q_2$ mg·kg⁻¹) of Cu, Zn and Ni over time after compost application. These trends suggested
that the decrease of the total concentrations of these trace metals could be due to an uptake of
extractable fraction in trees or a leaching in soil profile. Indeed, La Niña phenomenon recorded
exceptional rainfall rates with cumulated precipitations of 100 mm in 24 h and a mean of rainfall 57 %
in excess in 2020-2021 compared to mean values (Météo NC, 2021). Thus, the possibility of a leaching
of extractable trace metals in soil profile was not negligible. For Cr, the results were quite different
since the total extractable EDTA fractions ($Q_1 + Q_2$ %) increased over time after compost application.
Although the *Q*₁ and *Q*₂ values retrieved from computation of the kinetic curves of this latter trace metal were quite low (**Table 4-2; Figure 4-2**), these results suggested a possible specific influence of *in-situ* compost application on the behavior of Cr in amended soils with an *in-situ* production of extractable Cr.

Moreover, our results indicated that *in-situ* compost mineralization influenced the fate and behavior of the studied trace metals. One possible driver of this influence could be the labile organic matter released upon compost *in-situ* mineralization that can represent a source of soluble organo-metal complexes in amended soils (Balabane et Van Oort 2002; Walter et al. 2002; Achiba et al. 2010). However, the formation of stable metal-organo-complexes was also reported upon long-term compost mineralization as a possible way to decrease trace metals mobility in amended soils (Walter et al. 2002; Farrell et al. 2010; Lwin et al. 2018; Kaninga et al. 2020). Consequently, the maturity stage of the compost applied in soils could represent a key parameter on the actual impact of compost application on trace metal mobility in amended soils (Rodríguez-Espinosa et al. 2023). This latter point deserves to be further considered in future studies aimed at better understanding both the short and long terms influence of metal-rich compost application on trace metals behavior in amended soils.

### II.3.3.2. Assessing influence of repeated compost application: comparison t11-1 and t3-2

The possible influence of repeated compost application on the total concentrations and total extractable EDTA pools (*Q*₁ + *Q*₂) of trace metals was assessed by comparing the results obtained on amended soils before and after a second compost application (respectively t₁₁-₁ and t₃-₂). As total concentrations decreased after 11 months of the first compost application in soils, total concentrations increase once again with a second application of compost (**Table 4-1**). Indeed, total concentrations of Cr, Ni, Cu and Zn increased between t₁₁-₁ and t₃-₂ in amended soils, *i.e.*, from 57 to 125 mg kg⁻¹, 32 to 51 mg kg⁻¹, from 28 to 35 mg kg⁻¹ and from 96 to 126 mg kg⁻¹, respectively.

A similar increase trend was observed for the total extractable pools (*Q*₁ + *Q*₂ mg kg⁻¹) for all trace metals (**Table 4-2**). For Cr, this pool increased significantly from 0.82 mg kg⁻¹ 11 months after the first compost application (t₁₁-₁) to 1.34 mg kg⁻¹ after the second compost application (t₃-₂), from 3.54 to 5.73 mg kg⁻¹ for Ni, from 13.82 to 15.24 mg kg⁻¹ for Cu and from 20.50 to 26.77 mg kg⁻¹ for Zn. Total extractable fractions (*Q*₁ + *Q*₂%) showed slightly different trends with quite similar percentage between the two sampling times, *i.e.*, from 1.44 (t₁₁-₁) to 1.07% (t₃-₂) for Cr, from 10.93 (t₁₁-₁) to 11.32% (t₃-₂) for Ni, from 21.38 (t₁₁-₁) to 21.29% (t₃-₂) for Zn and a slightly decrease trend from 50.06 (t₁₁-₁) to 42.99% (t₃-₂) for Cu (**Table 4-2**). Our results indicated thus an increase of total concentrations and total extractable EDTA pools (*Q*₁ + *Q*₂ mg kg⁻¹) raising from compost applications. Those increase of both total
Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES

and extractable concentrations were already reported in studies with repeated applications of compost
in soils (Topcuoğlu 2005; Cambier et al. 2019). Our results indicated that repeated compost application
would generate a significant trace metal contamination of soil of both total and extractable
concentrations. Trace metals could thus accumulate or transfer in water-soil-plant system. Repeated
applications of compost should be thus monitored in order to evaluate the behavior of trace metal
extractable fractions at long term.
### II.4. Conclusion

In this study, we assessed the impact of compost application on trace metals dynamics in soils. Application of a Ni- and Cr-rich compost led to a 3-times and 4-times increase in Ni and Cr total concentration, respectively, in the amended soils after two-years. It also led to a significant increase of the total extractable EDTA pools of the studied trace metals, i.e., 4.9-times for Ni, 1.7-times for Zn, 1.5-times for Cr and 1.3-times for Cu in the amended soils compared to their control. In the amended soils, the extractable EDTA fractions of the studied trace metals could finally be ranked as Cr < Ni < Zn < Cu, with respective mean values of 1%, 11%, 21%, 48%. The enhanced mobility of anthropogenic Cu and Zn was more related to the rapidly extractable EDTA pool ($Q_1$). By contrast, the enhanced mobility of geogenic Ni and Cr was more related to the slowly extractable EDTA pool ($Q_2$), likely because of their major association with mineral species inherited from ultramafic Ferralsols, i.e., goethite, chromite, that undergo increased (bio)dissolution due to enhanced microbiological activity after compost application. Regarding these results, 1h- and 24h-EDTA extractions representing respectively the rapidly and total extractable pools, could be relevant to perform in order to highlight the short- and long-term risks of trace metal release in the environment.

Thus, the environmental risk associated with application of a Ni- and Cr-rich compost might act on the short timescale when related to Cu and Zn and on the long timescale when related to Ni and Cr. On the one hand, the decrease of the extractable EDTA pools of Ni, Cu and Zn with extended time after compost application suggests a potential attenuation of this environmental risk due to a decreasing mobility of trace metals with *in-situ* compost mineralization in the amended soils. On the other hand, the decline over time of the total concentrations of trace metals after compost application could be the consequence of their partial leaching downward the amended soil and/or uptake by crops growing on the amended soil. Repeated compost applications on the studied soil led to enhanced trace metals mobility, especially for Ni and Cr. This latter point emphasizes the potential risk that could arise from the transfer of trace metals across the soil-plant system. Despite the low concentration involved, there could be a particular concern with the occurrence of Cr(VI) that increased up to 4-times in the amended soils two-years after compost application. Further investigations should be undertaken in order to better understand and quantify the long-term dynamics of Ni and Cr in agricultural soils after application of such Ni- and Cr-rich compost. The actual pathway of Ni and Cr across the soil-plant system needs also to be further monitored in order to better assess the possible consequence of compost application on the environmental and health risks resulting from their potential leaching towards the water and/or food resources.
Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES

