The effects of nitrogen addition on soil organic carbon decomposition and microbial C-degradation functional genes abundance in a Pinus tabulaeformis forest

Nitrogen (N) deposition affects soil organic carbon (SOC) decomposition, therefore altering the global terrestrial carbon (C) cycle. However, it remains unclear how N deposition affects SOC decomposition by regulating microbial community composition and function, especially C-cycling functional gene composition. We investigated the effects of N addition (0, 3, 6, and 9 g N m−2 y–1) on the composition of soil microbial C-cycling functional gene, SOC-degrading enzyme activities, and CO2 emissions in a Pinus tabulaeformis forest. Under low N addition (3 or 6 g N m−2 y–1), labile C-degradation gene abundances were significantly increased. Under high N addition (9 g N m−2 y–1), C-cycling functional gene abundance and diversity were significantly decreased. These effects were related to the changes in soil NO3–-N, dissolved organic C, total N, and microbial biomass C contents. Furthermore, low N addition stimulated the activities of SOC-degrading enzyme and CO2 emissions, whereas high N addition had the inhibitory effect. C-degradation gene abundances were significantly correlated with the SOC-degrading enzyme activities and CO2 emissions. Increase in CO2 emission rates were related to the high microbial functional potentials for labile C degradation under low N addition, whereas the lower CO2 emission rates were related to the low microbial functional potentials for labile as well as recalcitrant C degradation under high N addition. Our study indicated that N deposition may change SOC decomposition by altering the abundance of labile C and recalcitrant C degradation genes.