# Influence of biochar and compost and their interaction on carbon and nitrogen mineralization in a Saharan sandy soil

Elie Le Guyader, H. Oueriemmi, Hugues Clivot, Mohamed El Mazlouzi, Alexandra Guillaneuf, Maxime Gommeaux, B. Marin, I.R. Zoghlami, Gonzague Alavoine, M. Sbih, et al.

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## To cite this version:

Elie Le Guyader, H. Oueriemmi, Hugues Clivot, Mohamed El Mazlouzi, Alexandra Guillaneuf, et al. Influence of biochar and compost and their interaction on carbon and nitrogen mineralization in a Saharan sandy soil. Geoderma Régional, 2025, 42, pp.e01002. 10.1016/j.geodrs.2025.e01002. hal-05249894

HAL Id: hal-05249894

https://univ-reims.hal.science/hal-05249894v1

Submitted on 11 Sep 2025

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# Influence of biochar and compost and their interaction on carbon and nitrogen mineralization in a Saharan sandy soil

E. Le Guyader<sup>a,*</sup>, H. Oueriemmi<sup>b</sup>, H. Clivot<sup>c</sup>, M. El Mazlouzi<sup>a</sup>, A. Guillaneuf<sup>a</sup>, M. Gommeaux<sup>a</sup>, B. Marin<sup>a</sup>, I.R. Zoghlami<sup>b</sup>, G. Alavoine<sup>c</sup>, M. Sbih<sup>d</sup>, B. Boumaraf<sup>e</sup>, A. Tirichine<sup>f</sup>, V. Kavvadias<sup>g</sup>, M.L. Aubertin<sup>h</sup>, D. Sebag<sup>h</sup>, T. Adatte<sup>i</sup>, X. Morvan<sup>a</sup>

## ARTICLE INFO

### Keywords:

Exogenous organic matter

C mineralization

Nitrogen

Salinity

Rock-Eval®

## ABSTRACT

Biochar and compost co-application in agricultural soils is of growing interest due to their complementary nature, combining high carbon (C) sequestration potential and nutrient inputs. An organically depleted sandy soil sampled in an oasis in southern Tunisia was selected to evaluate the impact of date palm-based biochar and compost as soil amendments and to assess the biodegradability of these organic products sole or in mixtures. Carbon and Nitrogen (N) mineralization dynamics in soils amended or not with biochar and/or compost were monitored during 112 days. Soil samples were taken at the start and after 56 and 112 days of incubation for soil organic matter quantification and characterization using Rock-Eval® thermal analysis.

The apparent mineralized organic carbon fraction of the compost was ≈16.7 % after 112 days. The mineralization rate constants of the labile fractions of sole compost and biochar were 0.012 and 0.037 day⁻¹ respectively following model fittings on C mineralization data. The proportion of their recalcitrant fractions were 77 and 98 % of initial C for compost and biochar, respectively. The comparison of model fits obtained for products alone vs mixtures showed no differences, suggesting no significant interaction between biochar and compost in mixtures on C mineralization. However, in the soil amended with pre-incubated biochar-compost mixtures, lower cumulated CO₂ emissions were found compared with the same products inputs applied individually. The nitrogen supply of the compost was estimated at 53 ± 28 kg N·ha⁻¹ in the short-term in the soil amended with ≈77 t·ha⁻¹ of date palm compost. Biochar pre-incubated with compost increased the soil N supply to 106 ± 45 kg N·ha⁻¹ compared with the unamended soil.

The low thermal stability of the initial pure compost highlighted the low stabilization of the organic matter during the composting process. The high salt content of the compost does not seem to inhibit organic matter decomposition in a saline non-sodic soil. However, improving the composting process with date palm residues is needed to optimise its carbon stability, and to avoid salinization of the soil and water in arid areas.

## 1. Introduction

In oasi an agroecosystems of the Middle East and North Africa, date palm (Phoenix dactylifera L.) plays a key ecological role, providing

suitable conditions beneath the canopy for other crop production and livestock farming (Allam et al., 2013; de Grenade, 2013). The date palm crop constitutes the main source of income for rural populations in harsh arid environments. The evolution of climatic conditions and agricultural

<footnote><sup>a</sup> Université de Reims Champagne-Ardenne, GEGENA, 51100 Reims, France</footnote>

<footnote><sup>b</sup> Institute of arid regions, Medenine, Eremology and Combating Desertification Lab. (LR16IRA01), 4100, Medenine, Tunisia</footnote>

<footnote><sup>c</sup> Université de Reims Champagne-Ardenne, INRAE, FARE, UMR A 614, Reims, France</footnote>

<footnote><sup>d</sup> Laboratory for improving agricultural production and protection of resources in dry areas University of Batna, Algeria</footnote>

<footnote><sup>e</sup> Laboratory of promoting innovation in agriculture in arid regions, University of Mohamed Khider, Biskra, Algeria</footnote>

<footnote><sup>f</sup> National Institute of Agronomic research of Algeria, Touggourt, Algeria</footnote>

<footnote><sup>g</sup> H.A.O. DEMETER - Institute of Soil and Water Resources, Department of Soil Science of Athens, Lykovrisi, Attiki, Greece</footnote>

<footnote><sup>h</sup> IFPEN, Earth Sciences and Environmental Technologies Division, Rueil-Malmaison, France</footnote>

<footnote><sup>i</sup> ISTE, Institut des Sciences de la Terre, Université de Lausanne, GEOPOLIS, Lausanne, Switzerland</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: elie.le-guyader@univ-reims.fr (E. Le Guyader).</footnote>
expansion in North Africa (Rodríguez-Caballero et al., 2017) presents new challenges in arid regions, such as desertification, sand encroachment, water scarcity, intense drought episodes, soil and water salinization (Bestelmeyer et al., 2015; Marlet et al., 2009). The ecological functions provided by date palm oases could serve in combatting desertification and sand encroachment in hot drylands (Mihi et al., 2019). With the aim of maintaining high productivity in these agroecosystems, best management practices such as organic fertilization would require high amounts of manure, but this organic amendment is more and more scarce in Saharan regions, due to the livestock reduction (El Janati et al., 2021).