<table><caption>**Table SI 4-11:** Mass balance calculation for the rapidly extractable Q₁ (mg kg⁻¹), slowly extractable Q₂ (mg kg⁻¹) and total extractable (Q₁ + Q₂; mg kg⁻¹) pools and total concentrations of Ni, Cr, Cu and Zn arising from the compost in soil t₃⁻₁. * represents the concentration calculated by considering the percent volume of soil and compost in the amended soil. The mass balance calculations could only be performed for soil t₃⁻₁ in which 9kg dry weight (DW) of compost were directly added in the planting hole of 0.5x0.5x0.5m with a soil bulk density of 1.3 g.cm⁻³</caption><thead><tr><th colspan="2">Soil t<sub>3-1</sub></th></tr></thead><tbody><tr><td>Amount of compost input (kg DW)</td><td>9</td></tr><tr><td>% volume of soil in amended soil</td><td>95</td></tr><tr><td>% volume of compost in amended soil</td><td>5</td></tr></tbody></table><table><thead><tr><th rowspan="2">mg.kg<sup>-1</sup></th><th colspan="4">Ni</th><th colspan="4">Cr</th><th colspan="4">Cu</th><th colspan="4">Zn</th></tr><tr><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th><th>Total</th><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th><th>Total</th><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th><th>Total</th><th>Q<sub>1</sub></th><th>Q<sub>2</sub></th><th>Q<sub>1</sub>+Q<sub>2</sub></th><th>Total</th></tr></thead><tbody><tr><td>Compost*</td><td>0.97</td><td>1.06</td><td>2.03</td><td>21</td><td>0.11</td><td>0.06</td><td>0.17</td><td>62</td><td>0.70</td><td>0.67</td><td>1.37</td><td>4</td><td>8.20</td><td>3.05</td><td>11.25</td><td>11</td></tr><tr><td>Control*</td><td>0.46</td><td>0.56</td><td>0.97</td><td>12</td><td>0.04</td><td>0.07</td><td>0.10</td><td>43</td><td>7.08</td><td>4.74</td><td>11.82</td><td>27</td><td>6.03</td><td>7.25</td><td>13.46</td><td>117</td></tr><tr><td>Amended soil measured</td><td>2.46</td><td>2.73</td><td>5.19</td><td>44</td><td>0.27</td><td>0.69</td><td>0.96</td><td>102</td><td>8.52</td><td>6.81</td><td>15.33</td><td>30</td><td>15.74</td><td>10.30</td><td>26.04</td><td>125</td></tr><tr><td>Measured/theoretical ratio</td><td><strong>1.73</strong></td><td><strong>1.74</strong></td><td><strong>1.73</strong></td><td><strong>1.35</strong></td><td><strong>1.71</strong></td><td><strong>5.63</strong></td><td><strong>3.55</strong></td><td><strong>0.97</strong></td><td><strong>1.10</strong></td><td><strong>1.26</strong></td><td><strong>1.16</strong></td><td><strong>0.98</strong></td><td><strong>1.11</strong></td><td><strong>0.98</strong></td><td><strong>1.05</strong></td><td><strong>0.97</strong></td></tr></tbody></table>
<figure><img src="image_70.png" /><figcaption>Chapitre 4

MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC DANS DEUX SOLS CONTRASTES</figcaption></figure>

**Figure SI 4-12:** Extractable Cr(VI) (mg.kg⁻¹) measured in the control soils (P0) and in the amended soils 3 months after the first (t3-1) and the second (t3-2) compost application
### III. Influence des paramètres du sol sur les cinétiques de mobilisation des ETM

#### III.1 Introduction

L'influence de l'apport de compost sur les réservoirs extractibles à l'EDTA a été évaluée sur deux types de sols contrastés, i.e. un Ferralsol développé sur un substrat ultrabasique et un Fluvisol développé sur un substrat d'anciennes alluvions sédimentaires. Pour ces deux types de sols, une augmentation significative des réservoirs extractibles EDTA contenus dans les sols a été montrée après apport de compost. Dans le cas des Ferralsols, l'augmentation des réservoirs extractibles EDTA de chrome et de nickel avait pour origine une remobilisation de ces ETM depuis le sol, alors que l'augmentation des réservoirs extractibles EDTA du cuivre et du zinc provenait d'une mobilisation de ces deux ETM depuis le compost. Dans le cas du Fluvisol, l'augmentation des réservoirs extractibles EDTA des quatre ETM était associée à leur remobilisation depuis le compost. Sur ce dernier sol, une différence a toutefois été observée pour le nickel et le chrome dont l'augmentation de la mobilité était associée au réservoir extractible sur le long terme ($Q_2$) ; cela suggérait une contribution possible d'une remobilisation des fractions géogènes de ces deux ETM suite à l'altération des phases porteuses minérales contenues dans le compost.

L'objectif de cette partie du travail était d'approfondir ces différents résultats en évaluant les liens entre les paramètres physico-chimiques de ces deux types de sols et l'évolution des réservoirs extractibles EDTA de ces ETM suite à l'apport de compost riche en nickel et chrome caractéristique du territoire. Dans un premier temps, l'analyse de ces deux sols par diffraction des rayons X a permis de souligner leurs différences du point de vue de leur composition minéralogique et donc des phases potentiellement porteuses d'ETM. Dans un second temps, l'analyse en composantes principales (ACP) a permis de souligner la contribution des principaux paramètres physico-chimiques des sols à l'évolution des réservoirs extractibles EDTA des ETM selon le type de sol.

#### III.2. Etude minéralogique des deux types de sols

L'étude minéralogique des sols permet d'identifier les phases minérales potentiellement porteuses des ETM et/ou celles potentiellement réactives vis-à-vis des ETM en solution. Sur les Ferralsols développés sur substrats ultrabasiques (Partie I), les principaux minéraux porteurs d'ETM sont les oxydes de fer et de manganèse. L'analyse DRX (Figure 4-3) a permis de mettre en évidence des caractéristiques minéralogiques similaires sur les trois parcelles sylvicoles sur Ferralsol avec la présence majoritaire de goethite (FeOOH) et d'hématite ($\alpha$-Fe$_2$O$_3$). Cette composition minéralogique est caractéristique des Ferralsols développés sur des substrats ultrabasiques (Becquer et al. 2006; Fandeur et al. 2009b; Dublet
et al. 2012). Une légère différence est notée pour le Sol2 (bleu) et le Sol3 (rouge) dans lesquels du talc ($Mg_3Si_4O_{10}(OH)_2$) est détecté alors que ce minéral semble absent du Sol1 (noir). Cette différence de composition minéralogique entre les trois Ferralsols est confirmée par les concentrations légèrement plus élevées en magnésium et en silicium dans le Sol2 et le Sol3 par rapport au Sol1 (Table 4-1). Ceci suggère ainsi une composition minéralogique différente entre les trois sites d'études des Ferralsols dérivés des substrats ultrabasiques.

Le Fluvisol étudié dans la Partie II, est composé principalement de quartz ($SiO_2$) et d'albite ($NaAlSi_3O_6$). Cette composition minéralogique est représentative des sols développés sur d'alluvions volcanosédimentaires (Latham et al. 1978).

<figure><img src="image_71.png" /><figcaption>**Figure 4-3:** Analyse par diffraction aux rayons X des sols témoins sans fertilisation des Ferralsols développés sur substrats ultrabasiques, i.e. Sol1, Sol2 et Sol3 (Chapitre 4 Partie I) et du Fluvisol de la parcelle d'oranger, i.e. PO (Chapitre 4 Partie II)</figcaption></figure>

