The responses of soil nitrogen transformation to nitrogen addition are mainly related to the changes in functional gene relative abundance in artificial Pinus tabulaeformis forests

The increase of soil nitrogen (N) availability may alter soil microbial community composition and the natural N cycle in forest ecosystems. However, the responses of soil microbial nitrogen functional genes (NFGs) to N addition and their consequent effect on the N-cycle processes are poorly understood. In this study, soil samples were collected from an artificial Pinus tabulaeformis forest located in Loess Plateau (China) to which N at four different concentrations was added (0 [N0], 3 [N3], 6 [N6], and 9 [N9] g N m(-2) y(-1)) for 4 years. We quantified the relative abundance of NFGs using functional gene microarray GeoChip 5.0 and determined net N transformation and N2O emission rates in a 14-day incubation experiment. The results showed that N-3 and N6 treatments did not significantly affect the total relative abundance and diversity of NFGs assemblage but significantly increased the relative abundance of specific genes for the NH3 cycle (ureC, nirA, and nrfA), and nitrification (amoA) and denitrification (norB). These positive effects were related to the increase in soil organic C, NO3--N, and microbial biomass C (MBC). N9 treatment significantly decreased the relative abundance of all NFGs, and this negative impact was correlated with reduced dissolved organic C and MBC. Moreover, N addition significantly changed net N nitrification, mineralization, and N2O emission rates, and NFGs explained the higher variances in the N transformation processes than soil properties. Specifically, ammonia-oxidizing archaea (amoA-AOB) and MBC were the key factors related to net N nitrification; ureC, nirK, and MBC were the key factors related to net N mineralization; and narG and nirS were the key factors related to N2O emission. Our results show that global N deposition may mainly influence N transformation processes by regulating the corresponding NFG relative abundance, thereby affecting the N cycle in forest soils. (C) 2020 Published by Elsevier B.V.