Soil organic matter (SOM) management is associated with soil fertility, and its crucial role in sustaining agricultural productivity and carbon sequestration is of growing interest. Oasis agroecosystems are generally characterized by low SOM stocks, as high temperatures, irrigation practices and weak OM protection due to the lack of fine soil particles favour rapid OM turnover (Brahim et al., 2022). Malou et al. (2020) found that SOM contents in Arenosols from a semi-arid region were positively affected by organic inputs such as manure and household organic wastes, but crop residues had little effect. Recycling organic residues in agriculture *via* composting or pyrolysis transformation processes can influence the soil organic status and associated fertility properties with impacts on soil nutrients, organic carbon stocks and their composition (Chenu et al., 2019; Peltre et al., 2017). The amount of leaves of date palms was estimated at 2.4 tons of dried matter. ha⁻¹.year⁻¹ in monocropped date palm systems according to El Janati et al. (2021). The same authors also indicate that under current practices date palm residues are mostly piled or burned in most parts of the world, causing plant disease issues and potentially causing fires in oases. Transformation of these residues through composting with a nutrient-rich source such as manure showed promising results for increasing soil fertility and silage corn yields in South Morocco (El Janati et al., 2022). The high availability of date palm residues and compost at a cheaper price compared with manure may encourage the use of compost by farmers. Compost quality refers to its biological stability and its ability to sustain soil fertility by increasing SOM stocks. Organic carbon (C) to nitrogen (N) ratio from exogenous organic matter (EOM) appears relevant for predicting C and N mineralization after its application to soil (Nicolardot et al., 2001; van der Sloot et al., 2022). For example, Parnaudeau et al. (2004) found that Corg/N ratio of sludge lower than 10 strongly stimulated soil N mineralization. The recent development of analytical tools and indices for organic matter characterization improved the prediction of EOM stability in the soil (Chenu et al., 2015). For about twenty years, the Rock-Eval® thermal analysis was used to assess the thermal stability of soil organic carbon (SOC) and EOM (Albrecht et al., 2015; Disnar et al., 2003; Kpemoua et al., 2024). This method, combining pyrolytic, then oxidative heating (Behar et al., 2001), is also suitable to assess the stability of biochar, the carbonization product obtained after heating organic matter in an oxygen-limited environment (Olsson et al., 2019). This material has a high soil carbon sequestration potential (Azzi et al., 2024; Wang et al., 2016), but its properties are highly variable due to feedstock selection and pyrolysis conditions (Tomczyk et al., 2020).

Biochars produced from plants are not considered as N fertilizers due to their relatively poor N content and limited availability. Mostly due to its sorption properties of inorganic nitrogen and its influence on N transformation, biochar application alters soil nitrogen dynamics, but case-by-case studies are necessary to assess its effects depending on biochar type and environmental conditions (Clough et al., 2013). Co-application of biochar with other organic sources could reduce the mineralization of EOM in the soil due to protection mechanisms of EOM labile components (Jien et al., 2015; Teutscherova et al., 2017). Enriching biochar with compost may increase its bioavailable nitrogen content and may reduce N leaching and N₂O emissions from compost amended soils (Kammann et al., 2015). Lebrun et al. (2023) found that biochar and compost mixed at a mass ratio of 30:70 had synergistic

effects. These effects included less nitrogen loss through leaching and more plant growth than when compost was used alone. Changes of biochar surface properties, through higher negative surface functional groups and higher negative charges, may also occur when biochar is chemically or biologically activated by mixing with organic amendments before soil application (Antor et al., 2023). Previous studies suggested that biochar may not influence N mineralization in agricultural soils after one year of application (Castaldi et al., 2011). However, a meta-analysis reported an overall reduction (≈10 %) of soil inorganic nitrogen (SIN) at short-term (1 month) in biochar amended soils (Nguyen et al., 2017).

The primary objective of this laboratory scale study was to assess the biodegradability of organic amendments, biochar and compost derived from date palm wastes, in a OM-depleted cultivated sandy soil. C and N mineralization dynamics were studied *via* soil incubation experiments. We hypothesized that (i) compost amended soil will exhibit higher C and N mineralization than biochar amended soil, due to the lower stability of compost (ii) biochar could slow down C mineralization of compost and affect soil N mineralization, and (iii) pre-incubation of biochar and compost before soil amendment could enhance their potential interaction.

## 2. Materials and methods

### 2.1. Study area

The Kebili region in southeast of Tunisia is characterized by an arid climate with a mean rainfall of 102 mm year⁻¹ and an average potential evapotranspiration of 2750 mm year⁻¹ (National Meteorological Institute of Tunisia, 1985–2016 period). The date palm (*Phoenix dactylifera* L.) is the dominant crop due to its tolerance to soil salinity and extreme temperatures. The traditional irrigation method for supplying crops is flooding. The palm groves are equipped with drains connected to the Chott El Djerid saline depression. Irrigation water is collected in deep underground aquifers, such as the North-Western Sahara Aquifer System.

### 2.2. Soil collection

Topsoil (2–20 cm) of a Gypsisol according to the WRB classification was collected in December 2022 from an experimental plot in the El-Atilet oasis in the governorate of Kebili (GPS coordinates 33°57'04" N, 9°05'47"W). The first two centimeters were removed as they correspond to recent aeolian deposits. A composite sample was made from subsamples of 4 plots located under date palms, maintaining a distance of 1 m from the palm trunk to avoid high root density. These cultivated plots have not been amended for at least 3 years, and they were irrigated with groundwater by flood irrigation every 10 days. The soil was air dried and sieved <2 mm one week before incubation experiments.

### 2.3. Organic amendments preparation

The compost was produced in Gabes (Tunisia) by the Association for Saving Oasis of Chenini (ASOC). First, crushed date palm leaves were soaked in a basin of water for 4 days. These residues were then mixed with sheep manure (70:30 in volume) before setting up windrows (1.5 m high, 3 m wide and 20 m long). Windrows were turned manually every week to ensure homogenisation and aerobic conditions. Because of the arid climate, the mixture was irrigated every 7 to 10 days with the locally available groundwater. The product obtained after 5 months of composting was air-dried and crushed to pass 4 mm sieve.

Biochar from date palm residues (rachis) collected in the Murcia region (Spain) was obtained by slow pyrolysis. The residues were first dried at 90 °C for 48 h. Pyrolysis was maintained for 2 h at 450 °C ± 5 °C under constant nitrogen flow after pre-heating to 150 °C, with the temperature rising at a rate of 5 °C per minute up to 450 °C (Le Guyader
et al., 2024, 2025). The biochar was ground in an automatic mortar and sieved to <1 mm.

## 2.4. Soil physico-chemical analyses

Soil bulk density was measured *in situ* in 6 replicates in different sampled plots after removing soil surface (0–2 cm). Soil granulometry was determined with Robinson's pipette method, without decarbonation (NF X31–107, 2003). Total nitrogen was measured with the Kjeldahl method (NF ISO 11261, 1995). Gypsum content was determined by difference between soluble sulphates in a 1:4 extract (25 g soil:100 mL water) and in a saturated paste extract (Loeppert and Suarez, 1996). Cation exchange capacity (CEC) and exchangeable cations were measured using the Metson method (NF X31–130, 1999 and NF X31–108, 2024, respectively). Soil available phosphorus was determined by Olsen method (NF X 31–161, 2024). Soil pH and electrical conductivity (EC) were measured at a ratio of 10 g of soil to 50 mL of deionized water. Soil EC was also measured in a water extract of a saturated soil paste (ECe) with 250 g of soil and 95 mL distilled water. Sodicity indicators were calculated: sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP). SAR was calculated based on the cations concentrations in saturated soil paste extract, expressed in mmol. L⁻¹. ESP was evaluated on the basis of the sum of exchangeable cations (Saidi et al., 2008, Sumner et al., 1998), expressed as mmol.kg⁻¹ of soil. Exchangeable cations were the difference between extractable cations using ammonium acetate 1 mol.L⁻¹ and the water-soluble cations in the saturated paste extract.

