# Disentangling the effects of applying pig slurry or its digestate to winter wheat or a catch crop on dissolved C fluxes

Anne-Flore Didelot, Emilie Jardé, Thierry Morvan, Charlotte Lemoine, Florian Gaillard, Gaëlle Hamelin, Anne Jaffrézic

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## To cite this version:

Anne-Flore Didelot, Emilie Jardé, Thierry Morvan, Charlotte Lemoine, Florian Gaillard, et al.. Disentangling the effects of applying pig slurry or its digestate to winter wheat or a catch crop on dissolved C fluxes. Agriculture, Ecosystems & Environment, 2024, 378, pp.109285. 10.1016/j.agee.2024.109285. hal-04705430

HAL Id: hal-04705430

https://hal.science/hal-04705430v1

Submitted on 23 Sep 2024

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# Disentangling the effects of applying pig slurry or its digestate to winter wheat or a catch crop on dissolved C fluxes

Anne-Flore Didelotª, Emilie Jardéᵇ, Thierry Morvanᶜ, Charlotte Lemoineᶜ, Florian Gaillardᶜ,

Gaëlle Hamelinª, Anne Jaffrezicª

**Corresponding author:** Anne-Flore Didelot, [anneflore.didelot@agrocampus-ouest.fr](mailto:anneflore.didelot@agrocampus-ouest.fr)

UMR SAS, 65 Rue de Saint-Brieuc, 35042 Rennes France

**Keywords:** biogas digestate, DOC, DIC, winter crop

## Abstract

The anaerobic digestion industry, which is still developing, generates biogas from organic waste products. A co-product of this process, digestate, is increasingly produced and can be recycled on agricultural land as an alternative to mineral fertilizers. Biogas digestate is a recent product whose chemical composition differs from that of its source material, and additional data still need to be acquired on its effects on dissolved carbon fluxes. The objectives of this study were to assess (i) the effects of applying biogas digestate on dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) fluxes with different winter crops, (ii) the dynamics of DOC and DIC concentrations during the drainage season, and (iii) the annual dynamics of DOC and DIC fluxes along the soil profile. The study examined effects of applying biogas digestate, pig slurry,

<footnote>ª Institut Agro, INRAE, SAS, 35000, Rennes, France</footnote>

<footnote>ᵇ Univ Rennes, CNRS, Géosciences Rennes - UMR 6118, Rennes, France</footnote>

<footnote>ᶜ INRAE, Institut Agro, SAS, 35000 Rennes, France</footnote>
or a mineral fertilizer to winter wheat and two catch crops (mustard and a multispecies crop) on DOC and DIC fluxes in the soil. Lysimeters at 40 cm (topsoil) and 90 cm (subsoil) depths were monitored from 2014-2023, from November to March (i.e., 9 winter drainage seasons). During the drainage season, the DOC concentration was highest with digestate, and its timing depended on development of the cover crop: from the beginning of the drainage season for mustard and the multispecies crop and around February for wheat. Applying digestate increased the topsoil DOC fluxes (mean of 35.7 ± 13.7 kg.ha⁻¹ with digestate vs. 21.0 ± 6.7 kg.ha⁻¹ with the other treatments), particularly under mustard. Topsoil DIC fluxes were highest with pig slurry due to higher mineralization than that with digestate (mean of 59.1 ± 22.8 kg.ha⁻¹ with pig slurry vs. 46.2 ± 16.3 kg.ha⁻¹ with the other treatments). In the subsoil, DOC fluxes were low (6.2 ± 4.1 kg.ha⁻¹) and DIC fluxes were high (80.0 ± 45.7 kg.ha⁻¹), with no difference among treatments.

# 1. Introduction

The anaerobic digestion industry has developed throughout the 21st century in Europe (Scarlat et al., 2018), which counted more than 18,900 biogas plants in 2019 (Iglesias et al., 2021). Since the energy produced is sustainable, it could be an interesting contribution to one of the European Union's targets for 2030, which is to increase the percentage of renewable energy to 45% of the total energy consumption. Anaerobic digestion can generate biogas from organic waste products from livestock. Thus, digestate, a co-product of anaerobic digestion, is increasingly produced.
Despite the digestion, digestate retains high nutrients concentrations (Czekała, 2022) and can be recycled on agricultural land as an alternative to synthetic mineral fertilizers. Digestate also contains organic carbon (C), which could improve soil quality and increase the amount of C stored in the subsoil by providing dissolved organic C (DOC), which is then stabilized by interacting with the mineral phase (Rumpel and Kögel-Knabner, 2011). Storing organic C in the soil is one of the solutions showcased to reduce the impact of climate change and to preserve soils and food security, as in the French initiative "4 per 1000" launched in 2015 during the COP21 (Chenu et al., 2019).

The composition of digestate differs from that of the organic waste product from which it is derived. Most of the labile fraction of the initial product, such as carbohydrate-like molecules, is consumed by microorganisms during anaerobic digestion. Thus, the digestate concentrates recalcitrant molecules such as lignin and non-hydrolyzable lipids that have higher biological stability (Tambone et al., 2009). Because of this distinct composition, digestate may have different effects on the soil, and more specifically on fluxes of elements dissolved in soil water. An excess of these fluxes could lead to negative consequences and threaten aquatic ecosystems, since DOC can transport pollutants (Dunnivant et al., 1992) or form carcinogenic trihalomethanes during water purification (Li et al., 1996). Moreover, dissolved inorganic C (DIC) contains dissolved CO₂, some of which is emitted into the atmosphere when groundwater joins the surface water (Kindler et al., 2011). Thus, it is essential to assess effects of applying digestate on dissolved C fluxes in the soil.

To do so, it is also important to consider plants, which also influence these fluxes. Winter crops have many benefits, such as increasing the soil organic matter content, conserving mineral nitrogen (N), and preventing soil erosion and weed development (Hartwig and Ammon, 2002).
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Crops can provide organic matter to the soil via the degradation of residues (roots, leaves, stems) and root exudation (Kalbitz et al., 2000). Root exudates are low molecular weight compounds (Nguyen, 2009) and thus, can contribute to the DOC pool. About 30-60% of the carbon fixed by photosynthesis by annual plants is translocated to the roots and up to 70% of this carbon can be released into the soil (Neumann and Römheld, 2000). Crops can also provide inorganic C to the soil by root respiration (Viktor and Cramer, 2003).

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Despite a growing interest in studying digestate and the effects of applying it on the soil, this product is relatively new, and additional data still need to be acquired, particularly in association with different crop species. The objectives of this study were to assess (i) the effects of biogas digestate from pig slurry on DOC and DIC fluxes in different cropping contexts, (ii) the dynamics of DOC and DIC concentrations during the drainage season, and (iii) the annual dynamics of DOC and DIC fluxes along the soil profile. We compared the soil water collected by lysimeters at 40 and 90 cm depths for three treatments: biogas digestate from pig slurry, raw slurry, and mineral nitrogen (the control). We hypothesized that there could be more DOC when applying digestate due to its recalcitrant chemical composition.

