Long-term nitrogen and phosphorus fertilization regulates straw carbon sequestration by altering soil stoichiometric imbalance and microbial nutrient limitations

Straw return is widely adopted in North China to enhance soil organic carbon (SOC) storage and crop productivity, yet its C sequestration efficiency has declined in recent years. Nitrogen (N) and phosphorus (P) fertilization regulates straw decomposition and soil C turnover, but the long-term effects and mechanisms of fertilization regimes on straw-derived C sequestration remain unclear. Soils from a 42-year long-term fertilization experiment with five gradient fertilization rates (F1, no fertilizer; F2, 180 kg N ha −1 yr −1 +120 kg P 2 O 5 ha −1 yr −1 ; F3, 360 kg N ha −1 yr −1 +240 kg P 2 O 5 ha −1 yr −1 ; F4, 540 kg N ha −1 yr −1 +360 kg P 2 O 5 ha −1 yr −1 ; F5, 720 kg N ha −1 yr −1 +480 kg P 2 O 5 ha −1 yr −1 ) were incubated with 13 C-labeled maize straw for 276 days to investigate straw-derived C turnover. Results showed that long-term combined N and P fertilization significantly increased soil C, N, and P pools while reducing soil and microbial C/N/P ratios. Fertilized treatments exhibited higher cumulative CO 2 emissions and greater straw-derived and native soil-derived CO 2 effluxes than F1, with the highest emissions observed under F4. Straw-derived C incorporation into SOC was significantly higher in F3-F5 than that in F1 and F2, but showed no significant difference between F4 and F5. Soil respiration and straw C turnover were closely associated with soil nutrient status, stoichiometric characteristics, and microbial resource-use efficiencies, with C and P limitations emerging as important influencing factors. Overall, long-term combined N and P fertilization affected straw-derived C sequestration and native SOC dynamics, whereas higher fertilization rates stimulated native SOC mineralization, suggesting a potential trade-off between straw-derived C stabilization and native SOC preservation. These findings highlight the need to optimize N and P fertilization intensity under straw-return management to improve SOC sequestration and soil sustainability in intensive croplands.