Water-soluble elements were also analyzed with a soil:water ratio of 1:5 (w:v) after shaking for 1 h in control soil, soil amended with 3 % compost and soil amended with 1.5 % biochar (by weight of soil) at day 0 of incubations to assess the quantity of water-soluble elements provided by the two studied EOM.

## 2.5. Physico-chemical analyses of organic amendments

The total carbon, hydrogen and nitrogen content of the biochar was measured by SOCOR company (Dechy, France) by dry combustion using an elemental analyzer (NF EN ISO 21663, 2020). Mineral content of biochar and compost was determined after 6 h heating at 550 °C in a muffle furnace (NF EN 15935, 2021). Residues from calcination were finely crushed before field portable X-ray fluorescence (FPXRF) analyses to determine elemental concentrations in compost samples. A portable Thermo Scientific Niton XL3t 980 Geometrically Optimized Large area Drift Detector (GOLDD+) energy-dispersive XRF analyzer was used. Elemental S and Cl concentrations were calculated via the built-in algorithm and a factory calibration (Mining mode). The quartz sand content was measured in the composts using 5 g dry samples after calcination, decarbonation with HCl and removal of water-soluble elements after column filtration, followed by dry sieving >50 µm. Total P, K, Mg, Ca, and Na in compost and biochar were analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) after mineralization with concentrated nitric acid and filtering <0.45 µm before analysis. Compost soluble salts were analyzed after hot water (100 °C) extraction during 30 min. Potential CEC of biochar was measured after pH adjustment to 7 and washing of samples with water until EC <0.2 mS.cm⁻¹ (Munera-Echeverri et al., 2018). Biochar pH and EC were determined at a ratio of 5 g of material to 50 mL of deionized water (Singh et al., 2017), while for compost we used the method described above for soils.

## 2.6. Characteristics of the soil and organic amendments

The sandy soil selected was slightly alkaline (Table 1). The EC of the saturated paste extract exceeds 4 mS.cm⁻¹ (see Supplementary material, Table S1), indicating that this soil is saline according to Allison et al. (1954). The soil was not sodic because SAR was lower than 13 mmol.L⁻¹

<caption>**Table 1**</caption>

Main physico-chemical properties of the soil (2–20 cm) and organic amendments (mean ± standard deviation, n = 3).

<table><thead><tr><th>Parameters</th><th>Unit</th><th>Soil</th><th>Compost</th><th>Biochar</th></tr></thead><tbody><tr><td>Particle size distribution</td><td>% Sand</td><td>82.1</td><td>–</td><td>–</td></tr><tr><td></td><td>% Silt</td><td>4.1</td><td>–</td><td>–</td></tr><tr><td></td><td>% Clay</td><td>3.5</td><td>–</td><td>–</td></tr><tr><td>Limestone content</td><td>%</td><td>7.1 ± 0.1</td><td>12.9 ± 1.4</td><td>ND</td></tr><tr><td>Total carbon</td><td>%</td><td>1.24 ± 0.05</td><td>22.98 ± 3.3</td><td>62.5</td></tr><tr><td>Organic carbon</td><td>%</td><td>0.37 ± 0.03</td><td>21.3 ± 3.1</td><td>ND</td></tr><tr><td>Total N</td><td>%</td><td>0.041 ± 0.007</td><td>1.15 ± 0.25</td><td>0.55</td></tr><tr><td>C/N</td><td>–</td><td>11.5</td><td>18.5</td><td>113.6</td></tr><tr><td>Total H</td><td>%</td><td>ND</td><td>ND</td><td>2.32</td></tr><tr><td>H/C</td><td>–</td><td>ND</td><td>ND</td><td>0.04</td></tr><tr><td>pH (water)</td><td>–</td><td>7.56 ± 0.06</td><td>7.4 ± 0.1</td><td>9.7 ± 0.1</td></tr><tr><td>Electrical conductivity</td><td>mS.cm<sup>-1</sup></td><td>1.95 ± 0.03</td><td>20.2 ± 1.3</td><td>7.6 ± 0.3</td></tr><tr><td>Cation exchange capacity</td><td>cmol<sup>+</sup>. kg<sup>-1</sup></td><td>3.9</td><td>ND</td><td>126 ± 5</td></tr><tr><td>Available phosphorus (Olsen)</td><td>mg.kg<sup>-1</sup></td><td>7.7 ± 0.6</td><td>–</td><td>–</td></tr></tbody></table>

ND: not determined.

and ESP lower than 15 % (Allison et al., 1954). This suggest that the soil's salinity does not result in the sodization of the soil exchange complex. Hence, it can be classified as sandy saline non-sodic soil. The soil contained low organic carbon (0.37 ± 0.03 %) and low available phosphorus (7.7 ± 0.6 mg.kg⁻¹) contents (Table 1).

The date palm compost C<sub>org</sub>/N ratio was 18.5, its electrical conductivity (EC) was very high (20.2 ± 1.3 mS.cm⁻¹), and the pH was slightly alkaline (7.4 ± 0.1) (Table 1). Sulfur and chlorine contents in the compost were high, 6.9 ± 0.1 % and 5.7 ± 0.3 % in weight, respectively (Supplementary material, Table S2). The soluble salt fraction was measured at 12.0 ± 0.5 % of compost dry matter (data not shown). The compost soluble fractions of K (70.6 ± 3.5 %) and Na (41.2 ± 3.1 %) were considerably higher than those of Ca (2.0 ± 0.2 %) (data not shown). Quartz sand accounted for 22 % of compost dry matter.

Date palm biochar had an alkaline pH, high EC and high K content (Table 1, Table S2). Its potential CEC was high (126 ± 5 cmol<sup>+</sup>.kg<sup>-1</sup>).

## 2.7. Incubation conditions and experimental design

Incubation experiments were carried out to measure the effect of compost and/or biochar inputs on soil CO₂ emissions and N mineralization dynamics. Soils were amended with biochar (B) and/or compost (Cp) with a fixed dose for compost (3 % by dry weight, equivalent to application of 77 t. ha⁻¹ assuming a soil bulk density of 1.29 g. cm⁻³ for the top 0–20 cm) and two different doses for biochar (0.75 and 1.5 % by weight). In Saharan regions, manure inputs reach ≈40 t. ha⁻¹ yr⁻¹ (El Janati et al., 2021), but the applied dose was higher due to the low organic matter and total N contents of the compost used. Application of a high dose of compost could also be recommended at a low frequency, as positive effects on soil functions were observed 5 years after the addition of 150 to 450 tons of fresh compost per hectare in a semi-arid area (Hernández et al., 2015). The carbon and nitrogen added by the compost represent 6.4 gC.kg⁻¹ soil and 0.35 gN.kg⁻¹ soil. Lower application doses for biochar were chosen due to the difficulty of large-scale production in Saharan regions (high cost, lack of infrastructure).

The following treatments were conducted in triplicate: unamended control soil (S), S + Cp, S + 0.75% B, S + 1.5% B, S + 0.75% B + Cp, S + 1.5% B + Cp, S + 0.75% B + Cp* and S + 1.5% B + Cp*. Biochar-compost mixtures were tested at respective dry weight ratios of 1:4 and 1:2.

