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Archaeological anthrosols constitute a heritage of long-term carbon storage and soil fertility. Their anthropogenic features are affected by the type and intensity of ancient human activities. Human activities can follow a gradation of disturbance intensity, with lower intensity related to a weak human pollution of natural soil, and stronger intensity related to anthropogenic materials inputs (e.g. refuse pits). Soil properties are indeed deeply modified by the addition of objects (e.g. bones, ceramic) and of organic matter with distinct chemical composition and biological stability (e.g. charcoal). The aim of the study was to establish a new analytical approach to distinguish intensities of human activities, based on organic matter characteristics. To this end, we studied intertropical soil profiles (0-120 cm) from Cameroon, Brazil and Bolivia, with spatial or temporal intensity variations of human activities. We used standard compositional parameters (hydrogen index, HI, and oxygen index, OI) and advanced thermal parameters (I-index and R-index) from Rock-Eval® pyrolysis, as well as magnetic susceptibility, to characterize anthrosols.Results demonstrated the potential of Rock-Eval® pyrolysis parameters to identify human activities changes. First, the deviation of I-index (delta-I) between our samples and a reference value from natural sites informed about the intensity of human impacts, allowing for the distinction between artificial infilling of refuse pits and soil profiles with no or few human impacts. Second, positive HI:OI correlation established the importance of charcoal as main organic C source. The magnetic susceptibility informed about the presence of burnt soils in a Brazilian and one of the Bolivian sites. The combination of all these parameters, when represented with soil depth, allowed for the estimation of temporal changes in Brazilian and Bolivian sites. The topsoils were similar for all sites, relative to a low intensity of human activities or to the resumption of natural pedogenesis, thereby alleviating the effects of ancient human activities on organic matter characteristics. In contrast, the subsoils exhibited higher intensities of ancient human activities, with even higher values of intensities in Bolivian sites, thereby evidencing the long-term conservation of their effects on organic matter characteristics.To conclude, anthropogenic activities may durably affect organic matter characteristics in tropical sites, even after several centuries. Beyond being of interest for archaeological research, this new approach raises questions about the long-term consequences of our current human activities.

Quantifying Soil Organic and Inorganic Carbon (SOC & SIC) separately in carbonate soils involves successive pretreatments and/or measurements to separate the two carbon forms. The Rock-Eval® (RE) thermal analysis has been developed in the 70’s by IFPEN to study oil bearing rocks with a ramped heating pyrolysis and oxidation. The RE has been increasingly used over the past 20 years to quantify and characterize SOC without pretreatments and with a single analysis in any kind of soils. To improve the SOC and SIC quantifications by RE, Disnar & al. (2003) and Sebag & al. (2022a; 2022b) suggested statistical corrections of the TOC and MinC standard parameters, which are SOC and SIC content estimators respectively. However, few applications have focused on SIC quantification and no adjustments of the RE standard analysis cycle have been investigated. This study aims at adjusting the RE standard analysis protocol to quantify SOC and SIC. For this, a panel of carbonate soils with a wide range of SOC and SIC contents was analyzed by RE. The SOC and SIC quantifications by RE were compared to those obtained by elemental analysis (EA, standard method to quantify soil carbon). A too high SIC amount in the RE crucible (> 4 mg) led to an underestimation of the SIC content, even after correcting the MinC parameter. The higher the SIC amount in the RE crucible, the more the SIC content was underestimated. The standard analysis cycle was too short to achieve a complete thermal decomposition of the SIC amount in the crucible leading to an underestimation of the SIC content. Thus, we propose to extend the last oxidation isotherm from 3 min to 5 or 7 min to achieve the complete SIC thermal decomposition. Moreover, the temperature limit used during the pyrolysis to distinguish the CO and CO2 emitted by organic matter thermal cracking and carbonate thermal decomposition phase is often inappropriate as the thermoresistant part of SOC and the main part of SIC decompose simultaneously after 550°C. Thus, we suggest to stop the pyrolysis phase at the onset of SIC decomposition to drag all SIC signal during the oxidation phase only. This modification of the standard analysis protocol for carbonate soils allows us to avoid some of the statistical corrections suggested by Disnar & al. (2003) and Sebag & al. (2022a; 2022b). Finally, we propose a new RE analysis protocol to simplify the calculations and improve the accuracy of SOC and SIC quantifications in carbonate soils. Disnar, J.R., B. Guillet, D. Keravis, C. Di-Giovanni & D. Sebag, 2003. Soil organic matter (SOM) characterization by Rock-Eval pyrolysis: scope and limitations. Organic Geochemistry 10.1016/S0146-6380(02)00239-5Sebag, D., V. Lamoureux-Var, I. Kowalewski, D. Pillot & H. Ravelojoana, 2022a. Procédé pour la quantification et la caractérisation du carbone dans les sols. IFP Energies Nouvelles Patent No. 3121225. France.Sebag, David; Lamoureux-Var, Violaine; Kowalewski, Isabelle; Ravelojoana, Herman; Lefrançois, Noémie, 2022b. Improved quantification of SOC and SIC in Rock-Eval® thermal analysis. SOM (8th International Symposium on Soil Organic Matter), Seoul, Korea.

Quantifying both soil organic and inorganic carbon (SOC & SIC) is essential to understand carbon (C) dynamics and to assess the atmospheric C sequestration potential in calcareous soils. The procedures usually used to quantify SOC and SIC involve pretreatments (decarbonation, decarbonatation) and calculation of the difference between C contents estimated by elemental analysis on raw and pretreated aliquots. These procedures lead to analytical bias associated to pretreatments, measurement deviations associated to the sample heterogeneity, and cumulative errors associated to calculations. The Rock-Eval® thermal analysis, used in soil sciences since the 2000s, provides two parameters estimating the organic (TOC) and inorganic (MinC) C contents of a non-pretreated aliquot with a single analysis. Nevertheless, the Rock-Eval® protocol has been standardized in the 70s by IFP Energies Nouvelles for studying oil bearing rocks and is, thus, not perfectly suited for soil study. Previous studies suggested statistical corrections of the standard parameters to improve their estimations of C contents assessed by elemental analysis but only few of them focused on the estimation of inorganic C content using the MinC parameter. Moreover, none of them suggested adjustments of the standard Rock-Eval® protocol. This study proposes to adapt this protocol to optimize SOC and SIC quantifications in soil samples. Comparisons between SOC and SIC quantifications by elemental analysis and by Rock-Eval®, with and without statistical corrections of the standard TOC and MinC parameters, were carried out on a soil panel with a wide range of SOC and SIC contents. The results show that the standard Rock-Eval® protocol properly estimates SOC contents once the TOC parameter is corrected. However, it cannot achieve a complete thermal breakdown of SIC amounts > 4 mg leading to an underestimation of high SIC contents by the MinC parameter, even after correcting it. Thus, the final oxidation isotherm is extended to 7 min to complete the thermal breakdown of SIC before the end of the analysis.

