Objective To study the effects of N deposition on soil organic carbon and microbial community structure in subtropical evergreen broadleaved natural forest, in order to better understand the response of forest ecosystem to future N deposition and develop effective mitigation strategies.

Method An N addition experiment was designed to evaluate the effect of N deposition on soil carbon (C) and soil microbial communities in a natural Castanopsis carlesii forest in subtropical China. In this study, randomized block design was adopted with three-levels of N addition, viz. control (CK, 0 kg·hm⁻²·a⁻¹), low N (LN, 40 kg·hm⁻²·a⁻¹) and high N (HN, 80 kg·hm⁻²·a⁻¹).

Result Compared with the CK, the total microbial biomass examined by phospholipid fatty acid analysis (PLFA) were insignificantly increased in LN and HN treatment. The SOC concentration in LN treatment was 27.4% higher than that in CK treatment in 0-10 cm depth, while no significant effect being observed in HN. There were no significant effects of N addition on SOC concentration at the 10-20 cm soil depth in both LN and HN treatment. LN treatment significantly increased the total biomass and the biomass of each microbial component, while there was no significant effect in HN treatment at the 0-10 cm soil depth. Correlation analysis and random forest model analysis showed that increased dissolved organic carbon (DOC) concentration, total nitrogen (TN) concentration, C/N ratio and the increase of total microbial biomass under N deposition are the key factors driving soil organic C accumulation. Principal component analysis showed that N deposition significantly changed the microbial community at the 0-10 cm soil depth while not in the 10-20 cm depth.

Conclusion Short term elevated atmospheric N inputs can increase forest soil C storage by increasing DOC concentration and microbial biomass in subtropical natural forest, while the long-term effects of N deposition on soil C sequestration remains unknown.