Long-term, amplified responses of soil organic carbon to nitrogen addition worldwide.

Soil organic carbon (SOC) is the largest carbon sink in terrestrial ecosystems and plays a critical role in mitigating climate change. Increasing reactive nitrogen (N) in ecosystems caused by anthropogenic N input substantially affects SOC dynamics. However, uncertainties remain concerning the effects of N addition on SOC in both organic and mineral soil layers over time at the global scale. Here, we analysed a large empirical data set spanning 60 years across 369 sites worldwide to explore the temporal dynamics of SOC to N addition. We found that N addition significantly increased SOC across the globe by 4.2% (2.7%-5.8%). SOC increases were amplified from short- to long-term N addition durations in both organic and mineral soil layers. The positive effects of N addition on SOC were independent of ecosystem types, mean annual temperature and precipitation. Our findings suggest that SOC increases largely resulted from the enhanced plant C input to soils coupled with reduced C loss from decomposition and amplification was associated with reduced microbial biomass and respiration under long-term N addition. Our study suggests that N addition will enhance SOC sequestration over time and contribute to future climate change mitigation.