Assembly of root‐associated bacteria communities: interactions between abiotic and biotic factors

Summary Nitrogen ( N ) deposition in many areas of the world is over an order of magnitude greater than it would be in absence of human activity. We ask how abiotic ( N ) and biotic (plant host and neighborhood) effects interact to influence root‐associated bacterial ( RAB ) community assembly. Using 454 pyrosequencing, we examined RAB communities from two dominant alpine tundra plants, G eum rossii and D eschampsia cespitosa , under control, N addition and D . cespitosa removal treatments, implemented in a factorial design. We hypothesized that host would have the strongest effect on RAB assembly, followed by N , then neighbor effects. The most dominant phyla were P roteobacteria (mostly G ammaproteobacteria ), A ctinobacteria , B acteroidetes and A cidobacteria . We found RAB communities were host specific, with only 17% overlap in operational taxonomic units. Host effects on composition were over twice as strong as N effects. D . cespitosa   RAB diversity declined with N , while G . rossii   RAB did not. D . cespitosa removal did not influence G . rossii   RAB community composition, but G . rossii   RAB diversity declined with N only when D . cespitosa was absent. We conclude that RAB of both hosts are sensitive to N enrichment, and RAB response to N is influenced by host identity and plant neighborhood.