Long-term fertilizer postponing promotes soil organic carbon sequestration in paddy soils by accelerating lignin degradation and increasing microbial necromass

Soil organic matter (SOM) in paddy soils is critical for achieving high crop yield sustainability and mitigating climate change. Our previous study revealed that long-term fertilizer postponing (FP) increases yield by improving SOM. Therefore, in this study, the mechanism by which fertilizer postponing increase SOM were investigated through a long-term experiment (11 years). Long-term FP significantly increased SOM (by 14.5%) by increasing root residue input. FP increased soil phenol oxidase activity and peroxidase activity but did not affect β-glucosidase activity, indicating that FP accelerated lignin degradation rather than cellulose degradation. Metagenome analysis also showed that FP significantly increased the relative abundance of lignin degradation genes, such as soil catalase, cytochrome c peroxidase, and peroxidase genes, by activating the growth of related microorganisms. In addition, long-term FP significantly increased bacterial necromass C (by 17%) and fungal necromass C (by 13%) by improving microbial biomass. Redundancy analysis and random forest model further revealed that lignin degradation genes (representing the contribution of lignin) and bacterial necromass C were dominant in plant- and microbe-derived C, respectively. We confirmed that the soil NH4+-N content was the main driving factor for high lignin degradation and microbial necromass variation. Structural equation model and random forest model revealed that soil carbon sequestration was mainly through the increase of lignin degradation and microbial necromass under high root residues input and soil NH4+-N content. Overall, our findings suggest that long-term FP is a sustainable agricultural management strategy that improves SOM by accelerating lignin degradation and increasing microbial necromass.