### III.3. Caractéristiques physico-chimiques des deux types de sol

Ainsi, les deux types de sols étudiés possèdent une minéralogie spécifique suggérant des caractéristiques physico-chimiques singulières (Tableau 4-3). Ainsi, les concentrations en ETM totaux étaient plus élevées dans les Ferralsols que les Fluvisols, notamment le chrome (x500), le nickel (x230), le fer (x100), le zinc (x3) et le manganèse (x2). En revanche, le Fluvisol possède un pH proche la neutralité (environ 7), une CEC (x10), des concentrations en phosphore (x10), en potassium (x10), en magnésium (x2) et en carbone organique dissous (x5), supérieures à celles mesurées dans les Ferralsols. Seules les concentrations en cuivre sont identiques pour les deux types de sols.
<table><caption>**Tableau 4-3:** Paramètres physico-chimiques du Ferralsol et du Fluvisol, i.e. pH, carbone (g.kg⁻¹), C/N, carbone organique dissous (COD) (mg.kg⁻¹), capacité d'échange cationique au pH du sol (CEC) (cmol(+).kg⁻¹); concentrations totales en phosphore (P), potassium (K), magnésium (Mg), fer (Fe) (g.kg⁻¹); manganèse (Mn), nickel (Ni) et chrome (Cr) (mg.kg⁻¹).</caption><thead><tr><th></th><th>Ferralsol</th><th>Fluvisol</th></tr></thead><tbody><tr><td>pH</td><td>5.2 ± 0.2</td><td>7.1 ± 0.2</td></tr><tr><td>C/N</td><td>15.0 ± 1.0</td><td>10.7 ± 0.8</td></tr><tr><td>COD (mg.kg<sup>-1</sup>)</td><td>39.7 ± 3.2</td><td>194.7 ± 9.0</td></tr><tr><td>CEC (cmol(+).kg<sup>-1</sup>)</td><td>2.2 ± 0.2</td><td>21.7 ± 0.6</td></tr><tr><td>P (g.kg<sup>-1</sup>)</td><td>0.10 ± 0.02</td><td>1.35 ± 0.21</td></tr><tr><td>K (g.kg<sup>-1</sup>)</td><td>0.05 ± 0.03</td><td>7.4 ± 0.7</td></tr><tr><td>Mg (g.kg<sup>-1</sup>)</td><td>2.99 ± 1.80</td><td>5.43 ± 0.22</td></tr><tr><td>Fe (g.kg<sup>-1</sup>)</td><td>494 ± 59</td><td>50 ± 2</td></tr><tr><td>Mn (mg.kg<sup>-1</sup>)</td><td>4 690 ± 152</td><td>2 726 ± 43</td></tr><tr><td>Ni (mg.kg<sup>-1</sup>)</td><td>4 076 ± 878</td><td>18 ± 6</td></tr><tr><td>Cr (mg.kg<sup>-1</sup>)</td><td>20 603 ± 1 864</td><td>42 ± 7</td></tr><tr><td>Cu (mg.kg<sup>-1</sup>)</td><td>33 ± 5</td><td>28 ± 3</td></tr><tr><td>Zn (mg.kg<sup>-1</sup>)</td><td>319 ± 44</td><td>116 ± 7</td></tr></tbody></table>

L'ACP a permis de mettre en avant la contribution de ces paramètres physico-chimiques sur les réservoirs extractibles EDTA, i.e., Q₁, Q₂ et Q₁+Q₂ des ETM (Figure 4-4). Les deux premières dimensions de l'ACP représentent 75% de la variance de l'ensemble des variables (47% pour la dimension 1 et 28% pour la dimension 2) (Figure 4-4 A, B). Les variables qui contribuent et corrèlent positivement à l'Axe 1 correspondent à la capacité d'échange cationique, le pH, les concentrations totales en potassium, les réservoirs extractibles EDTA du cuivre. Les autres variables qui contribuent et corrèlent négativement à l'Axe 1 correspondent aux concentrations totales en fer, chrome, zinc et nickel (Figure 4-4 A, C). Ces variables correspondent aux paramètres physico-chimiques caractéristiques du type de sol. Les variables qui contribuent et corrèlent positivement à l'Axe 2 correspondent à la teneur en matière organique et les réservoirs extractibles EDTA du zinc, du nickel et du chrome (Figure 4-4 B, C). Ces variables correspondent à l'apport de compost dans les sols. Enfin, la projection des individus sur les deux premières dimensions de l'ACP (Figure 4-4 D), indique que le Ferralsol est caractérisé par les concentrations totales en fer, chrome, nickel, zinc et manganèse, alors que le Fluvisol est décrit par le pH, la capacité d'échange cationique et les concentrations en potassium. Lorsque ces deux types de sols sont amendés, ils se déplacent vers les composantes des réservoirs extractibles EDTA des ETM, i.e., nickel et chrome pour le Ferralsol et cuivre et zinc pour le Fluvisol. Pour les deux types de sols, il existe
une distinction marquée entre le témoin sans fertilisation et le sol amendé avec du compost. Ce résultat est similaire à celui de Laurent et al. (2020) qui ont étudié l'effet de l'apport de composts et de fertilisation minérale sur trois sols différents à la Réunion. Ces auteurs ont montré à l'aide d'ACP que le type de sol et de fertilisation influencent la solubilisation du cuivre et du zinc dans les sols. L'ACP permet ainsi de mettre en évidence, d'une part le rôle de la nature du sol (dimension 1) mais également de l'apport de compost (dimension 2) sur les réservoirs extractibles des ETM.

<figure><img src="image_72.png" /><figcaption>(B) Contribution of variables to Dim-2</figcaption></figure>

<figure><img src="image_73.png" /></figure>

Fig

<figure><img src="image_74.png" /><figcaption>(D) Individuals - PCA</figcaption></figure>

<figure><img src="image_75.png" /></figure>

**Figure 4-4:** Analyse en Composantes Principales (ACP) réalisée sur les Ferralsols et le Fluvisol avant et après amendement. Les paramètres physico-chimiques pris en compte dans l'ACP sont : pH, C/N, carbone organique dissous (DOC, mg.kg⁻¹), matière organique (OM, g.100g⁻¹), capacité d'échange cationique (CEC, cmol(+).kg⁻¹), concentrations totales (mg.kg⁻¹) : phosphore (P), potassium (K), magnésium (Mg), manganèse (Mn), fer (Fe), nickel (Ni), chrome (Cr), cuivre (Cu), et zinc (Zn) et réservoirs extractibles EDTA (Q₁, Q₂ et Q₁+Q₂ en mg.kg⁻¹) du nickel, du chrome, du cuivre et du zinc
### III.4. Conclusion

Les résultats obtenus mettent en évidence le rôle de la nature du sol et de l'épandage de compost sur la définition des réservoirs extractibles en chrome, nickel, cuivre et zinc. Si l'apport de compost tend à améliorer la fertilité chimique des sols (pH, CEC, concentrations en cations majeurs), il tend également à augmenter les réservoirs potentiellement extractibles en ETM, soit en mobilisant les ETM du compost et/ou en mobilisant les ETM du sol. Les deux sols étudiés présentent des caractéristiques physico-chimiques très distinctes notamment en concentrations totales d'ETM. Ces deux sols représentent ainsi probablement deux extrêmes pédologiques de Nouvelle-Calédonie. Néanmoins, la diversité pédologique et agronomique sur le territoire est importante. Il serait ainsi nécessaire d'appréhender la réponse de l'extraction des ETM aux apports de composts pour différentes conditions agro-pédoclimatiques.
IV. Conclusions et perspectives du chapitre

Dans ce chapitre, nous avons mis en évidence l'influence de l'apport de compost riche en nickel et en chrome géogènes sur deux types de sols contrastés. Malgré des caractéristiques physico-chimiques très contrastés des deux sols amendés, les fractions d'ETM extractibles à l'EDTA ont fortement augmenté après apport de compost quel que soit le sol considéré. Néanmoins, deux mécanismes distincts seraient à l'origine de cette mobilisation.

