Soil microbial community responding to moderately elevated nitrogen deposition in a Japanese cool temperate forest surrounded by fertilized grasslands

In order to examine the hypothesis that the soil microbial community in a nitrogen-limited forest responds to moderately elevated nitrogen deposition (< 10kg N ha-1 yr-1), correlations between nitrogen deposition and soil microbial properties were analyzed in a cool temperate forest surrounded by normally fertilized pasture grasslands in northern Japan. Three experimental plots were established in forest edges adjacent to the grasslands and the other three plots were in forest interiors at least 700 m away from the grasslands. Nitrogen deposition in each plot was measured from May to November 2018. In August 2018, litter and surface soil samples were collected from all plots to measure net nitrogen mineralization and nitrification rates as indicators of microbial activity, and microbial biomass and various gene abundances (i.e., bacterial 16S rRNA, fungal ITS, and bacterial and archaeal amoA genes) as indicators of microbial abundance. Nitrogen deposition in forest edges was 1.4-fold greater than that in forest interiors, whereas maximum deposition was 3.7 kg N ha−1. Nitrogen deposition was significantly correlated with net nitrogen mineralization and nitrification rates and 16S rRNA and bacterial amoA gene abundances. Microbial community structures analyzed for bacterial 16S rRNA and fungal ITS gene amplicons were different between litter and soil samples, but were similar between the forest edge and interior. Nitrogen deposition was also correlated with the soil carbon-to-nitrogen ratio and nitrate and ammonium contents. Thus, it was suggested that some soil microbial activities and abundances in a nitrogen-limited forest likely responded to moderately elevated nitrogen deposition. These findings provide primary information on soil microbial response to moderately elevated nitrogen deposition.