Pre-incubated treatments marked with an asterisk (*) consisted in mixing biochar and compost before soil application. This biochar enrichment method was assessed to induce slow nutrient release
behaviour. This mixture was humected with distilled water at 60 % of the dry weight and pre-incubated for 14 days at 25 °C. Then, the pre-incubated biochar-compost mixture was mixed with the soil and placed in the incubator with other treatments. Mixtures in the non pre-incubated treatments were humected in the same way and mixed with the soil at the start of soil incubations.

All incubation treatments were adjusted at a water content corresponded to the field capacity of the soil (pF = 2.5) and incubated at 25 ± 1 °C in the dark. The water content was regularly checked gravimetrically and adjusted with deionized water. A soil amount equivalent to 50 g dry weight was placed with or without organic amendments in airtight glass jars (0.72 L) for aerobic incubation. Water (10 mL) was separately placed into the jars to maintain a moisture-saturated atmosphere. The quantity of CO₂ emitted was measured 16 times between 2 and 112 days (Fig. 1), by means of 10 mL NaOH (1 M) traps, which were titrated with HCl 0.5 mol.L⁻¹ after precipitating carbonate with a BaCl₂ solution. In the pre-incubated treatments, the CO₂ emitted was measured 4 and 14 days after mixing biochar and compost.

Total extractable soil inorganic N (NO₂, NO₃ and NH₄) was determined 9 times (Fig. 1) by destructive sampling of three replicates per treatment. Nitrates were considered as the predominant form of soil inorganic N (SIN) because NO₂ and NH₄ concentrations in the soil were not detected. Samples of 25 g moist soil were suspended in 100 mL KCl (1 M) and shaken for 1 h (ISO/TS 14256-1:2003). The supernatant was filtered through a n°40 Whatman filter (8 µm mesh size) after 30 min of decantation. The extracts were analyzed for SIN using steam distillation method (Bremner and Keeney, 1965).

Soil inorganic nitrogen, expressed in mgN.kg⁻¹ dry soil, was determined as follows:

$$ \text{SIN (mgN.kg}^{-1}\text{)} = ((V_1 - V_0) \times \text{c[H}^{+}\text{]} \times \text{M}_N) / n \times \left( \frac{100 + W_{\text{H2O}}}{100} \right) \times \text{Fd} \quad (1) $$

Where Fd = dilution factor, V1 = volume of sulphuric acid for sample titration (mL), V0 = volume of sulphuric acid for blank titration (mL), c [H⁺] = sulphuric acid concentration (N), M_N = nitrogen molar mass = 14 g.mol⁻¹, n = mass of soil sampled, W_H2O = water content of the sample (%), measured after soil drying at 105 °C.

## 2.8. Carbon and nitrogen mineralization

The apparent fraction of applied C mineralized in the treatments with compost and/or biochar was calculated according to Levavasseur et al. (2022):

$$ \text{EOM mineralized C (mgC.g}^{-1} \text{ added C)} = \frac{\text{CMC}_{\text{treatment}} - \text{CMC}_{\text{control}}}{\text{Organic C applied}} \quad (2) $$

where CMC is the cumulative C-CO₂ emitted during incubation in mgC, assuming that there was no impact of EOM amendments on SOM mineralization (i.e. no priming effect).

The net nitrogen mineralization was calculated for all treatments by subtracting the amount of mineral nitrogen in the unamended soil at each sampling date and expressed in mg inorganic N.kg⁻¹ dry soil. For these calculations, we assumed that the nitrogen contained in the biochar is not bioavailable.

## 2.9. Carbon mineralization kinetic model

A two-compartment model was the basis for fitting the cumulated C mineralization dynamics from soil, compost and biochar after subtracting the C-CO₂ released from control soil for both EOM products, alone or mixed. A two-pool double-exponential model is commonly considered as a reasonable approach to represent the decay of such products with (i) a relatively labile pool with a high mineralization rate and (ii) a recalcitrant pool with a residence time of centuries to millenia (Leng et al., 2019). Owing to the high residence time of the recalcitrant pool and the comparatively short duration of our incubation experiment (112 days), we considered the recalcitrant pool to be non degradable during the incubation and thus fitted the data for each OM (soil or EOM) using a more parsimonious approach based on a simple mono-exponential + constant decay model, which would translate for modelling the emitted C as follows:

$$ C_{\text{mineralized}} = C_0 \times (1 - \text{REC}) \times (1 - \exp^{-k_{\text{labile}} \times t}) \quad (3) $$

where Cmineralized is the amount of mineralized C from soil or EOM (mgC-CO₂.kg⁻¹) at time t (days), C₀ is the initial amount of potentially mineralizable C from soil or EOM (mg.kg⁻¹), REC is the fraction of the recalcitrant C pool and klabile is the mineralization rate constant of the labile pool (day⁻¹).

First, we fitted this simple model on C mineralization dynamics for the control soil. Secondly, we fitted separately two different models on apparent EOM mineralization data: one model based on incubation experiments with compost and biochar alone and a second model for mixtures, both models using the following combined equation:

$$ C_{\text{mineralized}} = C_{0 \text{ compost}} \times (1 - \text{REC}_{\text{compost}}) \times (1 - \exp^{-k_{\text{labile compost}} \times t}) + C_{0 \text{ biochar}} \times (1 - \text{REC}_{\text{biochar}}) \times (1 - \exp^{-k_{\text{labile biochar}} \times t}) \quad (4) $$

The two-compartment models were compared to simpler one-compartment models for soil and EOMs but the latter were proved to be statistically less effective in representing the data observed (data not shown).

<figure><img src="image_4.png" /><figcaption>Fig. 1. Monitoring of soil C and N mineralization and soil sampling strategy.</figcaption></figure>
## 2.10. Determination of soil organic and inorganic carbon

Rock-Eval® thermal analysis was used to quantify organic and inorganic carbon content in initial soil, compost and incubated samples at days 0, 56 and 112 (Fig. 1). The samples were previously ground (< 200 µm) and desalted. For desalination, approximately 100 mg of sample was shaken with 250 mL milli-Q water for 5 min, then the solid phase was separated from the solution by vacuum filtration and dried at 40 °C. All samples were analyzed (mass = 20–74 mg) using a Rock-Eval® 6 thermal analysis device. The analytical protocol consisted of two phases: (i) pyrolysis by heating from 200 °C to 650 °C at 25 °C. min⁻¹ in an inert atmosphere (N₂); (ii) oxidation of residual carbon from 300 °C to 850 °C at 20 °C. min⁻¹ in an artificial atmosphere (N₂/O₂: 80/ 20). Total organic carbon (TOC in wt%) and mineral carbon (MINC in wt%) were calculated by integrating the amounts of HC, CO, and CO₂ produced during thermal cracking of OM between defined temperature limits (Behar et al., 2001; Lafargue et al., 1998). A correction was made to calculate the TOC content of the samples based on their respective soluble inorganic fraction.

The thermograms related to hydrocarbon compounds, also called S2, were used to calculate (1) hydrogen index (HI) corresponding to the amount of hydrocarbons formed during the thermal pyrolysis (in mg HC per g TOC), and (2) the R-index representing the most thermally resistant and refractory pools of hydrocarbon compounds removed above 400 °C during the pyrolysis stage (Sebag et al., 2016; Malou et al., 2020).

