Delayed flooding after green manure incorporation enhances microbial necromass formation and stabilization in paddy soils

Green manuring is a cost-effective strategy to enhance soil organic carbon (SOC) sequestration in paddy systems, yet its effectiveness depends on water management–induced redox dynamics that regulate microbial decomposition and utilization of plant inputs. Here, we used a 13 C-labeled microcosm experiment to test how brief drainage prior to flooding influences green manure-derived C transformation and native SOC mineralization. Four treatments were applied: continuous flooding without (Control) or with green manure incorporation (GM), and delayed flooding by five days without (DW) or with green manure incorporation (GMDW). Compared with GM, GMDW reduced priming-induced CO 2 and CH 4 emissions from native SOC by 12.4% and 97.1%, respectively, while increasing green manure-derived 13 C-CO 2 emissions (+21.0%) and strongly suppressing 13 C-CH 4 emissions (-96.6%). GMDW enhanced the incorporation of green manure C into microbial necromass, increasing 13 C-bacterial and fungal necromass C by 64.0% and 204.5%, respectively, compared with GM. Therefore, the efficient accumulation of microbial necromass and the suppression of priming effects jointly underpin green manure–derived C sequestration. This shift was driven by transient aerobic conditions that stimulated plant-C degradation, increased green manure–derived dissolved organic C by 80.9%, and favored r-strategist and fungal-dominated communities, thereby accelerating microbial growth and necromass formation. Subsequently, under flooded conditions, the declined abundance of microbial C-degrading genes suggests a lower potential risk of necromass C decomposition, promoting necromass preservation and increasing the necromass accumulation coefficient. In contrast, delayed flooding without green manure enhanced microbial C-degrading gene abundance and potentially elevate the risks of native organic C decomposition. Collectively, owing to the readily decomposable character of milk vetch, delayed flooding induces a metabolic shift that accelerates the conversion of plant-derived C into microbial necromass during aerobic phases, and enhances its stabilization under subsequent anoxic conditions. This mechanism provides a practical strategy to increase SOC formation in paddy soils.