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## 2. Materials and methods

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### 2.1 *Study site and experimental design*

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The study site, EFELE, forms part of the SOERE PRO network (https://www6.inra.fr/valor-pro) (see Morvan et al. (2020) for a complete description). EFELE is located near Rennes, in western France (48.1019444° N, -1.7955555° W), and its experiment, started in 2012, compares effects of applying pig slurry or digestate derived from pig slurry on soil and water quality to those of

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mineral N fertilization. The experiment follows a complete randomized block design with 4 replicates, with each block consisting of 9 plots of 113 m² each.

The crop rotation (Table 1) consists of a 2-year succession of maize (Zea mays) and winter wheat (Triticum aestivum) with a N catch crop of white mustard (Sinapis alba) or a multispecies catch crop composed of white mustard, rapeseed (Brassica napus), clover (Trifolium alexandrinum), and phacelia (Phacelia tanacetifolia) sown at the beginning of September (2 months after the wheat harvest). The mineral fertilizer, pig slurry and digestate treatments were applied every year in April to the growing wheat or before the sowing of maize (Table 2).

Organic waste products application rates were determined by their N fertilizer value, in compliance with the European Union Nitrates Directive (91/676/EEC). The annual application rates and the inputs of mineral and organic N varied among the 9 years of the experiment analyzed in the present study (2014-2023) (Table 2). The rate of mineral N fertilizer applied to the crop was calculated using the mineral N balance-sheet method recommended in France (Machet et al., 2017).

<table><caption>Table 1. Two-year crop rotation of winter wheat (W), a catch crop (Cc, mustard or a multispecies crop composed of mustard, rapeseed, clover, and phacelia), and maize (M). The drainage season is represented in dark gray. Mineral nitrogen fertilizer, pig slurry, or digestate from pig slurry is applied in April of each year (circled).</caption><thead><tr><th>J</th><th>F</th><th>M</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th><th>W</th></tr></thead><tbody><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

The 2020-2021 season was unusual, because a catch crop of mustard instead of winter wheat was sown after the maize harvest. Unlike the other seasons with a mustard catch crop, the latter was sown later, at the end of September, and its cover developed less.
<table><caption>Table 2. Application rates and amounts of organic carbon (C) and mineral nitrogen (N) applied to the given crop for the three treatments (mineral N fertilizer, pig slurry, or digestate from pig slurry) from 2014-2022. Mineral N equals the sum of the ammoniacal N of the organic waste product and the N from the mineral fertilizer.</caption><thead><tr><th rowspan="2">Treatment</th><th rowspan="2">Content</th><th colspan="2">2014</th><th colspan="2">2015</th><th colspan="2">2016</th><th colspan="2">2017</th><th colspan="2">2018</th><th colspan="2">2019</th><th colspan="2">2020</th><th colspan="2">2021</th><th colspan="2">2022</th></tr><tr><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th><th>Maize</th><th>Wheat</th></tr></thead><tbody><tr><td rowspan="2">Mineral fertilizer</td><td>N (kg N.ha<sup>-1</sup>)</td><td>102</td><td>120</td><td>96</td><td>80</td><td>126</td><td>145</td><td>120</td><td>124</td><td>149</td><td>126</td><td>145</td><td>120</td><td>124</td><td>149</td><td>126</td><td>145</td><td>120</td><td>124</td><td>149</td></tr><tr><td>Application rate (t.ha<sup>-1</sup>)</td><td>25</td><td>32.8</td><td>23.4</td><td>22.5</td><td>21.2</td><td>40</td><td>41</td><td>41</td><td>41</td><td>48.1</td><td>25</td><td>32.8</td><td>23.4</td><td>22.5</td><td>21.2</td><td>40</td><td>41</td><td>41</td><td>48.1</td></tr><tr><td rowspan="2">Pig slurry</td><td>Organic C (kg C.ha<sup>-1</sup>)</td><td>519</td><td>1023</td><td>724</td><td>796</td><td>684</td><td>617</td><td>537</td><td>1880</td><td>120</td><td>519</td><td>1023</td><td>724</td><td>796</td><td>684</td><td>617</td><td>537</td><td>1880</td><td>120</td></tr><tr><td>Mineral N (kg N.ha<sup>-1</sup>)</td><td>114</td><td>160</td><td>98</td><td>89</td><td>102</td><td>92</td><td>101</td><td>226</td><td>148</td><td>114</td><td>160</td><td>98</td><td>89</td><td>102</td><td>92</td><td>101</td><td>226</td><td>148</td></tr><tr><td rowspan="2">Digestate</td><td>Application rate (t.ha<sup>-1</sup>)</td><td>28</td><td>27.3</td><td>40</td><td>24.4</td><td>24.7</td><td>40</td><td>41</td><td>41</td><td>41</td><td>41</td><td>28</td><td>27.3</td><td>40</td><td>24.4</td><td>24.7</td><td>40</td><td>41</td><td>41</td><td>48.8</td></tr><tr><td>Organic C (kg C.ha<sup>-1</sup>)</td><td>556</td><td>694</td><td>361</td><td>641</td><td>414</td><td>570</td><td>884</td><td>967</td><td>682</td><td>556</td><td>694</td><td>361</td><td>641</td><td>414</td><td>570</td><td>884</td><td>967</td><td>682</td></tr><tr><td></td><td>Mineral N (kg N.ha<sup>-1</sup>)</td><td>132</td><td>135</td><td>92</td><td>111</td><td>103</td><td>110</td><td>121</td><td>176</td><td>214</td><td>132</td><td>135</td><td>92</td><td>111</td><td>103</td><td>110</td><td>121</td><td>176</td><td>214</td></tr></tbody></table>

## 2.2 Soil description

The soil is classified as a Luvisol-redoxisol derived from aeolian silt deposited on schist material (see Morvan et al. (2020) for details). The main characteristics of the 0-30 cm layer at the beginning of the experiment were 14.2% clay, 71.0% silt, 14.8% sand, 1.1% organic C, a C:N ratio of 9.6, and pH of 6.1 (slightly acidic).