Pour les Ferralsols dérivés des substrats ultrabasiques, utilisés en sylviculture, les fractions extractibles à l'EDTA étaient multipliées par plus de 100 pour le nickel, 16 pour le zinc, 6 pour le chrome et 4 pour le cuivre, dans les sols amendés en comparaison aux sols témoins sans fertilisation. Si les fractions extractibles à l'EDTA du cuivre et du zinc étaient imputables à l'apport de compost, celles du nickel et du chrome ont été attribuées à une mobilisation ces ETM du sol, probablement en raison d'une altération physico-chimique et/ou biologique des phases minérales porteuses de ces ETM. Ainsi, l'augmentation des réservoirs extractibles des quatre ETM étudiés, particulièrement nickel et chrome, suggèrent un risque environnemental et sanitaire non négligeable pour l'utilisation de ce compost sur les Ferralsols. La sylviculture, implantée à plus de 50% sur des Ferralsols dérivés de substrats ultrabasiques en Nouvelle-Calédonie, apparaît de prime abord comme une filière judicieuse pour valoriser ce compost étant donné l'absence de contamination directe vis-à-vis d'une production alimentaire. Cependant, les modifications de la fertilité chimique et biologique de ces sols en réponse à l'apport de compost, et de la modification induite des réservoirs extractibles des ETM font apparaître la nécessité de suivis à plus long terme de l'impact des épandages de compost et de leur fréquence sur la dynamique du nickel et du chrome. Est notamment questionnable, la persistance d'un effet compost sur l'activité biologique et les réservoirs extractibles des ETM entre deux apports.

Dans le cas du Fluvisol, présentant un fond pédo-géochimique naturel faiblement concentré en ETM, conduit à une contamination anthropique par des effets de concentrations. Ainsi, après seulement deux apports, les concentrations totales en nickel et en chrome ont significativement augmenté de plus de 3 et 4 fois, respectivement. Ce premier résultat soulève la question de l'accumulation des ETM, éléments non biodégradables, dans les sols amendés et de leur devenir sur un plus long terme. D'autre part, nous avons montré que les fractions extractibles EDTA du nickel, du chrome, du cuivre et du zinc augmentaient significativement dans les sols amendés. Les fractions mobilisables du cuivre et du zinc seraient plutôt liées au réservoir rapidement extractible EDTA (Q₁) suggérant un risque à court terme. Quant au nickel et au chrome, les fractions mobilisables seraient liées au réservoir moins rapidement extractible EDTA (Q₂) traduisant un risque à plus long terme. Cette différence de comportement entre le cuivre et le zinc, d'une part, et le nickel et le chrome, d'autre part, est liée à l'origine des ETM,
anthropique vs géogène, tel qu'indiqué dans le **Chapitre 3**. Cependant, les résultats suggèrent également une mobilisation du chrome, et dans une moindre mesure du nickel probablement issu du compost. Des effets de minéralisation du compost après épandage, en lien avec l'activité biologique pourraient conduire à la mobilisation d'une fraction de ces ETM qui n'était initialement pas mobilisable. Les résultats soulignent la nécessité d'évaluer l'évolution des différents réservoirs extractibles ; et suggèrent qu'il serait intéressant d'étudier la spéciation des ETM dans les sols suite aux apports de composts, afin de valider les hypothèses énoncées concernant le rôle des phases porteuses des ETM pour les deux types de sol.

D'autre part, et bien que minimes, nous avons également mis en évidence l'augmentation des concentrations en Cr(VI) avec l'apport répété de compost. Ces résultats préliminaires devraient être confirmés. En effet, le chrome hexavalent, mis en mis en solution pourrait être facilement assimilé par les plantes cultivées ou être lixivié le long du profil de sol. Il pourrait à terme représenter un risque agronomique (phytotoxicité) ou sanitaire (contamination des captages d'eau) qu'il conviendrait de mieux évaluer.

Il est important de noter que les résultats décrits représentent une évaluation préliminaire du rôle de l'apport de compost riche en Ni- et en Cr- géogènes sur l'évolution des réservoirs extractibles en ETM. Néanmoins, les extractions à l'EDTA représentent la totalité extractible des ETM et donc une borne haute des ETM susceptibles d'être présents en solution. De nombreux paramètres biogéochimiques contrôlent néanmoins le partitionnement solide : liquide des ETM. Aussi, il est possible que les concentrations en solution ne soient pas directement et simplement proportionnelles aux réservoirs extractibles. Il conviendra ainsi d'évaluer les mécanismes de solubilisation des ETM à partir de ces réservoirs extractibles.

Du point de vue des perspectives agronomiques, les résultats obtenus soulignent l'importance d'études complémentaires sur le moyen et long termes, destinées à mieux estimer le réel risque environnemental et sanitaire de transfert dans le système sol-eau-plante des formes solubles des ETM. La comparaison effectuée entre les paramètres physico-chimiques des Ferralsols et Fluvisol et l'évolution des ETM sur le long terme, met en avant des mécanismes de mobilisation des ETM en fonction du sol considéré. Les sols étudiés présentent des caractéristiques physico-chimiques très contrastées qui soulignent la diversité de la couverture pédologique de la Nouvelle-Calédonie. De ce point de vue, ils constituent une base intéressante pour prédire la mobilité des ETM suite à l'apport de compost riche en nickel et en chrome, qui est l'une des spécificités du territoire. Cependant, il apparaît nécessaire d'étudier les conséquences de l'utilisation d'un tel compost sur la mobilisation des ETM à long terme dans d'autres conditions agro-pédo-climatiques.
Les effets à long terme des apports de composts sur la biogéochimie des ETM restent relativement méconnus dans la littérature et encore plus particulièrement pour les modèles agronomiques tropicaux avec des spécificités pédologiques et climatiques. Alors même que les effets attendus d'un retour au sol du compost dépendent de la nature du compost et des contextes pédo-géologiques, il apparaît nécessaire d'approfondir ces connaissances. En France, l'observatoire de recherche en environnement étudiant le recyclage agricole des Produits Résiduaires Organiques (PRO), est un réseau de sites expérimentaux au champ de longue durée étudiant le recyclage agricole des PRO. Ce réseau, nommé SOERE PRO, a été créé au début des années 2000 par l'INRAE. Il vise à mettre en place des essais terrains sur des suivis sur plus de 20 ans pour mutualiser les données et évaluer l'effet agronomique et les potentiels risques du recyclage des produits résiduaires organiques par un retour au sol. Ceci permet alors d'appréhender au mieux les risques environnementaux et sanitaires liés à leur utilisation en agriculture. Cinq sites expérimentaux sont instrumentés selon différents contextes agro-pédo-climatiques : en France sur des grandes cultures (blé, maïs, colza, orge, betterave sucrière) en comparant des produits compostés, différents composts urbains, des fumiers de bovins et des effluents d'élevage ; à la Réunion sur la canne à sucre en comparant des effluents d'élevage et des déchets organiques urbains ; au Sénégal en maraîchage en comparant des fumiers de volaille, des boues urbaines et des digestats.
## V. Synthèse graphique du chapitre 4

<figure><img src="image_76.png" /></figure>

Risque environnemental : à court-terme pour Cu & Zn anthropiques ; à long-terme pour Ni & Cr géogènes
## VI. Références

Achiba W Ben, Lakhdar A, Gabteni N, et al (2010) Accumulation and fractionation of trace metals in a Tunisian calcareous soil amended with farmyard manure and municipal solid waste compost. *J Hazard Mater* 176:99-108. https://doi.org/10.1016/j.jhazmat.2009.11.004

ADEME (2017) Phytodisponibilité des ETM Pour les plantes potagères et extrapolations dans la quantification de l'exposition des consommateurs

Amery F, Degryse F, Van Moorleghem C, et al (2010) The dissociation kinetics of Cu-dissolved organic matter complexes from soil and soil amendments. *Anal Chim Acta* 670:24-32. https://doi.org/10.1016/j.aca.2010.04.047

