Ammonium nitrogen content is a dominant predictor of bacterial community composition in an acidic forest soil with exogenous nitrogen enrichment.

Soil pH is a dominant factor affecting bacterial community composition in acidic, neutral, and alkaline soils but not in severely acidic soils (pH<4.5). We conducted a nitrogen (N) addition experiment in the field in severely acidic forest soil to determine the response of the soil bacterial community and identified the dominant factor in determining community composition. Using a high-throughput Illumina HiSeq sequencing platform, we found that high levels of N addition significantly decreased soil bacterial diversity and altered the composition of the soil bacterial community. The addition of nitrogen increased the relative abundance of copiotrophic taxa (Proteobacteria and Actinobacteria phyla) but decreased the relative abundance of oligotrophic taxa (Acidobacteria, Verrucomicrobia, Planctomycetes, and WD272). In particular, the relative abundance of N-cycling-related microbes (e.g., Burkholderia and Rhizomicrobium genera) also increased upon addition of N. Our correlation analysis showed that soil ammonium nitrogen concentration, rather than pH or nitrate nitrogen concentration, was a key environmental parameter determining the composition of the soil bacterial community. However, these bacterial response behaviors were observed only in the dry season and not in the wet season, indicating that high temperature and precipitation in the wet season may alleviate the impact of the addition of N on soil bacterial diversity and community composition. These results suggest that the soil bacterial community shifted to copiotrophic taxa with higher N demands under increased N addition in severely acidic forest soil.