Nitrogen addition has contrasting effects on particulate and mineral-associated soil organic carbon in a subtropical forest

Increasing atmospheric nitrogen (N) deposition has substantially affected carbon (C) and nutrient cycling in forest ecosystems. However, the responses of different soil organic carbon (SOC) fractions with different turnover rates to N addition are highly divergent, and the underlying mechanisms remain elusive. In this study, we explored the responses of surface soil (0–10 cm) characteristics and microbial communities to six years of experimental N addition (0, 50, 100 and 150 kg N ha−1 yr−1) in a subtropical evergreen broadleaf forest in southern China. Our results showed that N addition led to significant soil acidification (pH from 5.3 to 4.9). Microbial biomass carbon and total microbial, bacterial and fungal abundance (phospholipid fatty acid, PLFA) were reduced by N addition, but extracellular enzymes involved in C, N and phosphorus (P) cycling were not responsive to N addition. Soil extractable Ca2+ concentration was depleted by N addition, while other extractable cations (Fe3+, Al3+, Mg2+, K+, Na+) were not affected. Moreover, N addition did not significantly change the C and N concentration of bulk soil. We further separated the bulk soil into particulate organic matter (>53 μm, POM) and mineral-associated organic matter (<53 μm, MAOM) fractions by wet sieving. Interestingly, C in the POM fraction was significantly increased by N addition, while C in the MAOM fraction was depleted by N addition. Correlation analysis and structural equation modeling results suggested that N addition may suppress microbial decomposition of plant inputs and thus lead to accumulation of C in the POM fraction, while it may reduce the mineral sorption of microbial necromass and products and thus cause depletion of C in the MAOM fraction. Taken together, our results highlighted the vulnerability of soil C in the stable MAOM fraction to N addition, and emphasized the role of soil metals (particularly extractable Ca2+) and pH in controlling soil C storage under N addition.