Nitrogen-induced acidification increases soil organic carbon accrual by promoting particulate organic carbon and microbial necromass under long-term experiment in the paddy soils of East China

Background and aims Nitrogen (N) addition can substantially affect soil carbon cycling in agroecosystems. Microbial necromass carbon (MNC) is widely recognized as a key contributor to soil organic C (SOC) fractions. However, the mechanisms underlying the responses of MNC and SOC fractions to N fertilization in paddy soils remain unclear. Methods A field experiment with four N rates, namely, 0, 300, 450, and 600 kg N ha ^–1 yr ^–1 was conducted to determine the effects of N addition on SOC fractions, soil microbial necromass carbon (MNC), enzyme activity, and microbial biomass in paddy soils with rice–wheat rotation. Results N addition increased SOC and POC concentrations by 2.88–8.41% and 14.6–41.2%, respectively, but did not affect MAOC. The ratio of MAOC to POC was reduced by N addition, indicating that N addition decreased SOC stability. N addition increased MNC concentration by 7.32–22.5% and its contribution to SOC by 4.14–13.7%. The activity of β-1,4- N -acetyl-glucosaminidase was decreased, while the activities of leucine amino peptidase and acid phosphatase were increased under P addition. Structural equation modeling and random forest revealed that N-induced decrease in soil pH promoted the accrual of MNC by increasing root and microbial biomass, consequently improving POC. Conclusions POC is likely more vulnerable to N addition than MAOC. N-induced acidification is the primary driver for promoting SOC accrual by increasing POC in paddy soils.