Pierre Barré

Soil organic carbon (SOC) immobilisation and sequestration have been found to be influenced by a wide range of environmental factors, notably the climate, stand characteristics, and soil properties. Recent research has suggested that some soil properties traditionally regarded as major predictors of SOC stocks (e.g. clay content) may be less influential than those pH–dependent, reactive constituents, including crystalline aluminium (Al) and iron (Fe) oxides, and basic cations. However, these soil properties have rarely been considered for explaining large-scale variation in SOC storage. Here, by taking the advantage of the RENECOFOR network double forest soil inventory at a 15–year interval over the French European territory, we investigate how both SOC storage and its temporal dynamics vary as a function of climate, stand characteristics, and soil properties. Furthermore, we used Rock-Eval® (RE) thermal fractionation to identify descriptors correlated with proxies of SOC biogeochemical stability, in particular the hydrogen index (HI – hydrocarbon proportion of SOC) and T50 CO2 PYR (thermal stability indicator positively correlated to SOC biogeochemical stability). While the effect of the cations only occurred when pH ≥ 5, the metal oxides increased SOC for both pH ≥ 5 and pH < 5 groups. These effects were also identified on the thermal stability of C (T50 CO2 PYR) for the 0–10 and 10–20 cm soil layers. No unequivocal relationship was found between HI and soil properties. When considering reactive soil constituents, clay content played a limited role in explaining variations of SOC and RE metrics. While HI was lower in coniferous forests, there was no other clear effect of climate nor stand characteristics. Regarding the C dynamics, only the 0–10 cm layer showed sufficiently high variations to be confidently analysed. For this layer, the expected variables (e.g. temperature or litter input) did not explain any of the variations of SOC or RE metrics. However, the composition of SOC, through tree species or HI, had an influence on the SOC dynamics, with lower SOC variation the more the carbon was labile. Additionally, the variation of SOC was highly influenced by the dynamics of holorganic layers, with a decrease of SOC content variation with an increase of the change of forest floor stock. Our results highlight the generally stronger role of soil factors than climatic or stand ones in explaining large-scale variation in forest SOC storage, largely due to the inclusion of refined soil geochemical properties beyond clay content.

Assessing the chemical composition of soil organic matter (OM) inputs is essential to understand and simulate soil OM decomposition dynamics and thus better evaluate the soil contribution to the global C balance. The Van Soest (VS) chemical extraction distinguishes four fractions of the OM namely “soluble”, “hemicellulose-”, “cellulose-” and “lignin-”like compounds. However, this procedure is time-consuming, costly, and requires acids, solvents and detergents. The Rock-Eval® (RE) thermal analysis consists of pyrolysis of the sample followed by oxidation of the residue. Since the 2000s, it has been increasingly used in soil science to quantify soil organic carbon (SOC) via the TOC parameter and to characterize SOC thermal stability, a proxy of biological stability, through various indices. Although several studies have investigated the relationship between SOC thermal stability and the chemical composition of soil OM, it has to be consolidated in order to identify and quantify the main components of OM, especially for OM of litters. In this study, we further explored this relationship by comparing the chemical composition (VS fractions) and the RE-derived parameters determined on various agricultural and forestry litters. The RE signals obtained before (bulk sample) and after the VS extraction (lignin-like compound residue) were compared. The effect of the VS extraction was reflected on the RE signals of the lignin-like compound residue. The thermolabile compounds emitted during the pyrolysis were lost and the CO and CO2 signals obtained during the oxidation resembled those of pure lignin. Correlation matrix between the RE parameters and the VS fractions were performed. The cellulose and hemicellulose proportions were positively correlated to the hydrocarbon compounds (HC) emitted below 340 °C (named A1, Spearman coefficient = 0.75, p-value