Key points• Carbon occurs in different forms in soils: in living organisms, dead organic matter with particulate organic matter, organicmolecules, and as inorganic carbon• Soil carbon stock and soil carbon storage are different concepts: a carbon stock is quantified at a given date while its storageis quantified over a period of time• The carbon stock distribution across the planet is heterogeneous, and complex biotic and abiotic processes underlie itsdynamics• For a given soil and climate, the largest stocks are found in forests and grasslands. In comparison, cultivated soils are carbondepleted• Land-use changes and agricultural practices have a major impact on soil carbon stocks: they may have negative, positive ordiverse effects on carbon storage, depending on climate and soil conditions• Soil carbon storage provides socioeconomic benefits as targeted by five Sustainable Development Goals

Identifying the drivers of soil organic carbon (SOC) stock changes is of the utmost importance to contribute to global challenges like climate change, land degradation, biodiversity loss, or food security. Evaluating the impacts of land use and management practices in agriculture and forestry on SOC is still challenging. Merging datasets or making databases interoperable is a promising way, but still has several semantic challenges. So far, a comprehensive thesaurus and classification of management practices in agriculture and forestry has been lacking, especially while focusing on SOC storage. Therefore, the aim of this paper is to present a first comprehensive thesaurus for management practices driving SOC storage (DATA4C+). The DATA4C+ thesaurus contains 224 classified and defined terms related to land management practices in agriculture and forestry. It is organized as a hierarchical tree reflecting the drivers of SOC storage. It is oriented to be used by scientists in agronomy, forestry, and soil sciences with the aim of uniformizing the description of practices influencing SOC in their original research. It is accessible in Agroportal (http://agroportal.lirmm.fr/ontologies/DATA4CPLUS, last access: 24 March 2022) to enhance its findability, accessibility, interoperability, and reuse by scientists and others such as laboratories or land managers. Future uses of the DATA4C+ thesaurus will be crucial to improve and enrich it, but also to raise the quality of meta-analyses on SOC, and ultimately help policymakers to identify efficient agricultural and forest management practices to enhance SOC storage.

International audience; PurposeUrbanization is a major driver of land use change and can affect the soil organic carbon (SOC) pools. This study aimed to understand the urbanization impact on SOC stocks and pools at profile scale (0–100 cm).MethodsThe SOC was studied at 0–30 and 0–100 cm depths in park and sealed soils of three French cities (Marseille, Nancy, and Nantes). Physical fractionation was performed to gain insight on the size of different SOC pools (particulate and organo-mineral soil fractions).ResultsThe SOC stocks were seven to ten times higher in parks than in sealed soils, but lower than in natural soils according to literature data. The contribution of the first 30 cm to profile SOC stock was around 40%, with strong heterogeneity, especially in sealed soils. Considering the whole 0–100 cm profile, SOC stocks in particulate organic matter fractions (light fraction > 50 µm) were 25–48 times higher in parks than in sealed soils, while SOC stocks in mineral-associated fractions ( 50 µm, particularly in sealed soils (11% in average at 0–100 cm depth). This fraction associated to sand is usually poor in SOC in natural or agricultural soils. In these urban soils, it might be bitumen, a dense organic artifact.ConclusionThe SOC stocks up to 100 cm depth and their heterogeneity pleaded to strengthen and expand SOC studies in all urban soils.

International audience; Soil organic carbon (SOC) contributes to agrosystem productivity. Understanding how farming practices implemented by smallholders affect the levels and distribution of SOC in carbon (C) pools with different stabilities is essential in sub-Saharan Arenosols where SOC mineralization is intense. The stability of SOC was studied by thermal (Therm-C), physical (particulate organic matter >50 μm, POM-C and fine soil fractions

International audience; Mid-Infrared reflectance spectroscopy (MIRS, 4000–400 cm−1) is being considered to provide accurate estimations of soil inorganic carbon (SIC) contents, based on prediction models when the test dataset is well represented by the calibration set, with similar SIC range and distribution and pedological context. This work addresses the case where the test dataset, here originating from France, is poorly represented by the calibration set, here originating from Tunisia, with different SIC distributions and pedological contexts. It aimed to demonstrate the usefulness of 1) classifying test samples according to SIC level based on the height of the carbonate absorbance peak at 2510 cm−1, and then 2) selecting a suitable prediction model according to SIC level. Two regression methods were tested: Linear Regression using the height of the carbonate peak at 2510 cm−1, called Peak-LR model; and Partial Least Squares Regression using the entire MIR spectrum, called Full-PLSR model. First, our results showed that Full-PLSR was 1) more accurate than Peak-LR on the Tunisian validation set (R2val = 0.99 vs. 0.86 and RMSEval = 3.0 vs. 9.7 g kg−1, respectively), but 2) less accurate than Peak-LR when applied on the French dataset (R2test = 0.70 vs. 0.91 and RMSEtest = 13.7 vs. 4.9 g kg−1, respectively). Secondly, on the French dataset, predictions on SIC-poor samples tended to be more accurate using Peak-LR, while predictions on SIC-rich samples tended to be more accurate using Full-PLSR. Thirdly, the height of the carbonate absorbance peak at 2510 cm−1 might be used to discriminate SIC-poor and SIC-rich test samples ( 5 g kg−1): when this height was > 0, Full-PLSR was applied; otherwise Peak-LR was applied. Coupling Peak-LR and Full-PLSR models depending on the carbonate peak yielded the best predictions on the French dataset (R2test = 0.95 and RMSEtest = 3.7 g kg−1). This study underlined the interest of using a carbonate peak to select suitable regression approach for predicting SIC content in a database with different distribution than the calibration database.

