Particulate organic carbon is more sensitive to nitrogen addition than mineral-associated organic carbon: A meta-analysis

Soil organic carbon (SOC) is heterogeneous and consists of multiple pools differing in physical and chemical properties. SOC stocks are expected to increase in response to nitrogen (N) enrichment. However, the comprehensive understanding and general conclusions about the responses of different SOC pools such as particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) to N addition are still lacking, in spite of the fact that this knowledge is of valuable for the accuracy of predictions of the vulnerability of SOC to N deposition. Here, we conducted a meta-analysis to explore the effects of N addition on POC and MAOC, as well as their relative proportions in bulk SOC. We found that N addition increased POC (20.6%) to a greater extent than MAOC (4.5%). Consequently, N addition significantly increased the proportion of POC in bulk SOC by 10.4%, but significantly decreased the proportion of MAOC in bulk SOC by 2.6%. The response of POC to N addition was negatively correlated with the response of pH and microbial biomass carbon (MBC), while the response of MAOC to N addition was positively correlated with the response of pH and MBC. Notably, the positive effect of N addition on the proportion of POC in bulk SOC decreased as the initial proportion of POC increased. Inversely, the negative effect of N addition on the proportion of MAOC in bulk SOC increased with the initial proportion of MAOC. Overall, our findings suggest that although N could enhance SOC stocks, the larger increment of POC would make the SOC pools more susceptible to future global changes. Our results also highlight the need to explicitly incorporate the differential responses of POC and MAOC into ecosystem models to improve predictions of SOC stock responses to global change.