Amir H, Pineau R (2003) Release of Ni and Co by microbial activity in New Caledonian ultramafic soils. *Can J Microbiol* 49:288-293. https://doi.org/10.1139/w03-039

Antoniadis V, Alloway BJ (2002) The role of dissolved organic carbon in the mobility of Cd, Ni and Zn in sewage sludge-amended soils. *Environ Pollut* 117:515-521. https://doi.org/10.1016/S0269-7491(01)00172-5

Avadí A, Benoit P, Bravin MN, et al (2022) Trace contaminants in the environmental assessment of organic waste recycling in agriculture: Gaps between methods and knowledge. *Adv Agron* 174:53-188. https://doi.org/10.1016/bs.agron.2022.03.002

Balabane M, Van Oort F (2002) Metal enrichment of particulate organic matter in arable soils with low metal contamination. *Soil Biol Biochem* 34:1513-1516. https://doi.org/10.1016/S0038-0717(02)00066-4

Bartlett R, James B (1979) Behavior of Chromium in Soils: III. Oxidation. *J Environ Qual* 8:31-35. https://doi.org/10.2134/jeq1979.00472425000800010008x

Becquer T, Quantin C, Boudot JP (2010) Toxic levels of metals in Ferralsols under natural vegetation and crops in New Caledonia. *Eur J Soil Sci* 61:994-1004. https://doi.org/10.1111/j.1365-2389.2010.01294.x

Becquer T, Quantin C, Rotte-Capet S, et al (2006) Sources of trace metals in Ferralsols in New Caledonia. *Eur J Soil Sci* 200-213. https://doi.org/10.1111/j.1365-2389.2005.00730.x

Becquer T, Quantin C, Sicot M, Boudot JP (2003) Chromium availability in ultramafic soils from New Caledonia. *Sci Total Environ* 301:251-261. https://doi.org/10.1016/S0048-9697(02)00298-X

Belon E, Boisson M, Deportes IZ, et al (2012) An inventory of trace elements inputs to French agricultural soils. *Sci Total Environ* 439:87-95. https://doi.org/10.1016/j.scitotenv.2012.09.011

Bermond A, Varrault G, Sappin-Didier V, Mench M (2005) A kinetic approach to predict soil trace metal bioavailability: preliminary results. *Plant Soil* 275:21-29. https://doi.org/10.1007/s11104-004-7599-1

Bolaños-Benítez V, van Hullebusch ED, Lens PNL, et al (2018) (Bio)leaching behavior of chromite tailings. *Minerals* 8: https://doi.org/10.3390/min8060261

Borch T, Kretzschmar R, Skappler A, et al (2010) Biogeochemical redox processes and their impact on contaminant dynamics. *Environ Sci Technol* 44:15-23. https://doi.org/10.1021/es9026248

Cambier P, Michaud A, Paradelo R, et al (2019) Trace metal availability in soil horizons amended with various urban waste composts during 17 years – Monitoring and modelling. *Sci Total Environ* 651:2961-2974. https://doi.org/10.1016/j.scitotenv.2018.10.013

Cerdeira-Pérez A, Monterroso C, Rodríguez-Garrido B, et al (2019) Implementing nickel phytomining in
a serpentine quarry in NW Spain. J Geochemical Explor 197:1-13.
https://doi.org/10.1016/j.gexplo.2018.11.001

Chen H, Dou J, Xu H (2017) Removal of Cr(VI) ions by sewage sludge compost biomass from aqueous solutions: Reduction to Cr(III) and biosorption. Appl Surf Sci 425:728-735.
https://doi.org/10.1016/j.apsusc.2017.07.053

Cheng J, Ding C, Li X, et al (2015) Heavy metals in navel orange orchards of Xinfeng County and their transfer from soils to navel oranges. Ecotoxicol Environ Saf 122:153-158.
https://doi.org/10.1016/j.ecoenv.2015.07.022

Ciavatta C, Antisari LV, Sequi P (1989) Determination of organic carbon in soils and fertilizers. Commun Soil Sci Plant Anal 20:759-773. https://doi.org/10.1080/00103628909368115

Clemente R, Escolar Á, Bernal MP (2006) Heavy metals fractionation and organic matter mineralisation in contaminated calcareous soil amended with organic materials. Bioresour Technol 97:1894-1901. https://doi.org/10.1016/j.biortech.2005.08.018

DAVAR (2010) Inventaire 2010 du verger calédonien et évaluation de la consommation de fruits frais

Døelsch E, Deroche B, Van de Kerchove V (2006) Impact of sewage sludge spreading on heavy metal speciation in tropical soils (Réunion, Indian Ocean). Chemosphere 65:286-293.
https://doi.org/10.1016/j.chemosphere.2006.02.046

Doelsch E, Masion A, Moussard G, et al (2010) Impact of pig slurry and green waste compost application on heavy metal exchangeable fractions in tropical soils. Geoderma 155:390-400.
https://doi.org/10.1016/j.geoderma.2009.12.024

Dublet G, Juillot F, Morin G, et al (2012) Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation. Geochim Cosmochim Acta 95:119-133.
https://doi.org/10.1016/j.gca.2012.07.030

Fandeur D, Juillot F, Morin G, et al (2009a) XANES evidence for oxidation of Cr(III) to Cr(VI) by Mn-oxides in a lateritic regolith developed on serpentinized ultramafic rocks of New Caledonia. Environ Sci Technol 43:7384-7390. https://doi.org/10.1021/es900498r

Fandeur D, Juillot F, Morin G, et al (2009b) Synchrotron-based speciation of chromium in an oxisol from new caledonia: Importance of secondary fe-oxyhydroxides. Am Mineral 94:710-719.
https://doi.org/10.2138/am.2009.3073

Fangueiro D, Bermond A, Santos E, et al (2005) Kinetic approach to heavy metal mobilization assessment in sediments: Choose of kinetic equations and models to achieve maximum information. Talanta 66:844-857. https://doi.org/10.1016/j.talanta.2004.12.036

Farrell M, Perkins WT, Hobbs PJ, et al (2010) Migration of heavy metals in soil as influenced by compost amendments. Environ Pollut 158:55-64. https://doi.org/10.1016/j.envpol.2009.08.027

Fischer L, Brümmer GW, Barrow NJ (2007) Observations and modelling of the reactions of 10 metals with goethite: Adsorption and diffusion processes. Eur J Soil Sci 58:1304-1315.
https://doi.org/10.1111/j.1365-2389.2007.00924.x

Garnier J, Quantin C, Echevarria G, Becquer T (2009) Assessing chromate availability in tropical ultramafic soils using isotopic exchange kinetics. J Soils Sediments 9:468-475.
https://doi.org/10.1007/s11368-009-0062-4

Garnier J, Quantin C, Martins ES, Becquer T (2006) Solid speciation and availability of chromium in ultramafic soils from Niquelândia, Brazil. J Geochemical Explor 88:206-209.
https://doi.org/10.1016/j.gexplo.2005.08.040
Gori A, Ferrini F, Fini A (2019) Reprint of: Growing healthy food under heavy metal pollution load: Overview and major challenges of tree based edible landscapes. Urban For Urban Green 45:126292. https://doi.org/10.1016/j.ufug.2019.02.009

Gunkel-Grillon P, Laporte-Magoni C, Lemestre M, Bazire N (2014) Toxic chromium release from nickel mining sediments in surface waters, New Caledonia. Environ Chem Lett 12:511–516. https://doi.org/10.1007/s10311-014-0475-1