Within agroforestry systems, trees greatly contribute to soil carbon (C) sequestration. We still know little about how this contribution varies according to tree species. Here, we quantified the soil C content derived from three main tree species, Eucalyptus robusta, Coffea arabica and Citrus clementina, used in rainfed rice-based agroforestry in the Highlands of Madagascar. A fully factorial controlled mesocosm experiment was performed, with seedlings of the tree species planted with or without rice in a Ferralsol. The natural 13C abundance in the soil was measured to assess tree- and rice-derived C. One year after planting, Eucalyptus robusta was the species exhibiting the highest tree-derived C values, with 148 g C m−2 on average. In contrast, Citrus clementina transferred only 12 g C m−2 to the soil. No significant contribution of Coffea arabica to the soil C stock was found. The presence of rice affected neither the growth of tree seedlings nor their abilities to provide C to the soil. The difference in soil C sequestration among tree species was significantly correlated with differential root biomass. Tree root biomass was significantly correlated with soil δ13C. This experimental assay highlighted the contrasting contributions of tree species to ecological processes such as soil C sequestration at the early stage of agroforestry establishment. Further studies considering tree density and spatial arrangement should be conducted under field conditions to gain further insights into tree contributions to soil C sequestration in agroforestry systems.

International audience; Continental carbon (C) sequestration plays a major role in climate change resilience, mitigation, and agro-ecological intensification. Soil Moisture Active Passive Global Daily Carbon Net Ecosystem Exchange dataset (SMAP L4) is used for monitoring daily terrestrial C stocks, given the high potential of semiarid areas for Sustainable Goals achievements, in terms of human issues and surface areas. Inter-and intra-annual variation of Global Primary Productivity (GPP), Net Ecosystem CO2 Exchange (NEE), Soil Heterotrophic Respiration (RH) and Soil Organic Carbon (SOC) were analyzed from 2015 to 2020, across the region of Maradi (Niger) and 15 sites with rainfed crops and vegetal mosaics. To quantify temporal trends of C indicators across years, Mann-Kendall were conducted based on yearly GPP days of min/max. It underlined declines of SOC in the northern part of Maradi (τ >-0.6), rising in the south (τ > +0.6). Univariate monitoring (e.g. 2162 daily measurements) of GPP and RH showed asymmetries in their distribution (positive skews, GPP ̅ =1.19 gC m-2 d-1 , Me=0.88 gC m-2 d-1 ; RH ̅ =0.679 gC m-2 d-1 ; Me=0.485 gC m-2 d-1). This result might be attributed to local variations of Plant Functional Types and microbial activities. Averaged NEE stressed a slightly negative C balance among sites (overall ̅ =-0.003 gC m-2 d-1), suggesting their storage capacity. Processing aspects being set up, a consolidation of those first results should be done. Satellite data records analysis argue for the implementation of Man-Environment observatories in Maradi as a pilot region, for both remote and local understanding of C behavior in agroecosystems and multiscale counseling.

Mid-Infrared reflectance spectroscopy (MIRS, 4000 – 400 cm-1) is being considered to provide accurate estimations of soil inorganic carbon (SIC) contents. Usually, the prediction performances by MIRS are analyzed using figures of merit based on entire test datasets characterized by large SIC ranges, without paying attention to performances at sub-range scales. This work aims to 1) evaluate the performances of MIR regression models for SIC prediction, for a large range of SIC test data (0-100 g/kg) and for several regular sub-ranges of SIC values (0-5, 5-10, 10-15 g/kg, etc.) and 2) adapt the prediction model depending on sub-ranges of test samples, using the absorbance peak at 2510 cm-1 for separating SIC-poor and SIC-rich test samples. This study used a Tunisian MIRS topsoil dataset including 96 soil samples, mostly rich in SIC, to calibrate and validate SIC prediction models; and a French MIRS topsoil dataset including 2178 soil samples, mostly poor in SIC, to test them. Two following regression models were used: a partial least squares regression (PLSR) using the entire spectra and a simple linear regression (SLR) using the height of the carbonate absorbance peak at 2150 cm-1.First, our results showed that PLSR provided 1) better performances than SLR on the Validation Tunisian dataset (R2test of 0.99 vs. 0.86, respectively), but 2) lower performances than SLR on the Test French dataset (R2test of 0.70 vs. 0.91, respectively). Secondly, our results showed that on the Test French dataset, predicted SIC values were more accurate for SIC-poor samples (< 15 g/kg) with SLR (RMSEtest from 1.5 to 7.1 g/kg, depending on the sub-range) than with PLSR prediction model (RMSEtest from 7.3 to 14.8 g/kg, depending on the sub-range). Conversely, predicted SIC values were more accurate for carbonated samples (> 15 g/kg) with PLSR (RMSEtest from 4.4 to 10.1 g/kg, depending on the sub-range) than with SLR prediction model (RMSEtest from 6.8 to 14 g/kg, depending on the sub-range). Finally, our results showed that the absorbance peak at 2150 cm-1 could be used before prediction to separate SIC-poor and SIC-rich test samples (452 and 1726 samples, respectevely). The SLR and PLSR regression methods applied to these SIC-poor and SIC-rich test samples, respectively, provided better prediction performances (R²test of 0.95 and RMSEtest of 3.7 g/kg). Finally, this study demonstrated that the use of the spectral absorbance peak at 2150 cm-1 provided useful information on Test samples and helped the selection of the optimal prediction model depending on SIC level, when using calibration and test sample sets with very different SIC distributions.

