Nitrogen addition-driven soil organic carbon stability depends on the fractions of particulate and mineral-associated organic carbon

Nitrogen (N) deposition greatly affects soil carbon (C) fractions, triggering changes in soil organic carbon (SOC) persistence and functionality. However, the responses of soil C fractions to N deposition remain unclear on a global scale. Here, we conducted a meta-analysis of 69 publications and explored the response of C fractions (particulate organic carbon, POC; mineral-associated organic carbon, MOC) to N addition. Our findings reveal that N addition significantly increases the particulate organic carbon (POC) and mineral-associated organic carbon (MOC) pools (32.3% and 8.8%, respectively). Nevertheless, we observed a notable increase in the fraction of POC (fPOC) and a decrease in the fraction of MOC (fMOC) (15.9% and -6.3%, respectively), indicating that N addition augments the SOC pool but decreases SOC stability worldwide. Moreover, the response ratios of POC and MOC were positively correlated with the duration of N addition. In terms of SOC increase, POC was the most important predictor under short-term N addition, whereas MOC significantly contributed to SOC accumulation after long-term N addition. Overall, our study provides solid evidence that N addition reduces SOC stability primarily through changes in POC and proposes a novel approach to predict the soil C-climate feedback for Earth System Models.