## 2.3 Climate data

Climate data were obtained from an automatic weather station located at the study site (Campbell Scientific CR10 0 0 logger and weather station). The climate is mild oceanic temperate, with a mild winter and warm summer. From 2014-2022, the mean annual temperature was 12.8°C, annual rainfall averaged 681 mm, and Penman potential evapotranspiration was 731 mm, respectively.
<table><caption>Table 3. Drainage sampling period, winter rainfall, and drainage for each drainage season studied (mean ± 1 standard deviation). Winter rainfall equals the sum of rainfall received during the sampling period (from 1 November to 31 March of the following year).</caption><thead><tr><th rowspan="2">Drainage season</th><th rowspan="2">Winter crop</th><th rowspan="2">Winter rainfall (mm)‡</th><th rowspan="2">Sampling period†</th><th rowspan="2">No. of sampling dates</th><th colspan="2">Drainage (mm)</th></tr><tr><th>40 cm depth</th><th>90 cm depth</th></tr></thead><tbody><tr><td>2014-2015</td><td>Winter wheat</td><td>295 (- 24)</td><td>8 Dec. 2014-24 Mar. 2015</td><td>7</td><td>226 ± 30</td><td>298 ± 45</td></tr><tr><td>2015-2016</td><td>Mustard</td><td>384 (+ 65)</td><td>5 Jan. 2016-14 Mar. 2016</td><td>6</td><td>213 ± 21</td><td>236 ± 46</td></tr><tr><td>2016-2017</td><td>Winter wheat</td><td>253 (- 66)</td><td>1 Feb. 2017-16 Mar. 2017</td><td>4</td><td>97 ± 8</td><td>27 ± 15</td></tr><tr><td>2017-2018</td><td>Mustard</td><td>367 (+ 48)</td><td>21 Dec. 2017-28 Mar. 2018</td><td>7</td><td>262 ± 33</td><td>281 ± 43</td></tr><tr><td>2018-2019</td><td>Winter wheat</td><td>279 (- 40)</td><td>27 Dec. 2018-20 Mar. 2019</td><td>3</td><td>149 ± 25</td><td>120 ± 13</td></tr><tr><td>2019-2020</td><td>Mustard</td><td>512 (+ 193)</td><td>14 Nov. 2019-23 Dec. 2019</td><td>5</td><td>170 ± 30</td><td>191 ± 35</td></tr><tr><td>2020-2021</td><td>Mustard</td><td>301 (- 18)</td><td>2 Nov. 2020-1 Mar. 2021</td><td>6</td><td>241 ± 44</td><td>221 ± 37</td></tr><tr><td>2021-2022</td><td>Winter wheat</td><td>220 (- 99)</td><td>13 Dec. 2021-30 Mar. 2022</td><td>5</td><td>135 ± 21</td><td>124 ± 31</td></tr><tr><td>2022-2023</td><td>Multispecies</td><td>377 (+ 58)</td><td>5 Dec. 2022-22 Mar. 2023</td><td>5</td><td>204 ± 51</td><td>158 ± 36</td></tr></tbody></table>

## 2.4 Drained water sampling

In pits opened in spring 2011 on six plots, two pairs of fiberglass wick lysimeters were installed in each plot, one in the topsoil (40 cm depth) and one in the subsoil (90 cm depth). Fiberglass wick lysimeters have been shown to be most effective at collecting soil leachate (Sansoulet et al., 2008) and can sample the large volumes of water necessary for subsequent analyses. Each lysimeter (area of 0.125 m²) is made from a 50 × 25 × 2 cm stainless steel plate, with the fiberglass wick unfolded onto the plate before applying the plate up against the soil layers above. The entire wick goes through a central hole in the plate and down a Tygon® tube to a receiving glass flask (10 L). Six flasks were equipped with ultrasonic sensors (Telemecanique Sensor XX918A3C2M12) to monitor the filling dynamics of the flasks and to collect the water rapidly after the end of the drainage, before its composition could change. Drainage is relatively

<footnote>† First date of collection – Last date of collection</footnote>

<footnote>‡ Values in parentheses are the difference compared to the 30-year mean (319 mm).</footnote>
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moderate under the climate conditions at the EFELE site, and the number of sampling dates
per season ranged from 3-10 depending on the amount of winter rainfall.

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## 2.5 Sample collection and analysis

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Drainage water was sampled as soon as possible after the end of the drainage event. Water samples were filtered at 0.45 µm and stored at 4°C until analysis. DOC and DIC concentrations were determined using a TOC-VCSH analyzer (Shimadzu Corp., Kyoto, Japan) using thermal oxidation.

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## 2.6 Water balance and calculation of fluxes

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DOC and DIC fluxes were calculated by multiplying the drainage estimated from the water balance by the DOC and DIC concentrations, respectively. The water balance was predicted using the STICS (multidisciplinary simulator for standard crops) model (Brisson et al., 1998, http://www6.paca.inra.fr/stics_eng), with the model being initialized for the first date of water collection by the lysimeters and predicting drainage from the soil layers as a function of soil field capacity.

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The 2019-2020 drainage season was unusual: due to high rainfall (512 mm) (Table 3) the groundwater table rose into the soil profile at the end of December, which disrupted drainage, and the water sampling had to be stopped early. Consequently, this season was excluded when calculating the fluxes (Table 4), but its undisrupted sampling dates (14 Nov.-23 Dec. 2019) were kept when analyzing concentration dynamics (Figures 2-4).

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## 2.7 Statistical analysis

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All statistical analysis was performed using R software (R Core Team, 2017). The influence of crop, fertilization type, drainage, and their interactions on annual fluxes of DOC and DIC were
assessed using analysis of variance (ANOVA). The assumptions of ANOVA were checked by using the Shapiro-Wilk and Bartlett's tests. When needed, the data were transformed to satisfy these assumptions. Differences were considered significant at p < 0.05, and means were compared using Tukey's range test. When the latter's parametric conditions could not be met, Kruskal-Wallis and Dunn's tests were performed instead.

Four statistical models were applied:

* Model 1: to the concentration dataset, by drainage date and depth:

$$Y = \mu + Tr_i + \varepsilon_i \quad (1)$$

* Model 2: to the seasonal cumulative fluxes of the entire dataset, by depth:

$$Z = \mu + Tr_i + Cr_i + Dr_i + (TrCr)_{ij} + (CrDr)_{jk} + (TrDr)_{ik} + \varepsilon_{ijk} \quad (2)$$

* Model 3: to the seasonal cumulative fluxes dataset, by crop and depth:

$$Z = \mu + Tr_i + Dr_i + (DrTr)_{ij} + \varepsilon_i \quad (3)$$

* Model 4: to the seasonal cumulative fluxes dataset, by drainage season, crop and depth:

$$Z = \mu + Tr_i + \varepsilon_i \quad (4)$$

Where Y is a response variable (DOC or DIC concentration on a given drainage date), Z is a response variable (total amount of leached DOC or DIC fluxes), μ is population mean, Dr is the amount of drained water, Cr is the type of crop, Tr is the organic waste product treatment, and ε is the residual error.
## 3. Results

### 3.1 Drained water

The cumulative volume of drained water ranged from 97 ± 8 (2016-2017) to 262 ± 33 mm (2017-2018) at 40 cm depth and 27 ± 15 (2016-2017) to 298 ± 45 mm (2014-2015) at 90 cm depth. At both depths, drainage varied greatly among seasons (Figure 1), in accordance with the high variability in rainfall (220-512 mm) (Table 3). Most drainage seasons started between November and January, except for 2016-2017, which started in February, lasted only one month, had much less rainfall (253 mm, compared to the mean of 332 mm), and resulted in very low drainage volumes, particularly at 90 cm depth (Table 3). The seasons with the lowest rainfall and drainage volumes, such as 2016-2017, 2018-2019, and 2021-2022, coincided with the growing of wheat (Table 3). Overall, similar amounts of water were collected at 40 (189 ± 54 mm) and 90 cm (184 ± 86 mm) during each drainage season; thus, water flowed through the soil profile and was not retained between the two depths.

