Divergent impact of long-term anthropogenic nitrogen inputs on global particulate and mineral-associated organic carbon

Abstract Background Soil organic carbon (SOC) represents the largest carbon sink in terrestrial ecosystems. Anthropogenic nitrogen inputs increase the levels of reactive nitrogen, significantly influencing SOC dynamics. Given the complexity of SOC, it remains unclear how its key components—particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)—respond to nitrogen addition over time, due to their fundamental differences in formation, persistence, and functions. We conducted a meta-analysis of 105 global studies and 499 observational results to investigate the responses of soil organic carbon components to long-term nitrogen addition. Results The results showed that nitrogen addition had a consistently positive impact on POC content, with the impact becoming increasingly pronounced with increasing nitrogen addition duration. Short-term nitrogen addition led to significant increases in MAOC content (9.47%), while long-term addition (over 20 years) resulted in a notable 12.19% reduction. The observed increase in aboveground biomass following nitrogen addition likely drives the rise in POC, whereas the inhibitory effects of long-term nitrogen addition on microbial activity may explain the negative effects on MAOC. Conclusions Overall, although long-term nitrogen addition will promote soil carbon sequestration to a certain extent, the stability of the soil carbon pool might be reduced. Therefore, future studies on the response mechanisms of soil carbon stability to nitrogen deposition need to take into account the impact of duration to improve our understanding of the spatial and temporal variations in soil carbon cycling amidst climate change challenges.