Combined application of nitrogen and phosphorus fertilizers increases soil organic carbon storage in cropland soils

Inorganic fertilization is indispensable in modern agriculture, yet its effects on soil organic carbon (SOC) storage and the underlying driving factors remain uncertain due to natural and anthropogenic interferences. In this study, bootstrap and random forest algorithms were employed to examine the effects of various inorganic fertilization regimes on SOC and crop yield, using a comprehensive dataset derived from 332 peer-reviewed publications. Moreover, the responses of SOC storage to agricultural management practices, climatic conditions, and initial soil properties under combined nitrogen (N) and phosphorus (P) fertilization were analyzed. Results indicated that inorganic fertilization generally increased crop yield and enhanced SOC sequestration. The increases in SOC and crop yield were significantly higher under combined N and P fertilization (i.e., NP and NPK fertilization) than under N fertilization alone. Straw return was the only agricultural management practice that significantly enhanced the annual SOC change rates. However, combined N and P fertilization increased SOC storage even without straw return, probably due to the enhanced plant-derived C inputs. Additionally, soil nutrient conditions, particularly soil P availability, were the key regulators of SOC turnover and storage under combined N and P fertilization. Microbial P limitation constrains the magnitude of SOC sequestration in cropland soils. In conclusion, our findings highlight the pivotal role of soil P availability in promoting SOC sequestration under combined N and P fertilization. Therefore, further efforts are required to determine the optimal amounts and ratios of N and P fertilizers to achieve higher soil C sequestration while sustaining crop yield.