Effects of long-term water and straw management on microbial necromass carbon accumulation in rice paddy fields

Microbial necromass carbon (MNC) is a key component of stable soil organic carbon (SOC) pools. Water management and straw returning are critical agricultural measures that affect SOC stability and carbon cycle. However, the long-term effect of these practices on microbial-derived SOC remains unclear. Here, a 12-year field experiment was conducted using mid-season drainage (MD) and continuous flooding (CF) with or without straw returning. The study aimed to evaluate how water and straw management affect MNC and its contribution to SOC. The results showed that straw returning in 2022 increased fungal necromass carbon (FNC) by 32.3 %, but reduced the contribution of bacterial necromass carbon (BNC) to SOC. In 2023, MD increased MNC and BNC by 9.2 % and 12.6 % respectively compared to CF. Straw returning increased MNC, FNC and BNC by 15.3 %, 14.2 % and 16.8 % respectively, but remarkably reduced the contribution of MNC to SOC in 2023. Notably, straw returning had a larger effect on FNC accumulation than BNC. Linear regression (LS) analysis identified TN, bacterial and actinomycetes PLFAs as the main drivers of MNC accumulation, whereas NO3–-N, fungal and AMF PLFAs were the primary factors affecting BNC accumulation.These findings highlight the importance of living microbial biomass in MNC accumulation. The partial least squares path model (PLS-PM) revealed that straw returning increased soil TN and TP, improved soil C/P and N/P ratio, alleviated soil phosphorus limitation, increased microbial PLFAs, and ultimately promoted MNC accumulation. Overall, mid-season drainage combined with straw returning is the most effective management strategy to promote the accumulation of MNC.