Pyrogenic carbon (PyC) is the carbonaceous solid residue of incomplete combustion of biomass. It is a continuum of mostly condensed and aromatic molecules. PyC persists for longer in soils relative to non-PyC organic carbon. However, estimates of PyC residence time vary greatly. The time and spatial scales investigated are not always adapted to the long-residence time and vertical and lateral mobility of PyC in the soil profile and the landscape. In addition, agricultural land-use and shallow slopes are under-represented in the PyC literature. We measured the concentrations and stocks of PyC down to 60 cm along three toposequences in a small agricultural catchment with shallow slopes and homogeneous soil parent material in the west of France. We used two methods (chemo-thermal oxidation – CTO and hydropyrolysis – HyPy) of PyC quantification that cover the intermediate to highly condensed part of the PyC continuum, and also measured the radiocarbon values in both total soil organic carbon (SOC) and the PyC fraction. There was likely little persistent PyC inputs to the catchment in the last 150 years which gave us access to the resultant, long term PyC distribution in the landscape. In particular, we aimed to investigate whether the vertical and horizontal distribution of PyC were similar or differed from SOC and whether they were affected by the soil types along the slope. Topographic position was not the main driver of PyC stocks in this landscape. The stock of PyC_CTO to 60 cm depth averaged 2.5 ± 0.22 t ha⁻¹ across topographic positions. PyC stocks were the highest in a Solimovic Cambisol at the toeslope (3.3 ± 0.26 t ha⁻¹), likely formed following changes in erosion dynamics with land-use. Contrary to previous reports, erosion redistributed already aged PyC without enrichment or depletion. PyC_HyPy concentrations in the topsoil decreased from upslope (median = 1.6, IQR = 0.22 g C kg⁻¹ soil) to downslope positions (median = 1.10, IQR = 0.40 g C kg⁻¹ soil), which we tentatively attribute to PyC_HyPy leaching following the destabilisation of mineral associations with iron oxides in the water-table affected portion of the transects. The subsoil (30–60 cm) represented between 37 % and 51 % of the PyC_CTO stock. PyC_HyPy proportion in SOC increased with depth and reached an average of 11 ± 3.3 % at 50–60 cm depth. PyC_HyPy had an uncalibrated radiocarbon age of 2520 to 9600 years BP at this depth, significantly older than bulk SOC at the same depth and than PyC_HyPy at 0–10 cm (1530 to 2630 years BP). These results confirm the long persistence of PyC in soils and point to a slow advection of PyC towards the soil depth under the pedoclimatic conditions of our study area. Future studies should assess whether erosion modalities and age and quality of PyC affect its fate during erosion events. Identifying the proportion of PyC produced which is quickly transported away from the watershed and that which remains and is stabilised in soils for millennia after a fire is an important knowledge gap that still needs to be investigated to close the terrestrial PyC budget.

Recycling organic waste products (OWPs) is known to influence soil physical, chemical, and biological properties, yet few studies have compared the long-term effects of different OWP type across multiple sites. This study examined the impacts of repeated OWP application on soil properties in two French long-term field experiments: QualiAgro and PROspective (20 and 18 years, respectively). The OWP included dehydrated urban sewage sludge (SLU), green waste and SLU compost, biowaste compost from source-separated municipal organic waste co-composted with green waste, municipal solid waste compost, farmyard manure from a dairy cow farm (FYM), and composted FYM from open-air composting on a concrete platform. The application of OWP led to increased soil nutrient levels and trace element availability, and stimulated microbial biomass and enzyme activities, while the response of nematode varied depending on site and OWP type. Biological properties were less affected than physico-chemical properties, though the OWP application enhanced soil microbial biomass and specific enzyme activities. The impact on soil nematode communities varied depending on OWP type and site. Strong correlations were observed among soil property changes, with exogenous carbon and nutrient inputs from OWP identified as key drivers. Larger changes were noted in QualiAgro, where OWP application rates were higher and initial soil quality lower. These findings highlight that OWP applications, depending on their type, rate, and initial soil conditions, can significantly alter soil properties.