Gustafsson JP, Persson I, Oromieh AG, et al (2014) Chromium(III) complexation to natural organic matter: Mechanisms and modeling. Environ Sci Technol 48:1753–1761. https://doi.org/10.1021/es404557e

Hargreaves JC, Adl MS, Warman PR (2008) A review of the use of composted municipal solid waste in agriculture. Agric Ecosyst Environ 123:1–14. https://doi.org/10.1016/j.agee.2007.07.004

Hernandez-Soriano MC, Jimenez-Lopez JC (2012) Effects of soil water content and organic matter addition on the speciation and bioavailability of heavy metals. Sci Total Environ 423:55–61. https://doi.org/10.1016/j.scitotenv.2012.02.033

Houles A, Vincent B, David M, et al (2018) Ectomycorrhizal Communities Associated with the Legume Acacia spirorbis Growing on Contrasted Edaphic Constraints in New Caledonia. Microb Ecol 76:964–975. https://doi.org/10.1007/s00248-018-1193-1

Houot S, Cambier P, Benoit P, et al (2009) Effet d'apports de composts sur la disponibilité de micropolluants métalliques et organiques dans un sol cultivé. Etude Gest des Sols 16:255–274

Houot S, Pons M-N, Pradel M, et al (2014) Valorisation des matières fertilisantes d'origine résiduaire sur les sols à usage agricole ou forestier

Kaninga B, Chishala BH, Maseka KK, et al (2020) Do soil amendments used to improve agricultural productivity have consequences for soils contaminated with heavy metals? Heliyon 6:e05502. https://doi.org/10.1016/j.heliyon.2020.e05502

Kaza S, Yao LC, Bhada-Tata P, Van Woerden F (2018) What a Waste 2.0 : A Global Snapshot of Solid Waste Management to 2050., Urban Development. World Bank., Washington DC

Kierczak J, Pietranik A, Pędziwiatr A (2021) Ultramafic geoecosystems as a natural source of Ni, Cr, and Co to the environment: A review. Sci Total Environ 755: https://doi.org/10.1016/j.scitotenv.2020.142620

Labanowski J, Monna F, Bermond A, et al (2008) Kinetic extractions to assess mobilization of Zn, Pb, Cu, and Cd in a metal-contaminated soil: EDTA vs. citrate. Environ Pollut 152:693–701. https://doi.org/10.1016/j.envpol.2007.06.054ï

Latham M, Quantin P, Aubert G (1978) Étude des sols de la Nouvelle-Calédonie Nouvel essai sur la classification, la caractérisation, la pédogénèse et les aptitudes des sols de Nouvelle-Calédonie. Paris

Laurent C, Bravin MN, Crouzet O, et al (2020) Increased soil pH and dissolved organic matter after a decade of organic fertilizer application mitigates copper and zinc availability despite contamination. Sci Total Environ 709:135927. https://doi.org/10.1016/j.scitotenv.2019.135927

Legros S, Doelsch E, Feder F, et al (2013) Fate and behaviour of Cu and Zn from pig slurry spreading in a tropical water-soil-plant system. Agric Ecosyst Environ 164:70–79. https://doi.org/10.1016/j.agee.2012.09.008

Legros S, Levard C, Marcato-Romain C-E, et al (2017) Anaerobic Digestion Alters Copper and Zinc Speciation. Environ Sci Technol 51:10326–10334. https://doi.org/10.1021/acs.est.7b01662
Löv Å, Sjöstedt C, Larsbo M, et al (2017) Solubility and transport of Cr(III) in a historically contaminated soil — Evidence of a rapidly reacting dimeric Cr(III) organic matter complex. Chemosphere 189:709–716. https://doi.org/10.1016/j.chemosphere.2017.09.088

Lwin CS, Seo BH, Kim HU, et al (2018) Application of soil amendments to contaminated soils for heavy metal immobilization and improved soil quality—a critical review. Soil Sci Plant Nutr 64:156–167. https://doi.org/10.1080/00380768.2018.1440938

Manceau A, Matynia A (2010) The nature of Cu bonding to natural organic matter. Geochim Cosmochim Acta 74:2556–2580. https://doi.org/10.1016/j.gca.2010.01.027

Maturi KC, Banerjee A, Kalamdhad AS (2021) Assessing mobility and chemical speciation of heavy metals during rotary drum composting of Ageratum conyzoides. Environ Technol Innov 24:. https://doi.org/10.1016/j.eti.2021.101871

Paradelo R, Villada A, Barral MT (2018) Chemical Fractionation of Trace Elements in a Metal-Rich Amphibolite Soil Amended with Municipal Solid Waste Composts. Waste and Biomass Valorization 9:1935–1943. https://doi.org/10.1007/s12649-017-9940-y

Pasquet C, Monna F, van Oort F, et al (2018) Mobility of Ni, Co, and Mn in ultramafic mining soils of New Caledonia, assessed by kinetic EDTA extractions. Environ Monit Assess 190:. https://doi.org/10.1007/s10661-018-7029-0

Payet N, van de Kerchove V (2007) Composts de déchets verts urbains réunionnais : recherche de la source d'ETM

Pribyl DW (2010) A critical review of the conventional SOC to SOM conversion factor. Geoderma 156:75–83. https://doi.org/10.1016/j.geoderma.2010.02.003

Quantin C, Becquer T, Rouiller JH, Berthelin J (2002) Redistribution of metals in a New Caledonia Ferralsol after microbial weathering. Soil Sci Soc Am J 66:1797–1804. https://doi.org/10.2136/sssaj2002.1797

Quevauviller P, Lachica M, Barahona E, et al (1996) Interlaboratory comparison of EDTA and DTPA procedures prior to certification of extractable trace elements in calcareous soil. Sci Total Environ 178:127–132. https://doi.org/10.1016/0048-9697(95)04804-9

Roba C, Roșu C, Piștea I, et al (2016) Heavy metal content in vegetables and fruits cultivated in Baia Mare mining area (Romania) and health risk assessment. Environ Sci Pollut Res 23:6062–6073. https://doi.org/10.1007/s11356-015-4799-6

Rodríguez-Espinosa T, Navarro-Pedreño J, Gómez Lucas I, et al (2023) Soluble Elements Released from Organic Wastes to Increase Available Nutrients for Soil and Crops. Appl Sci 13:. https://doi.org/10.3390/app13021151

Santos S, Costa CAE, Duarte AC, et al (2010) Influence of different organic amendments on the potential availability of metals from soil: A study on metal fractionation and extraction kinetics by EDTA. Chemosphere 78:389–396. https://doi.org/10.1016/j.chemosphere.2009.11.008

Sharma B, Vaish B, Monika, et al (2019) Recycling of Organic Wastes in Agriculture: An Environmental Perspective. Int J Environ Res 13:409–429. https://doi.org/10.1007/s41742-019-00175-y

Smith SR (2009) A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge. Environ Int 35:142–156. https://doi.org/10.1016/j.envint.2008.06.009

Sun SS, Ao M, Geng KR, et al (2022) Enrichment and speciation of chromium during basalt weathering: Insights from variably weathered profiles in the Leizhou Peninsula, South China. Sci Total Environ
822:153304. https://doi.org/10.1016/j.scitotenv.2022.153304

Tella M, Doelsch E, Letourmy P, et al (2013) Investigation of potentially toxic heavy metals in different organic wastes used to fertilize market garden crops. Waste Manag 33:184-192.
https://doi.org/10.1016/j.wasman.2012.07.021

Topcuoğlu B (2005) Effects of repeated applications of sewage sludge and MSW compost on the bioavailability of heavy metals in greenhouse soil. Polish J Environ Stud 14:217-222