Soil carbon (C), now more than ever, attracts the interest of the scientific community for its importance in combating climate change and achieving food security. As a result, its key role in agricultural soil fertility and in anthropogenic greenhouse gas emissions mitigation is high on international agendas. A key issue regarding the linkage between food security and carbon storage concerns the mineralization or the stability of soil organic matter (SOM). Rock-Eval® analysis was used to examine the thermal stability of SOM and these results were presented in details at the EGU General Assembly in 2020 (EGU2020-11229). Several indicators are used to further appreciate the quantity and quality of SOM: particle size fractionation (POM-C), determination of permanganate oxidizable carbon (POX-C) and carbon mineralization kinetics (Min-C). The results of both approaches are crossed and presented here. Soils were sampled from two soil layers (0-10 et 10-30 cm) in agricultural plots representative organic inputs practices in local agricultural systems (No input, +Millet residues, +Manure and +Organic wastes). Total soil organic carbon (SOC) concentrations ranged from 1.8 to 18.5 g C.kg-1 soil (mean ± standard deviation: 5.6 ± 0.4 g C.kg-1 soil) in the surface layer (0-10 cm) and from 1.5 to 11.3 g C.kg-1 soil (mean ± standard deviation: 3.3 ± 0.2 g C.kg-1 soil) in 10-30 cm deep layer. The soil organic matter in these Arenosols while positively affected by organic inputs is dominated by thermally labile forms. The POM-C fractions represent respectively 45 % and 24 % of the COS in the 0-10 cm and 10-30 cm soil layers respectively. Permanganate oxidizable carbon (POX-C) and mineralizable C (Min-C) averaged 254 ± 14 mg C.kg-1 soil and 10.7 ± 1.2 mg C-CO2 kg-1 soil in the 0-10 cm layer. Our results show that in different situations, the labile pools POM-C, POX-C and Min-C are linked to the active thermal pools A1 (highly labile pool), A2 (labile pool), A3 (resistant pool) and even A4 (refractory pool). The A3 and A4 pools, which are known to be relatively stable in more clayey soils, are in fact quickly mineralized in the sandy soils of this region. This intense mineralization of SOM promotes the recycling of nutrients which is excellent for productivity of these agrosystems, but not for mitigation of climate change in the long term.keywords: Sahel ; Arenosols ; Thermal stability ; Biogeochemical stability ; Rock-Eval analysis, POM-C ; POX-C ; Min-C.

International audience; Soil organic carbon (SOC) is essential for the productivity of agroecosystems and for mitigating climate change. Because the SOC contents of sandy soils are usually small, the effects of agricultural management upon SOC stocks in such soils have been insufficiently studied. In West sub-arid Africa, the coarse-textured soils (mostly Arenosols) are diversely managed by smallholders. In this study, we aimed to quantify SOC stocks in cultivated soils of that region, in a context where agricultural practices rely mainly upon organic inputs derived from various integrated crop-livestock systems. SOC stocks were estimated for the 0-30 cm depth in 1,813 plots in Senegal's groundnut basin. We found that SOC stocks in farmers' fields varied between 2.3 and 59.8 Mg C ha(-1) (mean +/- standard deviation, 14.6 +/- 0.14 Mg C ha(-1)). SOC stocks were influenced slightly by soil type, but were only weakly correlated to soils' clay and silt contents. SOC stocks differed significantly among the three studied village territories due to contrasting livestock-raising systems. Average stocks were significantly higher in plots close to housings (home-fields), which receive larger amounts of organic inputs, than in plots farther from the village (out-fields). Thus, the organic inputs to home-fields improves soil C stocks of these sandy soils in the short term. Innovative agricultural practices in the studied area probably need to target options for managing all fields optimally. Those options will require continuous application of organic products-a measure that will in turn require solutions for improving availability or management of local organic resources.

International audience; Rural development projects to develop sustainable agriculture need to be assessed before engaging smallholder farmers at large scale. Data on agricultural systems to produce food, provide income for smallholders and reduce greenhouse-gas (GHG) emissions are scarce in Sub-Saharan Africa countries. Here, we assessed the potential of agroecological practices (AP) on economic benefits to farmers, GHG balance derived from agricultural activities, and efficiency of economic investments to mitigate GHG. The study was based on a NGO rural development project, which promoted AP: agroforestry, compost, and System of Rice Intensification. The economic and GHG mitigation benefits were projected over a period of 20 years on three scenarios. Two scenarios, differing by two expected levels of AP adoption, were compared to a reference one, in which there was no NGO intervention. Socio-economic, yield and soil data were gathered on 192 farms during five growing seasons (2013–2018). The GHG balance was estimated with TropiC Farm Tool and EX-ACT. The GHG emissions were reduced in both scenarios compared to the reference one: −5.2 to −13.6 tCO2eq farm−1 yr−1 for scenario 1 and 2 respectively. At the regional scale, the projected amount of C saved per euro invested was estimated at −0.25 tCO2eq euro−1 and -0.41 tCO2eq Euro−1 (or € 4 to 2.5 tCO2eq−1) under scenario 1 and scenario 2. The annual cash flow of farmers increased over the 20 years. Our study highlighted the potential of AP for increasing productivity and profitability of smallholder agricultural systems for the Malagasy farmers, while simultaneously contributing to climate change mitigation.

(IF 5.57; Q1); International audience; Soil organic carbon (SOC) ensures soil quality and productivity of cultivated systems in the Sahelian region. This study uses Rock-Eval® pyrolysis to examine how cultural practices impact the quantity of SOC and quality of SOM in cultivated sandy soils in the Senegal groundnut basin. This cost-effective method provides information on SOC thermal stability, which has been shown to be related qualitatively to biogeochemical stability of SOC. We sampled soils within two villages in agricultural plots representative of local agricultural systems, and in two local preserved areas (tree plantation and shrubby savanna). SOC concentrations ranged from 1.8–18.5 g.kg−1 soil in the surface layer (0−10 cm) and from 1.5–11.3 g.kg−1 soil in the 10−30 cm layer. SOC contents of cultivated soils decreased significantly (p-value < 0.0001) according to field amendment, in the following order: addition of organic wastes> addition of manure > millet residues left after harvest > no organic input. We found that the quantity and the quality of SOC are linked, and that both depend on land-use and agricultural practices, especially upon the type of organic inputs. Quantity of SOC and quality of SOM are correlated strongly in the tree plantation (R² = 0.98) and in the protected shrubby savanna (R² = 0.97). They are also correlated significantly in cultivated soils receiving organic wastes (R² = 0.82), manure (R² from 0.74 and 0.91), or millet residues (R2 = 0.91) but not in soils that receive no organic inputs. Indexes based upon Rock-Eval® pyrolysis were represented in an I/R diagram that illustrates the level of SOC stabilization. The indexes of the studied soils were plotted against comparable results from literature. Thermal signatures of the Senegalese Arenosols show an inversion of I and the R indexes compared to data from the literature. This result highlights SOC stabilization as a function of soil depth. Indeed, the refractory pool in the studied soils (where refractory pool ranged from 7.7–21.3 % in the 0−10 cm layer, and from 12.5–24.3 % in the 10−30 cm) was more abundant than in Ferralsols in natural conditions, where refractory pool ranged from 2 to 9%. The soil organic matter in these Arenosols while positively affected by organic inputs, is dominated by more or less labile forms that mineralize quickly: a quality that is excellent for productivity of these agrosystems, but not for mitigation of climate change in the long term.

