Green Manure Coupled With Straw Returning Increases Soil Organic Carbon via Decreased Priming Effect and Enhanced Microbial Carbon Pump

Green manuring and crop straw returning are widely used to increase soil organic carbon (SOC) sequestration, while the pathways and drivers in native SOC mineralization and new SOC formation after implementing these practices remain unclear. Here, through a 10-locations' network field experiment in southern China, the effects of green manuring (Mv), rice straw returning (Rs), and their combination (MR) on soil C sequestration efficiency (CSE) were evaluated. A microcosmic experiment was performed to trace the components and participants of SOC mineralization and formation under the three practices. The network experiment showed that Mv, Rs, and MR annually improved SOC stock by 119.7, 477.0, and 830.2 kg C ha⁻¹, respectively, with MR having the highest CSE of 23.1%, followed by Rs (17.7%) and Mv (13.3%). The microcosmic experiment further revealed that the highest CSE in MR most likely resulted from the low mineralization of native SOC (positive priming effects, PE) and great formation of new SOC through microbial C pump (MCP). Therein, MR mainly downregulated the K-strategists of the microbial community (e.g., Gaiellales) to yield negative PE on recalcitrant native SOC, such as protein/amino sugar- and lignin-like molecules; meanwhile, MR had the highest bacterial and fungal MCPs, which were, respectively, led by r-strategists (e.g., Sporobacter) and molds (e.g., Cladosporium). The study highlights the advantages of mixing low- (Rs) and high-quality (Mv) residues for efficiently increasing SOC sequestration and firstly discovers the core microbes that dominate the mineralization and formation of SOC in paddy fields.