Long-term phosphorus addition enhances the contributions of plant lignin and microbial necromass to soil organic carbon in a rice–wheat rotation

Plant-derived compound (PDC) and microbial necromass carbon (MNC) are the two most important drivers in soil organic carbon (SOC) formation and accrual. Continuous phosphorus (P) addition affects SOC accumulation in paddy soils. However, the underlying mechanisms by which P addition affect the contributions of PDC and MNC to the accrual of SOC remain unclear. Here, a 12-year P addition experiment with four rates (0, 75, 150, and 300 kg P₂O₅ ha⁻² crop season⁻¹) was conducted in paddy soils with rice–wheat rotation. Our findings showed that particulate organic carbon (POC) increased by P addition (P < 0.05), but mineral-associated organic carbon (MAOC) was not affected (P > 0.05). P addition enhanced the content and SOC-normalized content of lignin phenols by 40.1–71.8 % and 34.2–49.5 %, respectively, which were coincided with a decrease in lignin degradation. P addition enhanced the MNC content because of higher microbial biomass and lower nitrogen (N) acquisition enzyme activity. Moreover, P addition promoted the MNC contribution to the accrual of SOC, primarily from the increased contribution of fungal necromass carbon (FNC). The decrease in amino sugars to lignin phenols ratio and increase in POC to MAOC ratio implied that P addition might reduce SOC stability and enhance SOC loss potential. Long-term P addition increased bacterial richness but decreased fungal richness compared to that of the control. Moreover, bacterial and fungal richness: biomass ratios significantly reduced by P addition. Random forest and structural equation modelling analysis revealed that fungal richness played a significant role in controlling the contributions of PDC and MNC to SOC in paddy soils. Furthermore, microbial richness: biomass ratio was another non-negligible factor regulating PDC and MNC accumulation. Overall, our findings suggest that long-term P addition increases plant- and microbial-derived compounds but the increase in PDC is greater than in MNC, indicating that plant-derived SOC plays a more key role in SOC sequestration in paddy soils under continuous P addition.