Mid-infrared reflectance spectroscopy (MIRS) is time- and cost-effective. It was used for quantifying soil inorganic carbon (SIC) concentration in France based on a national library, and performances were evaluated on an independent regional set. Our objective was to improve the accuracy of MIRS predictions based on common multivariate regression, through spiking (enrichment of the national library with some representative target samples) with possible extra-weighting (replication of spiking samples) and local calibration (only using calibration samples that are spectral neighbours of each target samples), which have not been fully explored yet, in combination especially. Global (i.e. common) calibration yielded accurate prediction (standard error of prediction, SEP, was approximate to 5 g kg(-1 )which could be improved when the library was completed with spiking samples (optimally 10 samples extra-weighted 40 times; SEP = 3.3 g kg(-1)). Using spiking samples only (without the library) yielded slightly less accurate results (SEP = 3.6 g kg(-1)). Prediction was more accurate using local calibration without spiking, but on a validation set that was reduced because some validation samples lacked calibration neighbours (SEP = 2.5-2.7 g kg(-1)). Local calibration with spiking (optimally 10 samples without extra-weight) yielded somewhat less accurate prediction but for the full validation set when few calibration neighbours were required (SEP = 2.7 g kg(-1)), or higher accuracy on the reduced validation set when many neighbours were required (SEP = 2.3 g kg(-1)). These accurate predictions demonstrated the usefulness of representative spiking and local calibration for rendering large soil spectral libraries fully operational, while extra-weighting had no additional benefit. Along with more exhaustive spectral libraries, this paves the way for extensive use of MIRS for SIC determination.

Soil organic carbon (SOC) is a key element in the functioning of agrosystems. It ensures soil quality and productivity of cultivated systems in the Sahelian region. This study uses Rock-Eval pyrolysis to examine how cultural practices impact SOC quantity and quality of cultivated sandy soils in the Senegal groundnut basin. Such thermal analysis method provides cost-effective information on SOC thermal stability that has been shown to be qualitatively related to SOC biogeochemical stability. Soils were sampled within 2 villages agricultural plots representative of local agricultural systems and for local preserved areas. Total SOC concentrations ranged from 1.8 to 18.5 g.kg-1 soil (mean ± standard deviation: 5.6 ± 0.4 g.kg-1 soil) in the surface layer (0-10 cm) and from 1.5 to 11.3 g.kg-1 soil (mean ± standard deviation: 3.3 ± 0.2 g.kg-1 soil) in 10-30 cm deep layer. SOC of cultivated soils significantly (p-value < 0.0001) decreased according to treatments in the following order: +organic wastes > +manure > +millet residues > no input. Our results show that the quantity and the quality of SOC are linked to each other and both depend on land-use and agricultural practices, especially the nature of organic inputs. This correlation is very strong in the tree plantation (R² = 0.98) and in the protected shrubby savanna (R² = 0.97). It remains important for cultivated soils receiving organic wastes (R² = 0.82), manure (R² > 0.75), or millet residues (R2 = 0.91) but it’s no more significant in no-input situations. The Rock-Eval based indexes were depicted in a I/R diagram that illustrate the level of SOC stabilization and plotted against comparable results from literature. The Senegalese sandy soils have thermal signatures showing an inversion of the I and the R indexes compared to data from the literature and highlighting SOC stabilization as a function of soil depth. Indeed, the studied soils were characterized by a more abundant refractory pool (A5 which ranged from 7.7 to 21.3 % in 0-10 cm layer and from 12.5 to 24.3 % in 10-30 cm, respectively) compared to other tropical soils. The SOC in these sandy soils while positively affected by organic inputs is dominated by labile forms that mineralize quickly which is excellent for the needs of productivity of these agrosystems but not for mitigation of climate change.Keywords: Soil organic carbon; Organic inputs; Thermal analysis; Agrosystems; West Africa

International audience; The priming effect (PE) in soil, when induced by a fresh carbon supply, is believed to result from two different mechanisms, “stoichiometric decomposition” and “nutrient mining”, and contributes to either long-term SOM stabilization or depletion. Understanding how to affect the balance between both mechanisms can provide valuable insight into agroecology, especially in Southern countries where organic matter management is the primary method of fertilizing cultivated plots. Therefore, the objective of this study was to investigate the bacterial actors in each mechanism and the influence of three drivers in Malagasy Ferralitic soils: the quality of the inducing substrate, the quality of the SOM, and the soil nutrient status. Three different agricultural Ferralitic soils, characterized by different levels of NH4 and PO4 availability, different SOM size fractionation profiles, and different bacterial communities, were amended with three types of 13C-labeled organic substrate (glucose, wheat residue, and rice residue) and the PE was measured at two incubation times (7 and 42 days). The results showed that a PE generated by stoichiometric decomposition was mainly induced by the polymerized fraction of crop residues, especially in soil enriched by decaying plant tissues, mineral N, and its associated decomposers guild such as Verrucomicrobia, α- and δ-Proteobacteria and Actinomycetes. Conversely, when this young SOM pool was reduced, the PE was mainly generated by N mining, especially when induced by soluble compounds and when N was limiting but P was available.

