Does Soil Microbial Community Respond to Moderately Elevated Nitrogen Deposition? A Correlation Analysis in a Cool Temperate Forest Surrounded by Pasture Grasslands in Northern Japan

Abstract We analyzed relationships between nitrogen deposition (deposition of nitrate and ammonium ions) and soil microbial properties, which were spatially varied in a cool temperate forest surrounded by normally fertilized pasture grasslands in northern Japan. The aim of the present study was to gain the primary information on soil microbial response to moderately elevated nitrogen deposition (< 10 kg N ha−1 y−1). We established three experimental plots in the forest edge adjacent to the grasslands and other three plots in the forest interior at least 700 m away from the grasslands. During May to November 2018, nitrogen deposition in each plot was measured. In August 2018, litter and soil (0−5 cm depth) samples were collected from all plots to measure net nitrogen mineralization and nitrification rates as indicators of microbial activity, and microbial biomass carbon and nitrogen and various gene abundances (i.e. bacterial 16S rRNA, fungal ITS, bacterial amoA, and archaeal amoA genes) as indicators of microbial abundance. Nitrogen deposition in the forest edge was 1.4-fold greater than that in the forest interior, even while the maximum deposition was 3.7 kg N ha−1. Nitrogen deposition was significantly correlated to the net nitrogen mineralization and nitrification rates and the 16S rRNA and bacterial amoA gene abundances. Microbial community structures in litter and soil samples were also analyzed using a high throughput DNA sequencer for the bacterial 16S rRNA and fungal ITS gene amplicons. Microbial community structures were different between litter and soil samples but were similar between the forest edge and interior. Significant correlations of nitrogen deposition to the soil carbon-to-nitrogen ratio, and the nitrate and ammonium contents were also observed. Thus, our results show that moderately elevated nitrogen deposition in nitrogen-limited forest edges likely stimulate microbial activities and abundances in soils.