<figure><img src="image_3.png" /></figure>

<caption>Figure 1. Mean cumulative amount of leached water collected by the lysimeters at (A) 40 cm and (B) 90 cm depths during each drainage season. See Table 3 for the standard errors.</caption>
<figure><img src="image_4.png" /><figcaption>218</figcaption></figure>

Figure 2. Dissolved organic carbon (DOC) concentrations (mg.L⁻¹) (mean of two lysimeter replicates) in topsoil (40 cm) water during the 9-year study under winter wheat (A to D), a mustard catch crop (E to H), or a multispecies catch crop (I) for the three treatments: mineral nitrogen fertilizer, pig slurry, or digestate from pig slurry. Sampling during the 2019-2020 drainage season was interrupted after 23 Dec. 2019. Error bars are standard deviations. The treatment effect was tested for each date using ANOVA. Significance: *** p < 0.001; ** p < 0.01; * p < 0.05; . p < 0.1; NS p > 0.1.
223 3.2 Effect of the crop on DOC and DIC concentrations

224 Trends in topsoil DOC concentrations differed by crop. For the seasons under wheat, except for
225 2016-2017 and 2021-2022, DOC concentrations increased from the beginning of the drainage
226 season, peaked in February-March, and then decreased (Figure 2.A-D). Mean DOC
227 concentrations of the four seasons started the season at 11.7 ± 6.6 mg.L⁻¹ and ended the season
228 at 16.7 ± 8.4 mg.L⁻¹. For the 2016-2017 season, whose drainage started later, mean DOC
229 concentrations were higher throughout the season (seasonal mean: 23.2 ± 7.2 mg.L⁻¹, 1.6 times
230 higher than that of the four seasons of 14.2 ± 7.0 mg.L⁻¹). During the 2021-2022 season, the
231 DOC concentrations remained relatively stable.

232 For the five seasons under the catch crop of mustard (Figure 2.E-H) or multispecies catch crop
233 (Figure 2.I), trends in mean topsoil DOC concentrations differed markedly from the those
234 observed under wheat and were repeatable. They started the season at 20.9 ± 8.7 mg.L⁻¹ and
235 continuously decreased to end the season at 9.7 ± 2.4 mg.L⁻¹ (seasonal mean: 14.5 ± 6.8 mg.L⁻¹).
236 Compared to the seasons under wheat, initial DOC concentrations under the catch crops were
237 1.8 times higher and 0.6 times lower at the end of the drainage season, with a similar seasonal
238 mean. Regarding the unusual 2020-2021 season, during which mustard was sown instead of
239 wheat after the maize harvest, DOC concentrations at the beginning of the season were lower
240 (14.4 ± 4.8 mg.L⁻¹) than the mean DOC concentration of the five seasons under a catch crop,
241 but the concentrations at the end of the season (8.6 ± 2.5 mg.L⁻¹) were similar.

242 Subsoil DOC concentrations remained low and stable during all seasons and did not differ
243 among the crops. Mean seasonal DOC concentrations under wheat (6.2 ± 9.1 mg.L⁻¹) were
244 similar to those under the mustard and multispecies catch crops (4.3 ± 6.1 mg.L⁻¹) (Figure S1.A-
245 I).
Regardless of the crop, mean seasonal DIC concentrations were 29.0 ± 16.8 mg.L⁻¹ in the topsoil (Figure 3) and 50.5 ± 24.1 mg.L⁻¹ in the subsoil (Figure S2). Trends in topsoil DIC concentrations differed from those of DOC concentrations. They did not vary during several seasons, but did decrease during three seasons of a mustard or multispecies catch crop (2015-2016, 2020-2021, and 2022-2023) and one season of wheat (2016-2017). Under wheat, mean DIC concentrations were 29.1 ± 12.1 mg.L⁻¹ in the topsoil (Figure 3.A-D) and 51.0 ± 24.5 mg.L⁻¹ in the subsoil (Figure S2.A-D). Under mustard and multispecies catch crops, DIC concentrations were 29.0 ± 19.0 mg.L⁻¹ in the topsoil (Figure 3.E-I) and 50.3 ± 24.0 mg.L⁻¹ in the subsoil (Figure S2.E-I).

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### 3.3 Treatment effects on DOC and DIC concentrations

To assess effects of the treatment on DOC and DIC concentrations, ANOVA model (1) was applied and detected significant differences among crops in the dynamics of DOC concentrations. In the topsoil under wheat, DOC concentrations differed little among treatments at the beginning of the drainage season but did differ later, being higher with digestate (mean of 17.6 ± 7.3 mg.L⁻¹ for the four seasons on the first date when differences became significant) than with pig slurry or mineral fertilizer (11.5 ± 4.2 mg.L⁻¹) (Figure 2.A-D). During the 2014-2015, 2016-2017, and 2018-2019 seasons, DOC concentrations increased under digestate in February-March. Under the mustard and multispecies catch crops, mean DOC concentrations at the beginning of the drainage season were significantly higher with digestate (30.1 ± 8.8 mg.L⁻¹) than with pig slurry or mineral fertilizer (16.4 ± 3.7 mg.L⁻¹) (Figure 2.E-I). DOC concentrations with digestate decreased consistently to become similar to those with pig slurry or mineral fertilizer at the end of the drainage season (9.7 ± 2.4 mg.L⁻¹). Mean DOC concentrations in the subsoil did not differ among treatments (Figure S1).