International audience; Organic carbon saturation in soils refers to the theoretical maximum soil organic carbon (SOC) that can be associated with and stabilized on fine silt plus clay particles (F < 20 μm). We reviewed the literature dealing with SOC distribution between soil fractions to evaluate carbon saturation for tropical soils and estimate the C storage potential of cropland. We collected 258 data points on SOC distribution between soil fractions in tropical soils from 84 sites in 27 countries. We used boundary line analysis to estimate the maximum stabilized SOC depending on soil group, clay type and land use. SOC storage potential was calculated as the SOC saturation deficit, the difference between the actual SOC content and the maximum stabilized SOC content. We found that the maximum SOC in the fine fraction of tropical soils (53 g C kg− 1 fine fraction) was lower than previous assessments of global SOC storage scale based mainly on temperate soils. The F < 20 μm fractions were closer to SOC saturation in forest soils than in croplands. The cropland had a higher soil C storage potential, but changing agricultural management practices did not fill the deficit that is calculated using the whole dataset. The deficit was much lower when it was estimated with grassland or cropland data only: this provides a more realistic estimate for SOC storage potential for croplands. The SOC content in the coarser fraction (F > 50 μm) did not depend on soil texture and significantly contributed to the total SOC, especially in sandy soils (41.3%). This is affected by changes in agricultural management practices. We concluded that, although the aim of increasing SOC stabilization originally arose from climate change mitigation strategies, it must now be more viewed as being more relevant to food security and local adaptation to climate change.

Increasing soil organic matter (SOM) is of primary importance for maintaining soil fertility and mitigating climate change. Leaving crop residues on top of soil is not always an efficient means of increasing SOM because (i) of the high mineralization of the crop residues, (ii) crop residues may increase the mineralization of existing SOM (priming effect) and (iii) wetting-drying cycles may increase mineralization of SOM and crop residues. Little research has been carried out into these mechanisms under Sudano-Sahelian conditions where the rainfall is mostly irregular with wetting-drying cycles during the transition between the wet and the dry season. To evaluate the effect of wetting-drying cycles on the mineralization of SOM and crop residues and the priming effect, an agricultural soil from the North Region of Cameroon with or without (controls) 13 C-labeled rice straw amendment as crop residues was either subjected to five wetting-drying cycles or maintained at constant water potential after a single rewetting event. Soil samples were incubated for 70 days at 28 °C and the CO 2 and 13 CO 2 emissions and mineral N were monitored. Adding straw (+ 833 μg C g − 1 soil) increased the cumulative CO 2 emissions from the soil (+ 921 μg C-CO 2 g − 1 soil). A positive priming effect was observed (+ 92 μg C-CO 2 g − 1 soil). Only the first wetting cycle created a mineralization flush of the SOM and the straw. This flush did not recur probably because of a lack of labile SOM. However an extra addition of straw after 28 days increased CO 2 emissions but did not result in further mineralization flushes after re-wetting while SOM mineralization was not limited by N availability. We conclude that SOM depletion under Sudano-Sahelian conditions was not explained by SOM mineralization enhancement due to multiple rewetting events or to priming effect following crop residues addition. Indeed, SOM depletion could rather be explained by a high level of mineralization of both, SOM and crop residues, when the soil reached its water retention capacity.

International audience; Mid-infrared reflectance spectroscopy (MIRS, 4000–400 cm−1) is being considered to provide accurate estimations of soil properties, including soil organic carbon (SOC) and soil inorganic carbon (SIC) contents. This approach has mainly been demonstrated by using datasets originating from the same area A, with similar geopedological conditions, to build, validate and test prediction models. The objective of this study was to analyse how MIRS performs when applied to predict SOC and SIC contents, from a calibration database collected over a region A, to predict over a region B, where A and B have no common area and different soil and climate conditions. This study used a French MIRS soil dataset including 2178 topsoil samples to calibrate SIC and SOC prediction models with partial least squares regression (PLSR), and a Tunisian MIRS topsoil dataset including 96 soil samples to test them. Our results showed that when using the French MIRS soil database, i) the SOC and SIC of French validation samples were successfully predicted using global models (R2 val = 0.88 and 0.98, respectively), ii) the SIC of Tunisian samples was also predicted successfully both using a global model and using a selection of spectral neighbours from the French calibration database (R2 test of 0.96 for both), iii) the SOC of Tunisian samples was predicted moderately well by global model (R2 test of 0.64) and a transformation by natural logarithm of the calibration SOC values significantly improved the SOC prediction of Tunisian samples (R2 test of 0.97), and iv) a transformation by natural logarithm of SOC values provided more benefit than a selection of spectral neighbours from the French calibration database for predicting Tunisian SOC values. Therefore, in the future, MIRS might replace conventional physico-chemical analysis techniques, or at least be considered as an alternative technique, especially when optimally exhaustive calibration databases will become available.

Les sols sont une ressource essentielle a preserver pour la production d'aliments, de fibres, de biomasse, pour la filtration de l'eau, la preservation de la biodiversite et le stockage du carbone. En tant que reservoirs de carbone, les sols sont par ailleurs appeles a jouer un role primordial dans la lutte contre l'augmentation de la concentration de gaz a effet de serre. Ils sont ainsi au centre des objectifs de developpement durable (ODD) des Nations unies, notamment les ODD 2 « Faim zero », 13 « Lutte contre le changement climatique », 15 « Vie terrestre », 12 « Consommation et production responsables » ou encore 1 « Pas de pauvrete ». Cet ouvrage presente un etat des lieux des sols africains dans toute leur diversite, mais au-dela, il documente les capacites de stockage de carbone selon les types de sols et leurs usages en Afrique. Il propose egalement des recommandations autour de l'acquisition et de l'interpretation des donnees, ainsi que des options pour preserver, voire augmenter les stocks de carbone dans les sols. Tous les chercheurs et acteurs du developpement impliques dans les recherches sur le role du carbone des sols sont concernes par cette synthese collective. Fruit d'une collaboration entre chercheurs africains et europeens, ce livre insiste sur la necessite de prendre en compte la grande variete des contextes agricoles et forestiers africains pour ameliorer nos connaissances sur les capacites de stockage de carbone des sols et lutter contre le changement climatique. Cet ouvrage est une contribution du reseau Carbone des sols pour une agriculture durable en Afrique (Casa). Les objectifs du reseau Casa s'inscrivent dans le cadre de l'Action commune de Koronivia pour l'Agriculture (Koronivia Joint Work on Agriculture, KJWA). Adoptee en 2017, lors de la 23e conference des Parties (COP 23), la KJWA fournit un cadre pour renforcer la place de l'agriculture dans les discussions internationales sur le climat, a la fois pour son role dans la reduction de la concentration en gaz a effet de serre dans l'atmosphere et sa necessaire adaptation face au changement climatique.