Vincent B, Jourand P, Juillot F, et al (2018) Biological in situ nitrogen fixation by an Acacia species reaches optimal rates on extremely contrasted soils. Eur J Soil Biol 86:52-62.
https://doi.org/10.1016/j.ejsobi.2018.03.003

Walter I, Martínez F, Alonso L, et al (2002) Extractable soil heavy metals following the cessation of biosolids application to agricultural soil. Environ Pollut 117:315-321.
https://doi.org/10.1016/S0269-7491(01)00181-6

Wassenaar T, Doelsch E, Feder F, et al (2014) Returning Organic Residues to Agricultural Land (RORAL) - Fuelling the Follow-the-Technology approach. Agric Syst 124:60-69.
https://doi.org/10.1016/j.agsy.2013.10.007

Xu S, Li L, Zhan J, Guo X (2022) Variation and factors on heavy metal speciation during co-composting of rural sewage sludge and typical rural organic solid waste. J Environ Manage 306:.
https://doi.org/10.1016/j.jenvman.2021.114418

Zhang M, Heaney D, Henriquez B, et al (2006) A four-year study on influence of biosolids/msw cocompost application in less productive soils in alberta: Nutrient dynamics. Compost Sci Util 14:68-80.
https://doi.org/10.1080/1065657X.2006.10702265
# Chapitre 5 ÉVOLUTION DE LA MOBILITÉ DU NICKEL, CHROME, CUIVRE ET ZINC APRÈS L'ÉPANDAGE D'UN COMPOST RICHE EN NICKEL ET EN CHROME EN AGRUMICULTURE
Le chapitre précédent a mis en évidence l'augmentation de la mobilisation des ETM, notamment du nickel et du chrome géogènes, suite à l'apport de compost sur deux sols contrastés. Le suivi sur Fluvisol échantillonné sur une parcelle en agrumiculture (Washington Navel), a été réalisé sur les deux premières années du cycle de culture. Nous avons mis en évidence une augmentation significative des concentrations totales en nickel et en chrome dans les sols amendés, en lien avec l'apport de compost riche en nickel et en chrome géogènes. En parallèle, nous avons également montré que les réservoirs extractibles à l'EDTA du nickel, chrome, cuivre et zinc augmentaient dans les sols amendés par le compost. Nous nous intéressons dans ce chapitre à l'évolution des concentrations extractibles au CaCl₂ et à l'EDTA dans les sols ainsi que la concentration en ETM mesurée dans les feuilles et les fruits selon différentes pratiques de fertilisation et de gestion des lignes de plantation afin d'appréhender le risque de disponibilité environnementale des ETM sur des parcelles agricoles à visée alimentaire sur le long terme.

Les objectifs du chapitre 5 consistent en l'étude du devenir du nickel, chrome, cuivre et zinc en condition de plein champ dans le système sol-plante d'un verger d'orangers. L'étude a été réalisée sur deux cycles de cultures en tenant compte des pratiques de fertilisation, notamment des apports de compost riche en nickel et en chrome, et des pratiques de gestion des lignes de plantations sur billon.

Ce chapitre se divise en trois parties qui visent à :

(I) Évaluer par des extractions chimiques, les fractions extractibles CaCl₂ et EDTA des ETM sur les différentes modalités de la parcelle, i.e. témoin sans fertilisation, témoin positif avec une fertilisation minérale, la dose 1 de compost respectant les flux annuels et sur 10 ans de nickel et de chrome autorisés selon la norme française NFU 44-095, et la dose 2 basée sur les besoins agronomiques de la plante. Ces modalités de fertilisation sont couplées à des pratiques de gestion des lignes de plantation : paillage et plantes de couverture. Cette partie est présentée sous la forme d'un article "Impact of Ni- and Cr-rich compost and soil management on CaCl₂ and EDTA metallic elements: a two years case study in a Navel orchard".

(II) Évaluer les flux d'ETM apportés au sein des sols de la parcelle d'oranger selon les différentes pratiques de fertilisation sur deux ans afin d'évaluer l'évolution potentielle des concentrations en ETM dans le sol suite à des apports répétés de compost tout au long du cycle de vie du verger.
Chapitre 5

EVOLUTION DE LA MOBILITE DU NICKEL, CHROME, CUIVRE ET ZINC

APRES L'EPANDAGE D'UN COMPOST RICHE EN NICKEL ET EN CHROME EN AGRUMICULTURE

(III) Évaluer les transferts des ETM au sein du système sol-plante en étudiant les concentrations en ETM contenues dans les feuilles et les oranges du verger, et en les confrontant aux concentrations en ETM des sols. Le risque sanitaire vis-à-vis de la consommation des oranges produites sur la parcelle en fonction des modalités est également discuté. Cette partie est rédigée sous la forme d'un article : "Soil management of line plantation greater impact on trace metal transfer than fertilization in an Navel orchard (New Caledonia)".

Remarque- Les parties I et III sont rédigées en anglais et la partie II en français.
# I. Impact of Ni- and Cr-rich compost and soil management on CaCl2- and EDTA-extractable trace metals: a two years case study in a Navel orchard
## I.1. Introduction

Worldwide, municipal solid waste production represents two billion tons per year and is increasing because of demographic growth (Kaza et al., 2018). Most of the municipal wastes are disposed in landfill (37%) or openly dumped (33%) and less than a quarter is recycled and composted (19%) or incinerated (11%) (Kaza et al., 2018). Since up to 50% of municipal solid wastes are organic, agronomical recycling offers a great and cheap solution to improve soil-organic matter content and enhance physical properties and microorganism activity in cultivated soils (Siedt et al., 2021). Composting sewage sludge, with green waste is a judicious way for recycling sewage sludge before spreading on soils (Smith 2009; Rehana et al. 2020). During this process, organic matter is mineralized and stabilized through intensive microbial activity, pathogen load representing sanitary and environmental risks tends to decrease, but non-biodegradable elements such as trace metals tend to concentrate (Greenway and Song 2002; Farrell and Jones 2009; Zhan et al. 2022). As a consequence, compost inputs in the soil are regulated by standards (NFU 44-051 France; AS445A Australia) which allow a maximal threshold trace metal concentration in organic amendments and/or maximal annual fluxes of contaminants. Copper (Cu) and zinc (Zn) are largely present in compost with sewage sludge in many countries (Lopes et al. 2011; Tella et al. 2013; Tella et al. 2016) and represent problematic contaminants because of their increase concentration after compost applications and their potential eco-toxicity (Benoît et al., 2014; Smith, 2009; Yoshida et al. 2018). However, geogenic trace metals, i.e., Ni or Cr, less present in compost can also accumulated in the amended soils depending on organic waste applied and on their concentration (Klein et al., 2023a; Tella, 2013).

When compost is spread on soil, it can be considered that the balance of fluxes through the volume of amended soil determines the variation of stock of each trace metal in this volume (Doelsch et al., 2016). Some studies showed that compost inputs could increase mobile and available forms of trace metals in the soils (Cornu et al., 2001; Hargreaves et al., 2008; Smith, 2009). In the review of Hargreaves et al. (2008) on municipal sewage sludge compost, several studies reported an increase of DPTA-extractable available forms of Pb, Cu, Mn, and Zn in amended soils. Interestingly Cambier et al. (2019) demonstrated that soil stocks of potentially CaCl2-extractable forms of Cu increased faster than from the inputs of EDTA-extractable Cu contained in composts which could be explained by the link of Cu with organic particles, protecting this metal from extraction. In parallel, Klein et al., (2023b) also showed through a mass balance with EDTA kinetic extractions, a significant increase of both Ni- and Cr-EDTA-extractable available forms in amended soils compared to EDTA-extractable trace metals from
compost or soil alone because of the potential reduction of mineral-bearing phases such as manganese oxides.

