Table 3_A 20% nitrogen reduction threshold optimizes soil carbon stability and microbial functional resilience under long-term straw return in a rice–wheat rotation.docx

Straw return has been widely adopted to enhance soil organic carbon (SOC) sequestration; however, the mechanisms by which reduced N input interacts with straw return to regulate SOC stabilization remain poorly understood. Here we evaluated graded N reduction (0, 20, and 30%) under continuous straw return in a nine-year rice-wheat double-cropping experiment in central China.

Soil carbon pools, aggregate stability, organic carbon functional groups, and microbial community structure and function were assessed using carbon fractionation, wet-sieving, FTIR spectroscopy, and metagenomic analyses. We found that 20% N reduction (80%NR) maintained grain yield comparable to conventional N fertilization while significantly increasing readily oxidizable organic carbon (ROOC, +18–26%), water-soluble organic carbon (WSOC, +12–21%), microbial biomass carbon (MBC, +15–24%), and carbon management index (CMI, +42–58% relative to NR).