Differential responses of particulate and mineral-associated organic carbon to nitrogen addition and their potential drivers in global grasslands

Nitrogen (N) addition can alter soil organic carbon (SOC) storage, but its effects on functionally distinct SOC fractions remain unclear in global grasslands. Using a meta-analysis of 237 paired observations from 41 studies across global grasslands, we assessed the responses of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) to N addition and explored their potential drivers. N addition significantly increased the POC and MAOC pools by 8.20% and 5.50%, respectively, and increased the POC:SOC ratio by 13.07%. POC showed a larger response to N addition than MAOC, suggesting that particulate and mineral-associated carbon pools differ in their sensitivity to N addition. The response of POC to N addition was significantly negatively correlated with the responses of soil pH and microbial biomass carbon (MBC), whereas the response of MAOC to N addition was significantly positively correlated only with the response of soil pH. Overall, our findings suggest that N addition reshapes SOC fraction composition by increasing the POC:SOC ratio in global grasslands. These results highlight the importance of considering both fraction-specific responses and their associated drivers when predicting soil carbon dynamics under increasing N deposition.