Nitrogen Availability Modulates Root-Mediated Soil Organic Carbon Formation

Living roots influence the formation of soil organic carbon (SOC) under straw incorporation, but the mechanisms underlying this process remain unclear. To elucidate these effects, we traced the fate of isotopically labeled maize straw into SOC via soil cores that were incubated in a soybean field. Our findings suggest that root ingrowth does not significantly affect the total SOC accumulation under straw incorporation but instead facilitates the allocation of SOC into stable SOC pools. However, the formation of stable SOC differs fundamentally between low-nitrogen (LN) and high-nitrogen (HN) soils. In LN soils, root ingrowth promotes the accumulation of stable SOC by promoting the transfer of straw-derived carbon into occluded particulate organic carbon (oPOC) and mineral-associated organic carbon (MAOC). In contrast, root ingrowth reduces the contribution of straw-derived C to stable SOC in HN soils, but this reduction may be offset by the incorporation of rhizodeposited C into oPOC and MAOC. Further research indicates that these different patterns are driven by a stoichiometric imbalance between the microbial demand for carbon and nitrogen and their resource availability. These findings provide mechanistic insights into how the presence of roots under straw incorporation influences SOC formation, deepening our understanding of root-mediated SOC sequestration.