## 2.11. Calculations and statistical analyses

We used R software (R Core Team, 2013) for all statistical analyses. Two-ways ANOVAs were carried out to test the effects of biochar dose and application method of biochar compost-mixture on carbon mineralization (i.e. non pre-incubated vs. pre-incubated). Tukey's HSD test (α = 5 %) was applied to separate the means.

The *nls* function implemented in the R package *stats* was used to determine the nonlinear least-squares estimates of the parameters from the nonlinear model fittings on C mineralization data (Bates, 1988). We also compared the fits obtained for EOM alone vs in mixtures using the *anova* function (R package *stats*) to test the hypothesis of no interaction between compost and biochar.

# 3. Results

## 3.1. Effects of organic amendments on soil chemical properties

Initial soil pH was significantly increased by the highest dose of

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Effects of organic amendments on initial soil pH. The vertical error bars indicate the standard deviation (n = 3). S = soil, B = biochar and Cp = compost. S + B₁.₅: soil amended with 1.5 % biochar by weight. Different letters indicate significant difference between treatments at 0.05 level, according to Tukey's honestly significant difference test.</figcaption></figure>

biochar (1.5 %; *p*-value <0.001), while it was only tendentially increased after addition of 0.75 % biochar (Fig. 2). Adding compost at a dose of 3 % had no effect on pH. Compost addition significantly increased soil EC, independently from the presence or absence of biochar (*p*-value <0.001) (Supplementary material, Fig. S1). Water soluble salts extracted from the S + Cp treatment showed an increase in Mg, K, Na and Cl contents, relatively to the soil control (Table 2). Biochar, to a lesser extent, increased the K, Mg and Cl soluble contents of the soil. Initial and final (day 112) available P content increased significantly with compost addition. The highest mean value was obtained in the soil amended with biochar and compost mixtures (Supplementary material, Table S3). Soil CEC was not significantly influenced by organic amendments (Supplementary material, Table S3).

## 3.2. Carbon and nitrogen mineralization from incubation data

The average cumulative CO₂ emission after 112 days in the soil control was 713 ± 139 mgC.kg⁻¹ (Fig. 3a). Treatments with biochar alone (S + B) reached cumulative values of 830 ± 125 and 923 ± 139 mgC.kg⁻¹ after 112 days, for biochar additions of 0.75 and 1.5 %, respectively, with no significant difference compared to the control. Biochar organic C mineralization in the soil was estimated to be respectively 2.5 ± 6.4 % and 2.2 ± 3.1 % in treatments S + 0.75%B and S + 1.5%B of the initial C added from biochar after 112 days (Fig. 3b). After 112 days, compost application (S + Cp) significantly increased CO₂ cumulated emissions, with a value 2.5 times higher than in the control soil (*p*-value <0.001). The apparent mineralized C fraction of the compost was 16.7 ± 4.6 % of the initial C added at the end of incubation (Fig. 3b).

Regardless of the biochar dose, treatments with compost and biochar showed the highest cumulative CO₂ emission. The differentiation from the control was detected after the first week (*p*-value <0.001). Also, CO₂ emitted in S + 1.5% B + Cp after 1 week was significantly higher than S + Cp (*p*-value <0.001) and S + 0.75% B + Cp (*p*-value <0.01). Pre-incubated treatments exhibited significantly lower cumulated CO₂ production than the mixture treatments with the same dose of non pre-incubated EOM (*p*-value <0.05) (Fig. 3a). None of the cumulative CO₂ emission curves reached a plateau at the end of the incubation period.

Average mineral N concentrations and variation in mineral N production were low in control soil during the first 56 days of the incubation period (<15 mgN.kg⁻¹; Fig. 3c). After 7 and 14 days of incubation, the treatments with biochar significantly reduced the SIN concentrations compared to the control (*p*-value <0.001), while after 35 days, only the highest biochar dose significantly reduced the SIN concentration (*p*-value <0.01). Nitrogen immobilization occurred in compost amended soil (S + Cp) between 21 and 35 days of incubation, when SIN concentrations are lower than in control soil (Fig. 3c). The duration and magnitude of N immobilization was higher in treatments S + 0.75%B + Cp and S + 1.5%B + Cp. However, pre-incubated treatments showed higher SIN concentrations than the control during the incubation period of between 4 and 21 days.

The difference between SIN concentrations at day 112 between control soil and S + 1.5%B + Cp* treatment was significant (*p*-value

**Table 2**

Effects of organic amendments on initial soil water-extractable salt concentrations (soil:water ratio was 1:5). S = soil, B = biochar and Cp = compost. S + B₁.₅: soil amended with 1.5 % biochar by weight. Different letters in a column indicate significant difference between treatments at 0.05 level, according to Tukey's honestly significant difference test (n = 3).

<table><thead><tr><th rowspan="2">Treatments</th><th colspan="6">Concentrations (mg.kg<sup>-1</sup>)</th></tr><tr><th>Ca</th><th>Mg</th><th>K</th><th>Na</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th></tr></thead><tbody><tr><td>Control soil</td><td>2228 <sup>ab</sup></td><td>136 <sup>c</sup></td><td>37 <sup>c</sup></td><td>160 <sup>b</sup></td><td>931 <sup>c</sup></td><td>7684 <sup>ab</sup></td></tr><tr><td>S + B<sub>1.5</sub></td><td>2167 <sup>b</sup></td><td>159 <sup>b</sup></td><td>284 <sup>b</sup></td><td>174 <sup>b</sup></td><td>1116 <sup>b</sup></td><td>7004 <sup>b</sup></td></tr><tr><td>S + Cp</td><td>2280 <sup>ab</sup></td><td>230 <sup>a</sup></td><td>405 <sup>a</sup></td><td>651 <sup>a</sup></td><td>2522 <sup>a</sup></td><td>8035 <sup>a</sup></td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a) cumulated carbon mineralized, (b) cumulated apparent carbon mineralized, (c) soil inorganic nitrogen dynamics and (d) apparent soil inorganic nitrogen dynamics over the 112-days incubation period (apparent dynamics representing the difference between soil+ EOM treatments and the control soil). Data represent means from 3 replicated samples. The vertical error bars indicate the standard deviation. Different letters indicate significant difference between treatments at 0.05 level, according to Tukey's honestly significant difference test. Treatments: control soil (S), soil amended with biochar (S + B), compost (S + Cp), mixture of biochar and compost (S + B + Cp) and mixture of biochar pre-incubated with compost (S + B + Cp*). S + B0.75: soil amended with 0.75 % biochar by weight. Soil inorganic nitrogen was not measured at the end point for treatments with the lowest biochar dose.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Kinetics of cumulated apparent carbon mineralization. B0.75: soil amended with 0.75 % biochar by weight. The data are represented by points, and the curves represent the two-compartment model fittings.</figcaption></figure>
<0.01). The additional mineral N calculated from the N release was estimated at 106 ± 45 kg N.ha⁻¹ with S + 1.5%B + Cp* treatment (soil depth of 0–20 cm and bulk density of 1.29 g.cm⁻³ were considered). At day 112, SIN concentration in S + Cp did not differ significantly from the control, probably due to high variability. Nitrogen mineralized from compost at day 112 of the incubation period was 6.0 ± 3.2 % of organic N added, either 21 mgN mineralized over 350 mgN.kg⁻¹ soil supplied (Fig. 3d).

