Green manure roots return drives saline-alkali soil organic carbon accumulation via microbial necromass formation

Green manure strongly affects saline-alkali soil organic carbon (SOC) sequestration. The mechanism by which green manure influences the contribution of plant and microbial-derived carbon (C) to SOC in wheat-green manure cropping system remains unclear. Herein, plant residue C (PRC), microbial, bacterial, and fungal necromass C (MNC, BNC, and FNC), enzyme activity and microbial community were determined under wheat fallow after harvest (CK), green manure roots return (GMR), and green manure shoots and roots return (GMRS) in a five-year field experiment. Compared with CK, GMR and GMRS increased SOC content by 12 % and 11 % at 0–20 cm, respectively. Specifically, GMR accelerated the lignin biotransformation by increasing the relative abundance of K-strategy fungi, caused a reduction in the contribution of plant residues to SOC by 16–31 %. While GMR increased MNC, especially BNC by 1.6–2.8 times, which was the primary driver of SOC sequestration. Comparatively, GMRS increased the relative abundance of r-strategy bacteria by 12–13 %, and C- and N-acquisition enzymes by 12–17 % and 56–68 % compare to CK. This in turn, increased the accumulation of PRC, but decreased MNC (especially FNC) contribution to SOC. Overall, green manure return strategies altered the contribution of plant residues and microbial necromass to SOC by regulating microbial life strategies. MNC (especially FNC) contributed more to SOC than PRC. Therefore, green manure specially root return is a viable option to drive SOC accumulation via microbial necromass formation in wheat-green manure cropping system in saline-alkali soils.