Anthropogenic nitrogen (N) additions to ecosystems can have wide-ranging impacts on biogeochemical cycles, biodiversity, ecosystem structure, and the emissions of greenhouse gases. Soil microbes are the keystone group regulating N in terrestrial systems, from nitrogen fixation to denitrification, and yet there is still significant ambiguity about how archaeal, bacterial, and fungal taxa respond to N additions. In this study we utilized long-term nitrogen treatments at two LTER sites, Cedar Creek (CC) in Minnesota and Kellogg Biological Station (KBS) in Michigan, to examine the response of microbial communities to anthropogenic N additions. Using high-throughput pyrosequencing techniques and quantitative PCR (qPCR) we quantified changes in soil microbial communities across the nitrogen gradients (ranging from 0 kg/ha/yr to 800 kg/ha/yr).
Results/Conclusions

We observed strong directional shifts in community composition at both sites; suggesting that nitrogen deposition has specific predictable impacts on microbial communities. For example, at CC and KBS acidobacteria experienced 10% and 20% declines, respectively as nitrogen increased, with other groups such as the proteobacteria increasing in relative abundance across the N gradients. In contrast to these large, community-level responses, we observed only small changes in archaeal and fungal abundances relative to bacteria; except at the highest N addition plots. At extreme levels of nitrogen (>200 kg/ha/yr) we saw decreases in both the relative abundances of archaea and fungi. This study represents the first large-scale sequencing effort to document N impacts on microbial communities and the results suggest that N additions have strong and predictable impacts on bacterial community structure.