### 3.3. Apparent carbon mineralization kinetics

The non-linear models fitted on mineralized C data from compost and biochar alone or in mixtures are shown in Fig. 4 and the model parameters obtained are reported in Table 3 for these EOMs and for the control soil.

Estimated decomposition constants ($k_{labile}$) from the labile compartments were 0.012 day⁻¹ for soil and compost (alone or in mixture), and 0.037 and 0.045 day⁻¹ for biochar alone and in mixture, respectively (Table 3). The proportion of carbon in the recalcitrant pool (REC) was equal to 0.75 for soil and 0.77 and 0.75 for compost alone and in mixture, respectively (Table 3). Biochar showed the largest fraction of recalcitrant carbon with REC = 0.98.

The comparison of the two models fitted on compost and biochar data alone vs in mixtures revealed no significant differences (Anova, *p-value* = 1), suggesting no significant interaction between biochar and compost in mixtures on mineralized C.

For the pre-incubated treatments, C mineralization of biochar-compost mixtures was measured during pre-incubation and summed to the cumulated CO₂ production after 112 days of incubation (Fig. 5). Carbon mineralization during the pre-incubation of the biochar-compost mixture reached 109 ± 10 and 159 ± 34 mgC.kg⁻¹ soil in treatments with 0.75 and 1.5 % biochar, respectively. When combined with cumulated CO₂ production during the 112 days of incubation, total C mineralized from pre-incubated treatments reached 1600 ± 125 and 1677 ± 152 mgC.kg⁻¹ for S + 0.75%B + Cp* and S + 1.5%B + Cp*, respectively. These cumulated values of C mineralization were found to be significantly lower than those of the non pre-incubated treatments (two-way Anova, *p-value* < 0.01) with an observed decrease between 19 and 17 % (Fig. 5).

### 3.4. Thermal signature of native and exogenous organic matter

Rock-Eval® thermal analysis revealed that the compost contained a high proportion of hydrocarbons released before 400 °C during the pyrolysis (low R-index = 0.27 ± 0.02; Fig. 6) with 72.7 ± 2.0 % thermolabile.

At day 0 of the incubation experiments, the S2 thermograms of control soil and S + 1.5%B exhibited a unimodal distribution, whereas the

<caption>**Table 3**</caption>

Model parameters obtained for the fitting of mineralized C from soil and organic amendments applied (n = 3). $k_{labile}$ is the apparent mineralization rate constant, SE is the standard error, RSS is the residual sum of square and REC is the estimated recalcitrant pool fraction of organic carbon.

<table><thead><tr><th></th><th>k<sub>labile</sub> ± SE (day<sup>-1</sup>)</th><th>p-value</th><th>REC fraction</th><th>p-value</th><th>RSS</th></tr></thead><tbody><tr><td rowspan="2">Soil</td><td rowspan="2">0.012 ± 0.001</td><td>2.07<sup>e-</sup></td><td>0.75 ±</td><td>&lt; 2<sup>e-</sup></td><td rowspan="2">6640</td></tr><tr><td>07</td><td>0.01</td><td>16</td></tr><tr><td rowspan="2">Compost</td><td rowspan="2">0.012 ± 0.001</td><td>1.20<sup>e-</sup></td><td>0.77 ±</td><td>&lt; 2<sup>e-</sup></td><td rowspan="2">30,800</td></tr><tr><td>15</td><td>0.01</td><td>16</td></tr><tr><td rowspan="2">Compost in mixture</td><td rowspan="2">0.012 ± 0.002</td><td>6.61<sup>e-</sup></td><td>0.75 ±</td><td>&lt; 2<sup>e-</sup></td><td rowspan="2">32,800</td></tr><tr><td>09</td><td>0.02</td><td>16</td></tr><tr><td rowspan="2">Biochar</td><td rowspan="2">0.037 ± 0.008</td><td>1.35<sup>e-</sup></td><td>0.98 ±</td><td>&lt; 2<sup>e-</sup></td><td rowspan="2">30,800</td></tr><tr><td>05</td><td>0.00</td><td>16</td></tr><tr><td rowspan="2">Biochar in mixture</td><td rowspan="2">0.045 ± 0.028</td><td>0.117</td><td>0.98 ±</td><td>&lt; 2<sup>e-</sup></td><td rowspan="2">32,800</td></tr><tr><td></td><td>0.00</td><td>16</td></tr></tbody></table>

<figure><img src="image_8.png" /></figure>

**Fig. 5.** Cumulated carbon mineralization measured in the soil amended with compost and biochar after 112 days of incubation. S + B0.75: soil amended with 0.75 % biochar by weight. The vertical error bars indicate the standard deviation (n = 3). Different letters mean significant difference (p < 0.05) between the treatments.

<figure><img src="image_9.png" /></figure>

**Fig. 6.** Pyrolysis S2 signal vs temperature for the compost with different thermal limits (A1 to A5) according to Malou et al. (2020). The curve represents the mean; the shaded area is the standard error (n = 3).

soil that received compost has a bimodal distribution (Fig. 7b). Compost addition led to a significant enrichment of labile pools, mainly emitted between 340 and 400 °C (A2 interval). Biochar addition did not affect labile pools, but it did enrich refractory pool (A4 interval; Fig. 7c).

Interactive effects between biochar and compost regarding soil C mineralization were investigated combining modelling and Rock-Eval® thermal analysis. The proportion of the recalcitrant fraction of compost (0.75–0.77) estimated from mineralization data was much higher than the Rock-Eval calculated R-index (0.27 ± 0.02). The values for estimating recalcitrant fractions with both methods were close for soil and biochar data (Fig. 8).

At the end of the incubation experiments, S2 thermograms of soils amended with biochar or compost were very similar to those of the control soil (Fig. 7b and c). In addition, S2 thermograms were very similar between days 56 and 112 in each treatment. The contribution of the labile HC pool to the S2 peak in S + Cp decreased from 30 to 18 % between the day 0 and the day 112 of incubation. Nevertheless, at the end of incubation, R-index in S + Cp was 0.62 ± 0.05, a lower value compared to the control soil and S + 1.5%B with R-index values of 0.71 ± 0.05 and 0.72 ± 0.01, respectively (Table S4). Biochar addition (S + 1.5%B) did not increase the soil R-index after 112 days of incubation. But the hydrogen index (HI) mean value in S + 1.5%B (104 ± 13 mgHC. g⁻¹ TOC; Table S4) treatment was considerably lower than that of the soil (249 ± 47 mgHC. g⁻¹ TOC).
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** S2 thermograms obtained by Rock-Eval® pyrolysis at days 0 (yellow, red and black lines), 56 (grey lines) and 112 (grey dotted lines) of incubation for (a) soil, (b) soil + compost and (c) soil + biochar with different thermal limits (A1 to A5) according to Malou et al. (2020). S + B₁.₅: soil amended with 1.5 % biochar by weight.</figcaption></figure>

# 4. Discussion

## 4.1. Influence of organic amendments on soil chemical properties

Compost did not significantly affect soil pH, probably due to its pH close to that of the soil. Biochar only significantly increased soil pH at the highest dose (≈39 t·ha⁻¹). This effect was observed at the start of incubations, but in alkaline soils its duration would be limited due to physical weathering and leaching of soluble elements (Ippolito et al., 2012; Naisse et al., 2015b).