Pyrogenic carbon (PyC) is the carbonaceous solid residue of incomplete combustion of biomass. It is a continuum of condensed and aromatic molecules. PyC persists for longer in soils relative to non-PyC organic carbon. However, residence time varies depending on the method used to estimate it. The time and spatial scales investigated are not always adapted to the long-residence time and vertical and lateral mobility of PyC in the soil profile and the landscape. In addition, agricultural land-use and shallow slopes are under-represented in the PyC literature. We measured the concentrations and stocks of PyC down to 60 cm along three toposequences in a small agricultural catchment with shallow slopes and homogeneous soil parent material in the west of France. We used two methods (chemo-thermal oxidation – CTO and hydropyrolysis – HyPy) of PyC quantification that cover the intermediate to highly condensed part of the PyC continuum, and also measured the radiocarbon values in both total SOC and the PyC fraction. There was no or little PyC inputs to the catchment in the last 150 years which gave us access to the resultant, long term PyC distribution in the landscape. In particular, we aimed to investigate whether the vertical and horizontal distribution of PyC were similar or differed from SOC and whether they were affected by the gradient of soil evolution along the slope. Topographic position was not the main driver of PyC stocks in this landscape. The stock of PyC_CTO averaged 2.5±0.22 t ha^-1 across topographic positions and was only slightly larger in a Solimovic Cambisol at the toeslope (3.3±0.26 t ha⁻¹), likely formed following changes in erosion dynamics with land-use. Contrary to previous reports, erosion redistributed already aged PyC without enrichment or depletion. Future studies should assess whether erosion modalities and age and quality of PyC affect its fate during erosion events. PyC_HyPy concentrations in the topsoil decreased from upslope (median = 1.6, IQR = 0.22 gC kg⁻¹ soil) to downslope positions (median = 1.10, IQR = 0.40 gC kg⁻¹ soil), which we attribute to PyC_HyPy dissolution following the destabilization of mineral associations with iron oxides in the water-table affected portion of the transects. The subsoil (30–60 cm) represented between 37 and 51 % of the PyC_CTO stock. PyC_HyPy proportion in SOC increased with depth and reached an average of 11±3.3 % at 50–60 cm depth. PyC_HyPy had an uncalibrated radiocarbon age of 2520 to 9600 years BP at this depth, significantly older than bulk SOC at the same depth and than PyC_HyPy at 0–10 cm. These results confirm the long persistence of PyC in soils and point to a slow advection of PyC towards the soil depth under the pedoclimatic conditions of our study area. Identifying the proportion of PyC produced which is quickly transported away from the watershed and that which remains and is stabilized in soils for millennia after a fire is an important knowledge gap that still needs to be investigated to close the terrestrial PyC budget.

Pyrogenic carbon (PyC) is the carbonaceous solid residue of incomplete combustion of biomass. It is a continuum of condensed and aromatic molecules. PyC persists for longer in soils relative to non-PyC organic carbon. However, residence time varies depending on the method used to estimate it. The time and spatial scales investigated are not always adapted to the long-residence time and vertical and lateral mobility of PyC in the soil profile and the landscape. In addition, agricultural land-use and shallow slopes are under-represented in the PyC literature. We measured the concentrations and stocks of PyC down to 60 cm along three toposequences in a small agricultural catchment with shallow slopes and homogeneous soil parent material in the west of France. We used two methods (chemo-thermal oxidation – CTO and hydropyrolysis – HyPy) of PyC quantification that cover the intermediate to highly condensed part of the PyC continuum, and also measured the radiocarbon values in both total SOC and the PyC fraction. There was no or little PyC inputs to the catchment in the last 150 years which gave us access to the resultant, long term PyC distribution in the landscape. In particular, we aimed to investigate whether the vertical and horizontal distribution of PyC were similar or differed from SOC and whether they were affected by the gradient of soil evolution along the slope. Topographic position was not the main driver of PyC stocks in this landscape. The stock of PyC_CTO averaged 2.5±0.22 t ha^-1 across topographic positions and was only slightly larger in a Solimovic Cambisol at the toeslope (3.3±0.26 t ha⁻¹), likely formed following changes in erosion dynamics with land-use. Contrary to previous reports, erosion redistributed already aged PyC without enrichment or depletion. Future studies should assess whether erosion modalities and age and quality of PyC affect its fate during erosion events. PyC_HyPy concentrations in the topsoil decreased from upslope (median = 1.6, IQR = 0.22 gC kg⁻¹ soil) to downslope positions (median = 1.10, IQR = 0.40 gC kg⁻¹ soil), which we attribute to PyC_HyPy dissolution following the destabilization of mineral associations with iron oxides in the water-table affected portion of the transects. The subsoil (30–60 cm) represented between 37 and 51 % of the PyC_CTO stock. PyC_HyPy proportion in SOC increased with depth and reached an average of 11±3.3 % at 50–60 cm depth. PyC_HyPy had an uncalibrated radiocarbon age of 2520 to 9600 years BP at this depth, significantly older than bulk SOC at the same depth and than PyC_HyPy at 0–10 cm. These results confirm the long persistence of PyC in soils and point to a slow advection of PyC towards the soil depth under the pedoclimatic conditions of our study area. Identifying the proportion of PyC produced which is quickly transported away from the watershed and that which remains and is stabilized in soils for millennia after a fire is an important knowledge gap that still needs to be investigated to close the terrestrial PyC budget.

