Investigating the complementarity of thermal and physical soil organic carbon fractions

Evaluating SOC biogeochemical stability is key to better predict the impact of SOC on both climate mitigation and soil health. This evaluation can be conducted using SOC partition schemes that allow us to quantify SOC fractions with different biogeochemical stability. However, most of these schemes are costly or time consuming and cannot be implemented on large sample sets. Two exceptions are  the widely used physical fractionation protocol allowing to separate particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) and the emerging thermal fractionation protocol distinguishing active SOC (Ca; MRT ~30 years) from stable SOC (Cs; stable at a centennial timescale). Here, we use analyzes conducted on samples from the French soil monitoring network (RMQS) to compare the results of thermal fractionations (Ca/Cs) performed on ca. 2000 samples, and physical fractionations (POC/MAOC) performed on ca. 1000 samples. Our results show that MAOC and Cs from one side and POC and Ca from the other side have different sizes. The most biogeochemically stable fractions (Cs and MAOC) are mostly influenced by soil characteristics whereas land cover and climate influence more substantially POC and Ca. However, the more stable fractions provided by both fractionation schemes (respectively the more labile fractions) do not have the exact same environmental drivers. Our results therefore suggest that both fractionation scheme gives complementary results. The relative contribution of these fractionation schemes to the evaluation of soil functions and OC stock evolution remains to be evaluated on soil monitoring networks and constitutes a promising research avenue.