Following compost application, soluble potassium and available phosphorus significantly increased, which would promote plant growth

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Recalcitrant index calculated by Rock-Eval (R-index) and modelling (REC) for the initial soil and organic amendments. The error bars of the R-index are the standard deviation of this parameter calculated from Rock-Eval analysis (n = 2 for soil and n = 3 for biochar and compost) and correspond to the standard errors obtained by modelling for the REC parameter.</figcaption></figure>

in the selected nutrient-depleted soil. However, the water-soluble fraction of the soil experienced a significant increase in the concentrations of undesirable salts, known to degrade soil quality and plant growth (Gondek et al., 2020; Saidi, 2012). The electrical conductivity of the compost exceeded the threshold (i.e. EC₅ > 5 mS·cm⁻¹) suggested by Gondek et al. (2020) for composts with a high concentration of soluble salts. Compost total sodium content (Table S2) was respectively 3 times and more than 10 times higher than the values reported by Ghouili et al. (2023) and El Janati et al. (2022) in composts produced from similar biomasses. Soil water-extractable Na quantity increased by 4 times after compost addition (Table 2). Total K and Fe were also higher compared with the same previous studies. As expected, soil water-extractable K quantity increased by 11 times after compost addition (Table 2). Biochar addition also increased soil water-extractable K content by nearly 8 times. These additions are directly related to the high levels of mineral matter measured in organic amendments, but the lower biochar input limited this contribution. Mineral nutrients such as K and Ca are required for plant growth and can help in reducing soil sodicity by replacing the exchangeable Na + (Gondek et al., 2020). Further research is needed to monitor the release of Na and Cl from compost-amended soil and their potential effects on soil functions, as well as groundwater salinization. Thus, particular care should be paid to the compost application rate in arid regions in soils already affected by salinization processes and to avoid contamination of salt-unaffected soils by the application of a rich-salt compost.

## 4.2. Biodegradability and thermo-degradability of EOM assessed by carbon mineralization and Rock-Eval® thermal analysis

Data obtained from soil CO₂ emissions during incubations showed a slight increase in emitted C but not significant with biochar inputs, whatever the dose. This was consistent with the study of Teutscherova et al. (2017), which reported that an addition of 10 g C·kg⁻¹ soil with holm oak biochar did not significantly affect soil respiration after 60 days of incubation. Since biochar alone may not induce significant increase of C mineralization, as reported in a 1-year incubation study of Miscanthus biochar inoculated with soil solution (Aubertin et al., 2021), our study suggested no significant interaction effect of soil and biochar on C mineralization. The stable organic carbon of date palm biochar was estimated at 98 %, which is in line with values ranging between 91 and 94 % of organic carbon persistence after 100 years reported in a recent reanalysis of a dataset collected from 129 observations (Azzi et al., 2024). High organic carbon persistence from date palm biochar was associated with a very low H/Corg ratio (Table 1) compared with biochars produced from a wide range of feedstocks and pyrolysis
temperatures (Xiao et al., 2016). Moreover, the authors showed that H/C<sub>org</sub> values <0.04 were generally associated with pyrolysis temperatures ≥500 °C, which was not verified in our study, since date palm biochar was produced at 450 ± 5 °C. The H/C<sub>org</sub> ratio of the date palm biochar from our study was very close to those obtained by Usman et al. (2015) with pyrolysis temperatures between 400 and 500 °C. The mineralization rate for the labile carbon pool of biochar (0.037–0.045 g C·kg<sup>-1</sup> day<sup>-1</sup>) was higher than the mean value of 0.013 g C·kg<sup>-1</sup> day<sup>-1</sup> estimated in a meta-analysis (Wang et al., 2016). This could be explained by the relatively low pyrolysis temperature and the experimental duration of <0.5 year, promoting the decomposition of the labile biochar fraction (Kuzyakov et al., 2014).

The apparent mineralized organic carbon fraction of compost (16.7 %) was in agreement with those of a previous study, which reported a median carbon mineralization equal to 18.1 % of added carbon with various composts studied in laboratory conditions after 90 days (Levavasseur et al., 2022). The estimated mineralization rate for compost (0.012 g C·kg⁻¹ day⁻¹) was rather consistent with mean values ranging from 0.001 to 0.01 g C·kg⁻¹ day⁻¹ reported from incubation data with different types of organic amendments modelled using a EOM decomposition model based on the residue decomposition module of the STICS model (Levavasseur et al., 2021). However, the proportion of the recalcitrant fraction of the compost estimated from mineralization data was much higher than expected based on its Rock-Eval calculated R-index, while the values for estimating recalcitrant fractions with both methods appeared rather consistent for soil and biochar data (Fig. 8). Given that (i) the R-index only counts for the HC released between 400 and 650 °C during the pyrolysis stage of the Rock-Eval® analysis (Fig. 6; Sebag et al., 2016) and (ii) thermo-resistant C fraction from compost was mainly emitted as CO₂ during the oxidation stage (Aubertin et al., 2024), we suggest that the remaining part of the recalcitrant fraction from compost was released during the oxidation stage of the thermal analysis.

Albrecht et al. (2015) monitored OM decomposition during the composting process and found an increase in the R-index from 0.34 to 0.50 after 128 days. Another mature compost made from biowastes exhibited a R-index of 0.42 (Ducasse et al., 2023). The Rock-Eval® signature of the mature compost studied matched that of fresh plant material as previously defined by Sebag et al. (2016), with a R-index of 0.27. Results from the thermal analysis evidenced a clear thermo-labile C fraction, as shown by the S2 peak before 400 °C (Fig. 6), that was present only in the initial S + Cp treatment. The removal of this thermo-labile fraction after 56 days of incubation confirms that this fraction was preferentially mineralized and hence, also corresponded to the biologically labile C fraction of the compost.

The characterization of the mineral and organic fractions in the compost pointed at issues relating to the composting process. First, the windrows were spread directly onto a sandy soil surface, allowing the sand particles to adhere when the windrows were turned and irrigated. Moreover, aeolian deposits may have enriched the compost mineral fraction since the windrows were not covered. Watering windrows with highly saline water was probably at the origin of the accumulation of salts, creating unfavourable conditions for OM decomposition by microorganisms. Improving the composting process would require the use of low salt water and a suitable infrastructure to avoid compost enrichment with salts and sand, as well as reducing evaporation from windrows.