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269 Mean DIC concentrations in the topsoil with pig slurry, digestate, and mineral fertilizer were

270 33.5 ± 16.3, 27.9 ± 19.5, and 25.7 ± 13.2 mg.L⁻¹, respectively (Figure 3). Mean DIC

271 concentrations in the subsoil did not differ among treatments (Figure S2).
<figure><img src="image_5.png" /></figure>

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273 **Figure 3.** Dissolved inorganic carbon (DIC) concentrations (mg.L⁻¹) (mean of two lysimeter replicates) in the topsoil (40 cm) water during the 9-year

274 study under winter wheat (A to D), a mustard catch crop (E to H), or a multispecies catch crop (I) for the three treatments: mineral nitrogen fertilizer,

275 pig slurry, or digestate from pig slurry. Sampling of the 2019-2020 drainage season was interrupted after 23 Dec. 2019. Error bars are standard

276 deviations. The treatment effect was tested for each date using ANOVA. Significance: *** p < 0.001; ** p < 0.01; * p < 0.05; . p < 0.1; NS p > 0.1.
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## 3.4 Inter-seasonal variability in DOC and DIC fluxes

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Annual DOC fluxes ranged from 10.5-50.5 kg.ha⁻¹ in the topsoil and 1.0-14.1 kg.ha⁻¹ in the subsoil, while annual DIC fluxes ranged from 26.0-86.9 kg.ha⁻¹ in the topsoil and 19.9-174.4 kg.ha⁻¹ in the subsoil (Table 4). Mean DOC fluxes were particularly low during the 2021-2022 season in both the topsoil (11.7 ± 1.6 kg.ha⁻¹) and subsoil (1.4 ± 0.7 kg.ha⁻¹). The dissolved C fluxes varied greatly depending on the drainage season and the crop.

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When applying ANOVA model (2), the Crop, Drainage, and Treatment had significant (p < 0.001) effects on topsoil DOC fluxes. The Treatment:Crop and Crop:Drainage interactions were also significant (p < 0.001), implying that the Treatment effect must be interpreted as a function of the Crop effect, and the Crop effect as a function of the Drainage effect. The Crop:Drainage interaction was in accordance with the fact that the seasons with lower rainfall and drainage volumes (2016-2017, 2018-2019, and 2021-2022 seasons) coincided with the growing of wheat (Table 3). Mean DOC fluxes in the topsoil were significantly (p < 0.001) higher for the three seasons under mustard (33.4 ± 11.2 kg.ha⁻¹) than for the season under the multispecies crop (27.1 ± 9.3 kg.ha⁻¹) or the four seasons under wheat (20.0 ± 9.5 kg.ha⁻¹). The mean DOC fluxes under the crops resulted not only from the Crop effect, but from that of the Crop:Drainage interaction. Thus, ANOVA model (3) was applied to test the Drainage effect separately for each crop. It revealed that the Drainage effect was significant for wheat (p < 0.001) and nearly so for mustard (p = 0.051), with no Treatment:Crop interaction for wheat but a significant Treatment:Crop interaction (p < 0.05) for mustard. This effect of the Treatment:Crop interaction led to the application of Model (4) to each drainage season (section 3.5).

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Applying ANOVA model (2) to subsoil DOC fluxes indicated that only Crop had a significant effect (p < 0.001). Mean DOC fluxes were 9.2 ± 4.4 kg.ha⁻¹ under mustard and 4.4 ± 2.6 kg.ha⁻¹ under wheat and the multispecies crop.

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For topsoil DIC fluxes, applying ANOVA model (2) revealed that Crop, Drainage, and Treatment had significant (p < 0.001) effects and no interactions. Mean DIC fluxes were significantly higher under mustard and the multispecies crop (62.4 ± 16.5 kg.ha⁻¹) than under wheat (38.5 ± 14.3 kg.ha⁻¹). In the subsoil, only Crop and Drainage had a significant effect (p < 0.001), with 120.1 ± 38.2 kg.ha⁻¹ under mustard, 68.1 ± 11.9 kg.ha⁻¹ under the multispecies crop, and 52.9 ± 33.2 kg.ha⁻¹ under wheat.

### 3.5 Treatment effect on DOC and DIC fluxes

When applying ANOVA model (2) without separation by crop, the Treatment effect was significant for both DOC (p < 0.001) and DIC fluxes (p < 0.001) in the topsoil but not in the subsoil. To test the Treatment effect while avoiding the Drainage effect, ANOVA model (4) was applied to each drainage season.