The monitoring and predicting of the extractable trace metals in soils is highly influenced by the chemical method used. Two levels of extractable forms of trace metals can be generally defined: (1) the readily available fraction usually estimated by using non-complexing neutral saline solution, i.e., $CaCl_2$; (2) the mobilizable fraction estimated by chelating agent, i.e., EDTA or DPTA (Cambier et al., 2019; Degryse et al., 2006). First fraction is often found to better predict plant uptake and the second one is more meaningful in soil-solution interaction (Degryse et al. 2003, 2009; Harmsen 2007; Rodrigues et al. 2010). Thus, Cambier et al. (2019) recently indicated prediction equations of $CaCl_2$ concentrations of Cu, Zn and Cd from EDTA concentrations.

In a sustainable development point of view and considering the needs of other nutrient sources than mineral fertilizers (Cordell et al. 2009), compost could be used as a cheap solution for fertilization of crops. Indeed, it has been successfully used in perennial orchards for replacing mineral fertilization (Baldi et al. 2010; Toselli et al. 2019). It could be associated with different agroecological practices to ensure orchard productivity as straw mulch or leguminous cover crop on plantation lines. However, there is a lack of knowledges on the impact of soil management on trace metal dynamics contained in the compost after its spreading in the soil. Soil management practices are expected to modify geochemical parameters that in turn may impact on the trace metal solubilization such as the biological activity, the nutrient content, the redox potential, the particulate and the dissolved organic matter content, or the pH. All those geochemical parameters are known to impact trace metals availability in the soil (Smith, 2009; Violante et al., 2010; Weng et al., 2002). Fu et al. (2021) showed modification of chemical composition of soil and subsequently crop because of cover cropping. Moreover, Saad et al., (2018) showed the phytoremediation capacity of cover crop to uptake and to accumulate Ni in the crop system which suggest that cover crop could enhance trace metal translocation on the companion plant due to the improvement of soil characteristics. In parallel, Su et al. (2021) demonstrated an increase in available forms of trace metals due to the creation of organo-complex dissolved organic carbon with trace metals, which allowed an easier bioaccumulation of trace metals in the plants following straw mulching.

In New Caledonia, because of geogenic contamination, composts exhibit especially high Ni and Cr concentrations, i.e., more than 300 and 1 000 mg kg⁻¹ respectively, exceeding up to 10-fold the maximum concentration defined in the French regulation (Klein et al., 2023). The valorization of local compost could be a cheap solution for New Caledonia to increase its dependence from imports and its
food security. These composts could be valorized for fruit production, especially in orange production, the highest fruit marketed production in New Caledonia with 34 500 orange trees and 1 500 tons of orange (DAVAR 2010, 2021). Fruit production could be a target for these composts, because fruits are known to generally less accumulated trace metals than leafy or root vegetables (Cheng et al. 2015; Roba et al. 2016; Gori et al. 2019). However, at both worldwide and local scales, nothing is known about the fate of geogenic Ni and Cr following compost spreading, especially considering agricultural soils with low pedogeochemical trace metal background and different soil management of plantation lines.

This study aimed to understand the fate of trace metals in soils regarding the fertilization, notably compost dose input, and the soil management on a 2-years field experiment. We studied CaCl₂- and EDTA-extractable trace metals, both geogenic Ni and Cr and anthropogenic Cu and Zn. We hypothesized that total and extractable EDTA concentrations in soils increase relative to compost inputs. In contrast, extractable CaCl₂ concentrations could be buffered by soil geochemical modifications induced by compost inputs. Soil management by modifying soil chemical parameters, influences availability of trace metals with an increase of CaCl₂- and EDTA-extractable concentrations of trace metals through straw mulch compared to cover crop.
## I.2. Material and methods

### I.2.1. Field experiment

The field experiment, a Navel orchard, is located in the IAC's agronomic research station in Pocquereux (21°43'895'S; 165°53'922'E; La Foa, New Caledonia). More than 75% of the Citrus production in New Caledonia is located in La Foa basin. The climate is subtropical (mean annual temperature 25°C), wet and hot season occurred between December to March, while a colder season occurs between June and August. A marked dry and relative hot season generally extends between September and November. However, between 2020 and 2021, a persistent La Niña phenomena have brought a lot of precipitation along the year with a mean annual cumulative rainfall of 2 500 mm (Météo NC, 2021). Exceptional rainfall rates of 57 % excess have been recorded in consequence with no marked dry season during these years. The soil is a clayey loam Fluvisol with 26 % of clay, 29 % of silt and 42 % of sand (Table 5-1).

<table><caption>Table 5-1: Physical and chemical parameters of soil control measured at t3-1, mean of the 3 blocs without fertilization with straw and cover crop. Numbers in brackets represent standard deviation (n=6).</caption><thead><tr><th>Clay (%)</th><th>Silt (%)</th><th>Sand (%)</th><th>pH<sub>H2O</sub></th><th>pH<sub>KCl</sub></th><th>C g kg<sup>-1</sup></th><th>C/N</th><th>Nassimilable mg kg<sup>-1</sup></th><th>Na mg kg<sup>-1</sup></th><th>P mg kg<sup>-1</sup></th><th>K mg kg<sup>-1</sup></th></tr></thead><tbody><tr><td>26</td><td>29</td><td>42</td><td>7.0<br>(0.4)</td><td>6.0<br>(0.6)</td><td>14.66<br>(1.08)</td><td>10.29<br>(0.25)</td><td>8.16<br>(3.92)</td><td>13 040<br>(1 039)</td><td>839<br>(114)</td><td>5 840<br>(850)</td></tr><tr><th>Elements</th><th>Ca</th><th>Mg</th><th>Fe</th><th>Mn</th><th>Ni</th><th>Cr</th><th>Cu</th><th>Zn</th><th></th><th></th></tr><tr><td>mg kg<sup>-1</sup></td><td>3 873<br>(375)</td><td>5 240<br>(828)</td><td>51 350<br>(6 396)</td><td>2 780<br>(280)</td><td>10.95<br>(1.37)</td><td>34.15<br>(7.72)</td><td>24.59<br>(4.73)</td><td>129.06<br>(27.71)</td><td></td><td></td></tr></tbody></table>

The studied orchard has an area of 0.72 ha with 476 trees. Washington Navel variety is grafted into the flying dragon graft-holder (Poncirus trifoliata var. monstrosa (T.Itô) Swing) (Mademba-Sy et al. 2012). The experimental field is divided in 3 replicated blocks with 8 modalities including 4 fertilization treatments: (1) soil negative control, without fertilization (0); (2) soil positive control, with a mineral fertilization as a background (FM); (3) dose 1 of compost (D1); (4) dose 2 of compost (D2).

Compost applied is a mixture of sewage sludge obtained from the wastewater treatment plant of Noumea (50%), and green waste from house-garden and communal parks maintenance of Mont-Dore city (50%), produced in the composting platform of Nakutakouin, New Caledonia (CDE pers. com., 2019). The dose 1 of compost input was calculated for respecting the annual and 10-years fluxes of nickel (Ni) and chromium (Cr) inputs defined by the French regulation (NFU 44-051). The dose 2 input was based on N, P, and K concentrations in composts and agronomical needs of the orchard but as a consequence, the quantity of Ni and Cr added largely exceed French regulations (Table 5-2).