### 4.3. Interactive effects of biochar and compost on soil carbon mineralization

The compost apparent carbon mineralization was not affected by biochar application regardless of the dose, suggesting no interaction between these EOMs. This is not in line with several studies, in which the authors measured a reduction of soil total mineralized carbon attributed to an apparent negative priming effect of SOC and/or compost in the presence of biochar (Aubertin et al., 2021; Jien et al., 2015;

Teutscherova et al., 2017). Several studies have reported a positive (Zimmerman et al., 2011), neutral (Cross and Sohi, 2011) and negative priming effect of biochar (Bamminger et al., 2014; Jien et al., 2015; Naisse et al., 2015a; Zheng et al., 2018). According to Zimmerman et al. (2011), priming direction and magnitude depend on biochar properties (selected feedstock and pyrolysis temperature, i.e. biochars produced at low temperatures and from grasses induced positive priming) and also caused a shift in soil microflora in biochar-amended soils (Rasul et al., 2022). Also, previous studies showed that biochar stimulates SOC mineralization during the first weeks, but concluded that the priming direction varied from positive to negative in the long-term (Liu et al., 2017; Maestrini et al., 2015). The shorter duration of the present study but also the variability of measured responses may have limited these observations and prevent us from assessing the potential significance of the observed trends.

In the pre-incubated treatments, however, lower cumulated CO₂ emissions were found compared with the same EOM inputs applied individually. This reduction may be associated with the protection of EOM and/or SOC during the pre-incubation time-period, possibly through physical mechanisms (Jien et al., 2018) thereby leading to an apparent negative priming effect on C mineralized. Abiotic interactions may have occurred during biochar-compost blending, leading to biochar surface functional groups modification and the formation of an organic coating on its surface (Hagemann et al., 2017). Our assumption is that water addition during pre-incubation promoted this mechanism and reduced compost decomposition compared with non pre-incubated treatments. This is consistent with a previous incubation study (Aubertin et al., 2021) as well as a co-composting study, highlighting a shift in microbial composition after biochar addition (Jindo et al., 2012). This highlights the different decomposition behaviour of biochar and compost when applied in soil individually or after a pre-treatment with mixing in wet conditions.

### 4.4. Mineralization of soil inorganic nitrogen

The nitrogen (N) mineralization rate of the compost was 6.0 ± 3.2 % of organic N added, which is considered high for soil improvers like compost (Lashermes et al., 2010). This is much lower than others EOM types like sewage sludges whose N mineralization rate can reach 28 to 43 % (Levavasseur et al., 2021). Due to the high compost dose applied in the soil, the N supply was estimated at 53 kg N·ha⁻¹ in S + Cp treatment, which is a substantial supply for crops. However, the net N mineralization was not significant in this treatment. We observed a slight N immobilization in the first month after compost application. This should be taken into consideration in relation to agronomic implications such as nitrogen requirement for crops and time of compost application. Date palm compost applied alone could result in negative effects on plant N uptake in initial stages due to low N availability. Hence, additional N fertilizer would be recommended to provide adequate N for crop growth, particularly at the initial stage if the compost is incorporated just before sowing.

A reduction in SIN was observed in biochar amended soil, particularly with the highest dose. This observation was previously reported after biochar application in both compost amended and unamended soil (Teutscherova et al., 2017). The authors suggested a limitation of the ammonification due to NH₄⁺ sorption onto the biochar surface. By contrast, another study showed that the addition of a wide range of biochars at a dose of 2 % increased soil N mineralization rate, without clear link between biochars properties (mineralizable-C) and soil N transformation (Phillips et al., 2022). Underlying mechanisms may therefore include inorganic nitrogen sorption or a shift in N microbial transformation. The phenomenon of inorganic N adsorption and/or immobilization mainly depends on biochar feedstock and pyrolysis temperature, soil type and biological N demand (Clough et al., 2013).

Interestingly, pre-incubated treatments compensated for the N limitation observed in compost amended soil at the initial stage. A shift in N
mineralization dynamics may have occurred following the pre-
enrichment of biochar with compost, resulting in a higher N availabil-
ity at short-term. The assumption made is that a time lag may have
modified microbial activity during compost and biochar pre-incubation.

4.5. *Limitations and perspectives*

C and N mineralization dynamics were assessed in controlled con-
ditions. Therefore, the role of soil wetting-drying cycles, mainly driven
by irrigation practices in arid regions, were not taken into consideration.
In the future, the authors recommend expanding the present study to
several types of soils and organic amendments.

Isotopic techniques were not used to partition the origin of organic
carbon mineralization between biogenic/native SOC and EOM pools.
Thus, changes in basal SOC decomposition rate in soil and EOM were
referred to as apparent mineralization.

5. Conclusion

The main objective of this study was to assess the biodegradability of
date palm compost in a typical cultivated alkaline soil of the Saharan
region. Nitrogen and carbon mineralization were monitored in parallel
over a 112-day incubation period without an additional N source.
Compost showed a medium degree of C mineralization (≈16.7 % of
added C after 112 days). Results from the Rock-Eval® thermal analysis
evidenced the presence of a labile C fraction that was preferentially
mineralized in the first 56 days of incubation. Also, salt content was very
high in the final product, assuming that compost application could have
undesirable effects on soil quality. Date palm biochar addition to soil did
not significantly increase emitted C-CO2. Its estimated recalcitrant C
pool was high (98 % of total C), indicating a low labile fraction despite a
pyrolysis temperature of 450 °C.

Nitrogen mineralization dynamics were marked by a limited N
reorganization one month after application in compost amended soil.
Available N tendingially increased after 112 incubation days compared
to the control, with an apparent N supply estimated at 53 kg N·ha⁻¹.
Hence the expected supply of nitrogen for crops with compost was high,
and partly due to the high dose applied (77 t·ha⁻¹), since the N content
in the compost was moderate. Biochar alone slightly reduced soil N
mineralization in a dose-dependent manner. Contrasted behaviour be-
tween the effects of organic amendments applied individually and pre-
incubated was observed for N mineralization. Thus, this enrichment
method appears viable in sandy soil as it doubled the soil N supply in the
short-term (≈112 days of incubation at 25 °C) while further maintaining
organic status.

CRediT authorship contribution statement

E. Le Guyader: Writing – original draft, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. H. Oueriemmi: Writing – review & editing, Data curation. H. Clivot: Writing – review & editing, Investigation, Formal analysis. M. El Mazlouzi: Writing – review & editing. A. Guillaneuf: Writing – review & editing. M. Gommeaux: Writing – review & editing, Supervision. B. Marin: Writing – review & editing, Supervision. I.R. Zoghlami: Writing – review & editing. G. Alavoine: Writing – review & editing. M. Sbih: Writing – review & editing. B. Boumaraf: Writing – review & editing. A. Tirichine: Writing – review & editing. V. Kavvadias: Writing – review & editing. M.L. Aubertin: Writing – review & editing, Investigation. D. Sebag: Writing – review & editing, Investigation. T. Adatte: Writing – review & editing, Data curation. X. Morvan: Writing – review & editing, Supervision.

Funding

This study was funded by PRIMA, a program funded by the EC under

the H2020 framework, and the ANR (ANR-21-PRIM-0004).

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgments

We thank Ali Bennour and Mohamed Moussa of the Institute of Arid regions of Medenine (Tunisia), for enabling the experiments and soil sampling. Rock-Eval® is a trademark registered by IFP Energies Nouvelles.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.geodrs.2025.e01002.

Data availability

Data will be made available on request.

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