In the topsoil, except for the 2021-2022 season, which showed no difference among treatments, DOC fluxes were always higher with digestate (mean of 35.7 ± 13.7 kg.ha⁻¹ for the eight seasons) than with pig slurry or mineral fertilizer, regardless of the crop (Table 4). DOC fluxes with pig slurry and mineral fertilizer for the eight seasons were similar (mean of 21.0 ± 6.7 kg.ha⁻¹) and nearly 2.0 times lower than those with digestate. DIC fluxes in the topsoil were significantly higher with pig slurry (59.1 ± 22.8 kg.ha⁻¹) than with mineral fertilizer or digestate (46.2 ± 16.3 kg.ha⁻¹). The Treatment effect, tested individually for each season to avoid the Drainage effect, was significant only for the 2014-2015 and 2021-2022 seasons, which were under wheat.
<table><caption>Table 4. Dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) fluxes (kg.ha⁻¹) (mean of two lysimeter replicates) in the topsoil and subsoil during the 8-season study under winter wheat, a mustard catch crop, or a multispecies catch crop for the three treatments: mineral nitrogen fertilizer, pig slurry, or digestate from pig slurry. The 2019-2020 season was excluded from the flux dataset. Letters indicate significant differences among DOC fluxes (lowercase) or DIC fluxes (uppercase) based on ANOVA for each drainage season, depth, and crop separately.</caption><thead><tr><th rowspan="2">Depth</th><th rowspan="2">Treatment</th><th rowspan="2">C flux</th><th colspan="4">Wheat</th><th colspan="3">Mustard</th><th rowspan="2">Multispecies</th><th rowspan="2">Mean</th></tr><tr><th>2014-2015</th><th>2016-2017</th><th>2018-2019</th><th>2021-2022</th><th>2015-2016</th><th>2017-2018</th><th>2020-2021</th></tr></thead><tbody><tr><td rowspan="3">40 cm</td><td rowspan="2">Mineral fertilizer</td><td>DOC</td><td>29.5 ± 4.5<sup>b†</sup></td><td>12.6 ± 0.3<sup>c</sup></td><td>14.9 ± 0.2<sup>b</sup></td><td>10.5 ± 0.2<sup>a</sup></td><td>26.2 ± 3.8<sup>b</sup></td><td>30.4 ± 4.4<sup>b</sup></td><td>21.7 ± 7.2<sup>b</sup></td><td>21.5 ± 0.5<sup>b</sup></td><td>20.9 ± 7.8</td></tr><tr><td>DIC</td><td>53.9 ± 0.8<sup>B</sup></td><td>28.2 ± 0.3<sup>A</sup></td><td>26.4 ± 1.9<sup>A</sup></td><td>30.3 ± 0.7<sup>B</sup></td><td>62.4 ± 7.6<sup>A</sup></td><td>37.6 ± 8.5<sup>A</sup></td><td>55.7 ± 4.1<sup>A</sup></td><td>65.1 ± 4.3<sup>A</sup></td><td>45.0 ± 15.9</td></tr><tr><td>Total</td><td>83.4</td><td>40.8</td><td>41.3</td><td>40.8</td><td>88.6</td><td>68</td><td>77.4</td><td>86.6</td><td>65.9 ± 21.6</td></tr><tr><td rowspan="3">40 cm</td><td rowspan="2">Pig slurry</td><td>DOC</td><td>23.7 ± 0.2<sup>b</sup></td><td>16.8 ± 0.3<sup>b</sup></td><td>16.8 ± 1.4<sup>b</sup></td><td>11.6 ± 1.7<sup>a</sup></td><td>25.3 ± 3.2<sup>b</sup></td><td>25.0 ± 0.8<sup>b</sup></td><td>28.9 ± 2.9<sup>ab</sup></td><td>20.8 ± 3.1<sup>b</sup></td><td>21.1 ± 5.7</td></tr><tr><td>DIC</td><td>72.4 ± 2.0<sup>A</sup></td><td>32.8 ± 2.5<sup>A</sup></td><td>28.6 ± 3.6<sup>A</sup></td><td>47.8 ± 1.2<sup>A</sup></td><td>86.9 ± 6.4<sup>A</sup></td><td>45.7 ± 3.1<sup>A</sup></td><td>81.3 ± 11.0<sup>A</sup></td><td>76.9 ± 15.2<sup>A</sup></td><td>59.1 ± 22.8</td></tr><tr><td>Total</td><td>96.1</td><td>49.6</td><td>45.4</td><td>59.4</td><td>112.2</td><td>70.7</td><td>110.2</td><td>97.7</td><td>80.2 ± 27.1</td></tr><tr><td rowspan="3">40 cm</td><td rowspan="2">Digestate</td><td>DOC</td><td>44.5<sup>a</sup></td><td>22.1 ± 1.6<sup>a</sup></td><td>23.6 ± 2.0<sup>a</sup></td><td>13.1 ± 1.7<sup>a</sup></td><td>49.6 ± 3.2<sup>a</sup></td><td>50.5 ± 2.3<sup>a</sup></td><td>43.1 ± 0.6<sup>a</sup></td><td>39.0 ± 1.3<sup>a</sup></td><td>35.7 ± 13.7</td></tr><tr><td>DIC</td><td>50.9 ± 2.3<sup>B</sup></td><td>28.3 ± 2.6<sup>A</sup></td><td>26.0 ± 1.7<sup>A</sup></td><td>36.8 ± 1.5<sup>C</sup></td><td>64.6 ± 11.7<sup>A</sup></td><td>40.1 ± 1.8<sup>A</sup></td><td>73.9 ± 3.8<sup>A</sup></td><td>58.9 ± 0.6<sup>A</sup></td><td>47.4 ± 17.2</td></tr><tr><td>Total</td><td>95.4</td><td>50.4</td><td>49.6</td><td>49.9</td><td>114.2</td><td>90.6</td><td>117</td><td>97.9</td><td>83.1 ± 28.9</td></tr><tr><td rowspan="3">90 cm</td><td rowspan="2">Mineral fertilizer</td><td>DOC</td><td>6.1 ± 0.1<sup>a</sup></td><td>5.6 ± 0.9<sup>a</sup></td><td>1.3 ± 1.4<sup>b</sup></td><td>1.0 ± 0.4<sup>a</sup></td><td>14.1 ± 5.6<sup>a</sup></td><td>10.7 ± 5.5<sup>a</sup></td><td>5.1 ± 2.5<sup>a</sup></td><td>3.4 ± 0.1<sup>a</sup></td><td>5.9 ± 4.9</td></tr><tr><td>DIC</td><td>105.2 ± 17.4<sup>A</sup></td><td>27.7 ± 3.6<sup>A</sup></td><td>30.9 ± 9.5<sup>A</sup></td><td>40.7 ± 5.6<sup>C</sup></td><td>131.1 ± 38.2<sup>A</sup></td><td>87.5 ± 9.8<sup>A</sup></td><td>99.8 ± 15.2<sup>A</sup></td><td>55.3 ± 9.3<sup>A</sup></td><td>72.3 ± 39.5</td></tr><tr><td>Total</td><td>111.3</td><td>33.3</td><td>32.2</td><td>41.7</td><td>145.2</td><td>98.2</td><td>104.9</td><td>58.7</td><td>78.2 ± 42.3</td></tr><tr><td rowspan="3">90 cm</td><td rowspan="2">Pig slurry</td><td>DOC</td><td>5.6 ± 1.7<sup>a</sup></td><td>5.9 ± 1.6<sup>a</sup></td><td>3.3 ± 0.6<sup>ab</sup></td><td>2.2 ± 0.3<sup>a</sup></td><td>5.7 ± 0.5<sup>a</sup></td><td>8.9 ± 1.9<sup>a</sup></td><td>5.7 ± 2.4<sup>a</sup></td><td>3.1 ± 0.2<sup>a</sup></td><td>5.1 ± 2.3</td></tr><tr><td>DIC</td><td>122.2 ± 2.6<sup>A</sup></td><td>30.6 ± 4.7<sup>A</sup></td><td>31.6 ± 2.5<sup>A</sup></td><td>52.1 ± 4.9<sup>A</sup></td><td>174.4 ± 17.5<sup>A</sup></td><td>87.9 ± 0.4<sup>A</sup></td><td>131.2 ± 3.3<sup>A</sup></td><td>77.6 ± 5.2<sup>A</sup></td><td>88.4 ± 49.8</td></tr><tr><td>Total</td><td>127.8</td><td>36.5</td><td>34.9</td><td>54.3</td><td>180.1</td><td>96.8</td><td>136.9</td><td>80.7</td><td>93.5 ± 52.0</td></tr><tr><td rowspan="3">90 cm</td><td rowspan="2">Digestate</td><td>DOC</td><td>5.0 ± 0.7<sup>a</sup></td><td>8.7 ± 5.0<sup>a</sup></td><td>6.4 ± 0.8<sup>a</sup></td><td>1.0 ± 0.3<sup>a</sup></td><td>10.3 ± 4.0<sup>a</sup></td><td>10.5 ± 3.8<sup>a</sup></td><td>11.5 ± 8.4<sup>a</sup></td><td>6.4 ± 2.2<sup>a</sup></td><td>7.5 ± 4.5</td></tr><tr><td>DIC</td><td>88.5 ± 2.3<sup>A</sup></td><td>19.9 ± 8.4<sup>A</sup></td><td>42.2 ± 4.2<sup>A</sup></td><td>42.9 ± 9.7<sup>B</sup></td><td>169.8 ± 25.2<sup>A</sup></td><td>74.6 ± 4.4<sup>A</sup></td><td>124.7 ± 38.7<sup>A</sup></td><td>71.5 ± 7.9<sup>A</sup></td><td>79.3 ± 48.8</td></tr><tr><td>Total</td><td>93.5</td><td>28.6</td><td>48.6</td><td>43.9</td><td>180.1</td><td>85.1</td><td>136.2</td><td>77.9</td><td>86.7 ± 50.7</td></tr></tbody></table>

<footnote>† Standard deviation</footnote>
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## 3.6 Total dissolved C balance between topsoil and subsoil

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The annual mean of total dissolved C (DOC+DIC) for mineral fertilizer and pig slurry was 65.9 ± 21.6 and 80.2 ± 27.1 kg.ha⁻¹, respectively, at 40 cm and 78.2 ± 42.3 and 93.5 ± 52.0 kg.ha⁻¹, respectively, at 90 cm (Table 4). According to linear regressions between total dissolved C at 40 cm and 90 cm, there was 1.8 times more dissolved C at 90 cm than at 40 cm with mineral fertilizer (R² = 0.668) and 1.6 times more with pig slurry (R² = 0.822) (Figure S3). For both treatments, there was much more DIC than DOC (22.1-168.7 kg.ha⁻¹ more) at 90 cm (Figure S4.B).