In the last years, soil organic carbon (SOC) dynamics have been explored for agronomic and environmental issues in different agro systems. Many soils of the world, especially in arid and semi-arid environments, contain large stocks of soil inorganic carbon (SIC) as carbonates. Yet, the SOC dynamics has been poorly investigated in these soils, due to the complexity of measurements and of the processes involved. Indeed, few previous studies have shown links between SIC and SOC dynamics. Theses interactions are initiated by biological activities, i.e. CO2 production, are explained through equilibrium equations between soil carbonates and bicarbonates. However, few data were available on the specific impact of SIC on SOC mineralization especially at increasing soil depth.Alley agroforestry systems increased SOC content in the tree rows without any change in the SIC content. The heterogeneity in organic inputs and SOC contents induced by alley agroforestry allows the investigation of the interactions between SIC and SOC on CO2 emissions.To assess contributions of SIC to CO2 emissions with depth, we incubated carbonaceous soil samples coming from an 18-year-old agroforestry system (both tree row and alley) and an adjacent agricultural plot. Soil samples were taken at four different depths: 0-10, 10-30, 70-100 and 160-180 cm. Total CO2 emissions, the isotopic composition (δ13C, ‰) of the CO2 and microbial biomass were measured. The SIC concentrations were from 48 to 63 g C kg-1 soil and the SOC concentrations from 4 to 17 g C kg-1 soil. The total amounts of CO2 emissions from soil were correlated to C contents and decreased with depth (from 183-569 µgC g-1 soil in top soil vs 21-25 µgC g-1 soil in subsoil).The contribution of SIC-derived CO2 was not homogenous along the soil profile. It represented about 20% in the topsoil and 60% in the subsoil of the total soil CO2 emissions. As the SOC content and the microbial biomass, the SOC-derived CO2 emissions were larger in the topsoil especially in the tree row compared to the alley and the agricultural plot. The SIC-derived CO2 emissions were also larger in topsoil and in tree rows at 0-10 cm than in alleys or agricultural plots (71 µg C g-1 soil vs 45-48 µg C g-1 soil) or in the subsoil (13-15 µg C g-1 soil), whereas the amount of SIC was similar in top and subsoil and in tree rows, alleys or agricultural soils. This indicate that CO2 emissions from SIC were linked to the SOC content and its mineralization. In addition, our results suggest that the measurement of soil respiration in calcareous soils could be overestimated if the isotopic signature of the CO2 is not taken into account. It also advocates more in-depth studies on carbonate dissolution-precipitation processes and their impact on CO2 emissions.Reference:Cardinael, R., Chevallier, T., Guenet, B., Girardin, C., Cozzi, T., Pouteau, V., and Chenu, C. 2019 Organic carbon decomposition rates with depth and contribution of inorganic carbon to CO2 emissions under a Mediterranean agroforestry system, Eur J Soil Sci, https://doi.org/10.1111/ejss.12908.

International audience; Aims: Mineral-associated organic matter, mainly derived from microbial by-products, persists longer in soil compared to particulate organic matter (POM). POM is highly recalcitrant and originates largely from decomposing root and shoot litter. Theory suggests that root traits and growth dynamics should affect carbon (C) accumulation into these different pools, but the specific traits driving this accumulation are not clearly identified.Methods: Twelve herbaceous species were grown for 37 weeks in monocultures. Root elongation rate (RER) was measured throughout the experiment. At the end of the experiment, we determined morphological and chemical root traits, as well as substrate induced respiration (SIR) as a proxy for microbial activity. Carbon was measured in four different soil fractions, following particle-size and density fractionation.Results: Root biomass, RER, root diameter, hemicellulose content and SIR (characteristic of N2-fixing Fabaceae species), were all positively correlated with increased C in the coarse silt fraction. Root diameter and hemicellulose content were negatively correlated with C in the POM fraction, that was greater under non N2-fixing Poaceae species, characterized by lignin-rich roots with a high carbon:nitrogen ratio that grew slowly. The accumulation of C in different soil pools was mediated by microbial activity.Conclusions: Our results show that root traits determine C input into different soil pools, mediated primarily by microbial activity, thus determining the fate of soil organic C. We also highlight that C in different soil pools, and not only total soil organic C, should be reported in future studies to better understand its origin, fate and dynamics.

International audience; Agroforestry systems have been much studied for their potential to store soil organic carbon (SOC). However, few data are available on their specific impact on potential SOC mineralization, especially at depth in subsoils. Moreover, many soils of the world, especially in arid and semi‐arid environments, also contain large stocks of soil inorganic carbon (SIC) as carbonates. Consequently, the organic carbon dynamics has been poorly investigated in these soils due to the complexity of measurements and of the processes involved. To assess mineralization rates of SOC with depth, we incubated soil samples coming from an 18‐year‐old agroforestry system (both tree row and alley) and an adjacent agricultural plot established on a calcareous soil in France. Soil samples were taken at four different depths: 0–10, 10–30, 70–100 and 160–180 cm. Total CO2 emissions, the isotopic composition (δ13C, ‰) of the CO2 and microbial biomass were measured. The SIC concentrations ranged from 48 to 63 g C kg−1 soil and the SOC concentrations ranged from 4 to 17 g C kg−1 soil. The contribution of SIC‐derived CO2 represented about 20% in the topsoil and 60% in the subsoil of the total soil CO2 emissions. The microbial biomass and the SOC‐derived CO2 emissions were larger in the topsoil, but the decomposition rates (day−1) remained stable with depth, suggesting that only the size of the labile carbon pool was modified with depth. Subsoil organic carbon seems to be as prone to decomposition as surface organic carbon. No difference in CO2 emissions was found between the agroforestry and the control plot, except in the tree row at 0–10 cm. Our results suggest that the measurement of soil respiration in calcareous soils could be overestimated if the isotopic signature of the CO2 is not taken into account. It also advocates more in‐depth studies on carbonate dissolution‐precipitation processes and their impact on CO2 emissions.