Pyrogenic carbon (PyC) is the carbonaceous solid residue of incomplete combustion of biomass. It is a continuum of condensed and aromatic molecules. PyC persists for longer in soils relative to non-PyC organic carbon. However, residence time varies depending on the method used to estimate it. The time and spatial scales investigated are not always adapted to the long-residence time and vertical and lateral mobility of PyC in the soil profile and the landscape. In addition, agricultural land-use and shallow slopes are under-represented in the PyC literature. We measured the concentrations and stocks of PyC down to 60 cm along three toposequences in a small agricultural catchment with shallow slopes and homogeneous soil parent material in the west of France. We used two methods (chemo-thermal oxidation – CTO and hydropyrolysis – HyPy) of PyC quantification that cover the intermediate to highly condensed part of the PyC continuum, and also measured the radiocarbon values in both total SOC and the PyC fraction. There was no or little PyC inputs to the catchment in the last 150 years which gave us access to the resultant, long term PyC distribution in the landscape. In particular, we aimed to investigate whether the vertical and horizontal distribution of PyC were similar or differed from SOC and whether they were affected by the gradient of soil evolution along the slope. Topographic position was not the main driver of PyC stocks in this landscape. The stock of PyC_CTO averaged 2.5±0.22 t ha^-1 across topographic positions and was only slightly larger in a Solimovic Cambisol at the toeslope (3.3±0.26 t ha⁻¹), likely formed following changes in erosion dynamics with land-use. Contrary to previous reports, erosion redistributed already aged PyC without enrichment or depletion. Future studies should assess whether erosion modalities and age and quality of PyC affect its fate during erosion events. PyC_HyPy concentrations in the topsoil decreased from upslope (median = 1.6, IQR = 0.22 gC kg⁻¹ soil) to downslope positions (median = 1.10, IQR = 0.40 gC kg⁻¹ soil), which we attribute to PyC_HyPy dissolution following the destabilization of mineral associations with iron oxides in the water-table affected portion of the transects. The subsoil (30–60 cm) represented between 37 and 51 % of the PyC_CTO stock. PyC_HyPy proportion in SOC increased with depth and reached an average of 11±3.3 % at 50–60 cm depth. PyC_HyPy had an uncalibrated radiocarbon age of 2520 to 9600 years BP at this depth, significantly older than bulk SOC at the same depth and than PyC_HyPy at 0–10 cm. These results confirm the long persistence of PyC in soils and point to a slow advection of PyC towards the soil depth under the pedoclimatic conditions of our study area. Identifying the proportion of PyC produced which is quickly transported away from the watershed and that which remains and is stabilized in soils for millennia after a fire is an important knowledge gap that still needs to be investigated to close the terrestrial PyC budget.