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With digestate, the dissolved C at 40 cm (83.1 ± 28.9 kg.ha⁻¹) and 90 cm (86.7 ± 50.7 kg.ha⁻¹) were similar and more balanced than those of the other treatments, with only 1.2 times more total C fluxes at 90 cm depth (R² = 0.794) (Figure S3).

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# 4. Discussion

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## 4.1 Independent and interacting effects of treatment and crop on DOC and DIC concentrations

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Mean DOC concentrations in the topsoil (14.4 ± 6.8 mg.L⁻¹, over nine seasons) were quite high despite a low organic C content (1.1%). Those concentrations were similar to those observed in three European cropland experiments (12 mg.L⁻¹) by Kindler et al. (2011). Mean DOC concentrations under mustard and mineral fertilizer (11.5 ± 3.5 mg.L⁻¹, over four seasons) were higher than those observed by Walmsley et al. (2011) on Irish cropland (Eutric Cambisol) under mustard in the topsoil with a similar fertilization rate (4.4 ± 0.2 mg.L⁻¹). In the subsoil, DOC concentrations (5.0 ± 7.4 mg.L⁻¹, over nine seasons) were lower than those observed by Kindler et al. (2011) (12 mg.L⁻¹). DIC concentrations over nine seasons in the present study were similar to those observed by Kindler et al. (2011) in both the topsoil (29.0 ± 16.8 vs. 35 mg.L⁻¹, respectively) and the subsoil (50.5 ± 24.1 vs. 57 mg.L⁻¹, respectively). As Herbrich et al. (2017)

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In the topsoil, the temporal dynamics of DOC concentrations depended on the type of crop, with the degree of response depending on the treatment, and the concentrations were particularly high for digestate. Two factors can explain the difference observed between cropping contexts.

First, regardless of the treatment, DOC concentrations at the beginning of the season under wheat (11.7 ± 6.6 mg.L⁻¹) were 2 times lower than those under a catch crop (20.9 ± 8.7 mg.L⁻¹), which suggests that the latter had a larger leachable DOC pool. The crop before the mustard and multispecies catch crop was wheat, and the crop before wheat was maize (Table 1). Abiven et al. (2011) observed that maize roots contained more lignin than wheat roots (212.0 ± 16.1 vs. 134.5 ± 13.8 mg C lignin.g C plant⁻¹, respectively) but that their lignin was less biodegradable (C-type lignin:V-type lignin ratio of 3.90 vs. 1.47, respectively). After the harvest of wheat in July, its roots decomposed in the soil and provided soluble molecules of low molecular weight that contributed to the DOC concentrations measured at the beginning of winter drainage, as well as DIC from microbial respiration (Dang et al., 2022; He et al., 2015).

Second, the mustard and multispecies catch crops were sown in September and had two additional months to develop and produce root exudates than the wheat, which was sown in November and had developed little when the drainage season started. Under wheat, DOC concentrations increased around February, which could correspond to the end of the tillering stage. Since tiller number and root development are positively correlated (O'Brien, 1979), the
higher root biomass may have increased root exudation, thus increasing DOC concentrations (Ge et al., 2015; Lu et al., 2004). The results of the 2020-2021 season tend to support this hypothesis. In this season, a mustard catch crop was sown instead of wheat after the harvest of maize, and it was also sown later than those during the other seasons with a catch crop and thus less developed when the drainage season started. DOC from the decomposition of maize roots and from fresh mustard exudates contributed less to the DOC pool, which resulted in lower DOC concentrations at the beginning of the 2020-2021 drainage season than those of the catch-crop-seasons that followed wheat and were sown earlier. These differences between cropping contexts could explain why topsoil DOC concentrations peaked at the beginning of the drainage season for mustard and the multispecies crop but in the middle of the drainage season for wheat.

Other factors can explain why digestate seemed to increase the differences between crops more than the other treatments did. The higher DOC concentrations with digestate could have been related to the digestate's chemical composition, since some digestates of pig slurry contain molecules with auxin-like properties that come from the anaerobic digestion of aromatic amino acids (Scaglia et al., 2017a, 2015b). The auxin effect of digestate is a lead that could be explored in further studies on the digestate used in this study. Auxin can stimulate root development (Saini et al., 2013), which can provide more DOC through exudation and absorb more nutrients. Nitrate absorption tends to basify the rhizosphere (Bloom, 1996; Morot-Gaudry et al., 2006), which can increase the solubility of organic matter (Andersson et al., 2000).
Regardless of the crop, pig slurry could have provided more DOC than digestate did when organic waste products were applied in spring (April-May). Based on the application rates (Table 2) and concentrations of DOC in pig slurry (6.0 mg.g⁻¹; Morvan, personal communication) and digestate (3.8 mg.ml⁻¹; Monard et al., 2020), we estimated that pig slurry and digestate could have provided ca. 200 and 135 kg.ha⁻¹ of DOC, respectively. Both treatments also could have provided a small amount of DIC; for example, pig slurry could have provided ca. 28 kg.ha⁻¹ of total inorganic carbon based on a concentration of 0.84 mg inorganic C.g⁻¹ (Morvan, personal communication). During the 8 months between treatment application and the beginning of drainage, some of the DOC provided by the treatment likely mineralized. During anaerobic digestion, microorganisms consume some of the more labile fraction, which increases digestate concentrations of more biologically stable molecules such as lignins, hemicellulose, and non-hydrolyzable lipids (Marcato et al., 2009; Tambone et al., 2009). These molecules can be a source of DOC to the topsoil without undergoing much mineralization. Since pig slurry may have provided more DOC, and this DOC was more bioavailable, it may have resulted in higher DIC production than digestate. In summary, digestate yielded a larger leachable DOC pool that could have been due to its less-degradable chemical composition and its potential influence on root development and exudation via auxin-like molecules. Further analyses on the dissolved organic matter composition are needed to determine which sources supplied the DOC pool.