Soil organic carbon (SOC) constitutes the largest terrestrial carbon stock. Both distribution and variation of SOC stocks are needed to constitute reference baseline for studies on temporal SOC change. Specifically, in volcanic areas, the spatial variation of soil andic properties usually explains the spatial variation of topsoil SOC contents, but SOC data for deeper soil layers are needed. We measured the andic properties (e.g. pyrophosphate extractable aluminium and allophane contents) and SOC stocks down to 200-cm depth in a 1 km2 micro-watershed covered by Arabica coffee agroforestry in Costa Rica. We used diffuse reflectance mid-infrared (MIR) spectroscopy to allow a large number of soil analysis. The objectives of our study were (i) to evaluate MIRS as a low-cost and rapid tool to measure andic properties and SOC in Andosols and (ii) to predict the level of SOC stocks down to 200-cm depth.

The assessment of the impacts of land-use and management on soil organic carbon (SOC) dynamics is a major environmental concern, as the soil carbon cycle underpins key ecosystem services. However, assessments based on short-term SOC dynamics face methodological and experimental difficulties. Hurisso et al. (2016) proposed a method to assess SOC dynamics by coupling two methods: Permanganate Oxidizable Carbon (POXC) and Basal Soil Respiration (BSR). This method has been used in laboratory on dried and re-wetted soil samples from temperate regions mainly. In our study, we adapted this method to the field and proposed a cost-effective in-field indicator combining the POXC and in situ Basal Soil Respiration (SituResp (R) method). We tested the indicator at four study sites (n = 169 points) within various tropical land-use and management contexts based on rubber, soybean and oil palm cropping systems respectively in Thailand, Cambodia and Indonesia. The results demonstrated the relevance, sensitivity and robustness of the POXC-SituResp (R) indicator to characterize the impact of a gradient of disturbance on SOC dynamics. The results also highlighted the potential of conservation agriculture (no-tillage and crops residues) and compost amendments to accumulate SOC. Rock-Eval (R) analysis showed that POXC-SituResp (R) indicator is negatively linked to excess of potentially mineralizable labile carbon. Carbon pools targeted by the POXC were specified by Rock-Eval (R) pyrolysis measurements to be a rather thermal resistant pool of SOC. Our study confirms that the integrated indicator based on POXC and BSR assess a relative carbon stabilization of SOC pools. This indicator can be measured in the field by a rapid and cost-effective method.

Huit ans de travaux de recherche sur les services écosystémiques dans une grande ferme caféière du Costa Rica (observatoire collaboratif Coffee-Flux, en système agroforestier à base de caféiers sous de grands arbres d'Erythrina poeppigiana, surface projetée de couronne de l'ordre de 16 %) ont suggéré plusieurs applications pour les agriculteurs et les décideurs. Il est apparu que de nombreux services écosystémiques dépendaient des propriétés du sol (ici des Andisols), en particulier de l'érosion, de l'infiltration, de la capacité de stockage de l'eau et des éléments nutritifs. Nous confirmons qu'il est essentiel de lier les services hydrologiques et de conservation au type de sol en présence. Une densité adéquate d'arbres d'ombrage (plutôt faible ici) permet de réduire la sévérité des maladies foliaires avec, en perspective, une réduction de l'usage de pesticides-fongicides. Un simple inventaire de la surface basale au collet des caféiers permet d'estimer la biomasse souterraine et la moyenne d'âge d'une plantation de caféiers, ce qui permet d'évaluer sa valeur marchande ou de planifier son remplacement. Le protocole de calcul actuel pour la neutralité carbone des systèmes agroforestiers ne prend en compte que les arbres d'ombrage, pas la culture intercalaire. Dans la réalité, si on inclut les caféiers, on se rapproche très probablement de la neutralité. Des évaluations plus complètes, incluant les arbres, les caféiers, la litière, le sol et les racines dans le bilan en carbone du système agroforestier sont proposées. Les arbres d'ombrage offrent de nombreux servies écosystémiques s'ils sont gérés de manière adéquate dans le contexte local. Par rapport aux parcelles en plein soleil, nous montrons qu'ils réduisent l'érosion laminaire d'un facteur 2, augmentent la fixation de l'azote (N2) atmosphérique et le pourcentage d'azote recyclé dans le système, réduisant ainsi les besoins en engrais. Ils réduisent aussi la sévérité des maladies foliaires, augmentent la séquestration de carbone, améliorent le microclimat et atténuent substantiellement les effets des changements climatiques. Dans notre étude de cas, aucun effet négatif sur le rendement n'a été enregistré., Eight years of studying coffee ecophysiology and monitoring ecosystem services (ES) in a large coffee farm in Costa Rica revealed several practical recommendations for farmers and policy makers. The cropping system studied within our collaborative observatory (Coffee-Flux) corresponds to a coffee-based agroforestry system (AFS) under the shade of large trees of Erythrina poeppigiana (16 % of canopy cover). A lot of ES and disservices depend on local soil properties (here Andisols), especially erosion/infiltration, water/carbon and nutrient storage capacity. Therefore, for ES assessment, the type of soil is crucial. An adequate density of shade trees (rather low here) reduced the severity of leaf diseases with the prospect of reducing pesticide-fungicide use. A simple inventory of the basal area at collar of the coffee plants allowed estimating the belowground biomass and the average age of the plantation, to judge of its market value and to decide when to replace it. Coffee farms are probably much closer to C neutrality than predicted by the current C-Neutral protocol, which only considers shade trees. More comprehensive assessments, including trees, coffee, litter, soil, and roots in the C balance of the AFS are proposed. Shade trees offer many ES if they are adequately managed in the local context. As compared to full sun conditions, shade trees may (i) reduce laminar erosion by a factor of 2, (ii) increase N2 fixation and the % of N recycled into the system, thus reducing fertilizer requirements, (iii) reduce the severity of leaf diseases, (iv) increase C sequestration, (v) improve the microclimate, and (vi) substantially reduce the effects of climate change. In our case study, no negative effect on coffee yield was found.