## 4.2 Annual dynamics of DOC and DIC fluxes along the soil profile depending on the treatment

In the topsoil, mean winter DOC losses from leaching (10.5-50.5 kg.ha⁻¹, over eight seasons) were consistent with those observed by Kindler et al. (2011) in three European cropland experiments (31 ± 12 kg.ha⁻¹). Under a catch crop, the DOC fluxes (31.8 ± 10.9 kg.ha⁻¹, over four
seasons) were higher than those observed by Walmsley et al. (2011) (22 ± 2 kg.ha⁻¹). In the subsoil, the DOC fluxes observed by Kindler et al. (2011) were higher (30 ± 2 kg.ha⁻¹) than those observed in the present study (6.2 ± 4.1 kg.ha⁻¹, over eight seasons). In the topsoil, DIC leaching losses (50.5 ± 19.5 kg.ha⁻¹, over eight seasons) were more than 2 times lower than those observed by Kindler et al. (2011) (112 ± 30 kg.ha⁻¹) and 6 times lower than those observed by Walmsley et al. (2011) (299 ± 39 kg.ha⁻¹). In the subsoil, DIC fluxes (80.0 ± 45.7 kg.ha⁻¹, over eight seasons) were lower than those observed by Kindler et al. (2011) (133 ± 7 kg.ha⁻¹).

We summarized the sources that contributed to the DOC and DIC fluxes at 40 and 90 cm depths and their annual dynamics to explain the differences observed among the three treatments (Figure 4). At 40 cm, DOC fluxes were higher under digestate (Figure 4.C), while DIC fluxes were higher under pig slurry (Figure 4.B). As mentioned, application of pig slurry or digestate in spring could explain these differences, with more DOC potentially provided by pig slurry which mineralized into DIC. Chantigny (2003) highlighted an increase in soil-soluble organic C after treatment application, followed by rapid decomposition, which could explain why the topsoil DOC fluxes of pig slurry and mineral fertilizer did not differ (Figure 4.A). In comparison, since digestate provided more recalcitrant DOC, the latter could persist longer in the soil. The crops also input C into the system. As mentioned, the mustard and multispecies catch crops likely had more DOC coming from fresh root exudation and decomposition from the roots of the previous crop than wheat did. Some of this DOC may have been mineralized as well, and root respiration could also have provided DIC (Viktor and Cramer, 2003).

At 90 cm depth, DIC fluxes were higher, DOC fluxes were lower, and treatments no longer differed. The decrease in DOC fluxes from 40 to 90 cm is consistent with the transfer of DOC
in the soil profile explained by Kaiser and Kalbitz (2012), in which DOC moving down soil layers undergoes sorption and precipitation reactions that temporarily immobilize it and also undergoes microbial biodegradation (Leinemann et al., 2018; Scott and Rothstein, 2014). The imbalance in total dissolved C with pig slurry and mineral fertilizer, with an excess of DIC in the subsoil, suggested an input of DIC between 40 and 90 cm. It may have come from the mineralization of older organic matter that remained associated with the reactive mineral phase (i.e., iron and aluminum oxides) and underwent desorption or dissolution (Kaiser and Kalbitz, 2012). Moreover, wheat and mustard roots can exceed 40 cm (Aziz et al., 2017; Panigrahi and Panda, 2003) and still produce DIC through respiration. The near balance in total dissolved C between 40 and 90 cm with digestate suggests that the DIC pool at 90 cm may simply have come from the DIC pool at 40 cm and decomposition of the biodegradable portion of the DOC pool from 40 cm, with no other input. However, studies by Schmidt and Martínez (2019) and Coward et al. (2018) observed preferential adsorption of aromatic or hydrophobic molecules on the reactive mineral phase. Thus, the aromatic molecules in digestate (Marcato et al., 2009) can be retained more easily in the soil than those in pig slurry and mineral fertilizer, which contain lower concentrations of aromatic molecules. Additional DIC from the mineralization of older organic matter (Kaiser and Kalbitz, 2012) and root respiration, which can increase if the roots are more developed due to the auxin effect (Scaglia et al., 2017a, 2015b), could have compensated for the C lost through sorption and precipitation and thus approached a balance in total dissolved C. These processes may explain why the difference between treatments in the topsoil decreased as the soil water passed through the soil profile. The DOC that became associated with the mineral phase could remain stored in the subsoil for a long period (Rumpel and Kögel-Knabner, 2011), particularly
475 with digestate, due to its potential high concentration of preferentially adsorbed molecules.

476 This hypothesis could be supported by analyzing the composition of the digestate used.
<figure><img src="image_6.png" /><figcaption>Figure 4. Conceptual diagram of the potential pools that contribute to the dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) pools measured at 40 and 90 cm depths and explain the differences between three treatments: (A) mineral nitrogen fertilizer, (B) pig slurry, and (C) digestate from pig slurry.</figcaption></figure>

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478

479
480 **Conclusion**

481 This study highlighted that application of digestate from pig slurry, depending on the cropping context, increased the topsoil DOC concentration, particularly under a catch crop. The differences in DOC concentrations among treatments, with higher concentrations with digestate application, occurred at distinct moments during the drainage season depending on the crop: from the beginning of the season for mustard and multispecies catch crops and around February for wheat. Topsoil DIC fluxes were higher with pig slurry. In the subsoil, the treatments did not differ, having low DOC fluxes and high DIC fluxes.

488 The present study provided new references for both DOC and DIC fluxes for several organic waste products and crops, with good repeatability over 9 drainage seasons. In particular, it added new insights into higher DOC fluxes after application of digestate than after that of its original pig slurry or a mineral fertilizer, which did not differ. This study also has the advantage of exploring dissolved fluxes in both the topsoil and the subsoil.

493

494 **Declaration of Competing Interest**

495 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 article.

497

498 **Acknowledgements**

499

500 The EFELE field experiment forms part of the SOERE PRO (network of long-term experiments to study impacts of recycling organic waste products), certified by ALLENVI (National Alliance for Environmental Research) and included as a service of the "Investment for the future" infrastructure AnaEE-France, overseen by the French National Research Agency (ANR-11-INBS-0001). We thank ADEME (French Environment and Energy Management Agency) for its
financial support, INRAE (National Research Institute for Agriculture, Food and the Environment) and CNRS (National Center for Scientific Research) for funding the DEDICASS
(Agronomic determinants of dissolved C, N and P fluxes: effect of crops and biogas digestate application) project. We acknowledge the Bretagne region and Institut Agro Rennes-Angers for co-funding Anne-Flore Didelot's Ph.D. program and the GIS APIVALE (Scientific Interest Group on an Integrated Approach of Effluents' Valorization) for its support. Finally, the authors thank
Michael Corson for proofreading the manuscript's English, the editors of *Agriculture*, *Ecosystems and Environment* and reviewers for their time.

References

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