Smooth brome invasion increases rare soil bacterial species prevalence, bacterial species richness and evenness

Plant and soil communities are tightly linked, but the mechanisms by which the invasion of an exotic plant and the resulting shifts in plant diversity and productivity influence soil bacterial community structure remain poorly understood. We investigated the effects of invasive smooth brome (Bromus inermis) on grassland soil bacterial community structure using massively parallel sequencing of the 16S rRNA gene to determine bacterial community richness, evenness, composition and beta diversity (UniFrac indices) of soils collected along a gradient of smooth brome abundance. We evaluated several hypotheses including: (a) that the declines in native plant diversity associated with smooth brome invasion would cause declines in bacterial community diversity and (b) that mechanisms driving smooth brome effects on bacterial community structure involved altered soil edaphic properties rather than preferential invasion in areas of high soil nitrogen and distinct soil microbial communities. Smooth brome invasion led to increased soil nitrogen, soil carbon and root biomass. Bacterial evenness and bacterial richness increased with increasing smooth brome cover, while bacterial beta diversity declined. We found no evidence of a dominant direct link between the alteration of soil edaphic properties by brome and the changes in the soil bacterial community. Rather, the main controls on the soil bacterial community were direct effects of pH and smooth brome that could not be linked to the edaphic changes. The most important effect of brome on the bacterial community was the selective suppression of dominant bacterial species, which allowed rarer bacteria to increase in relative abundance.Synthesis. Here, we show that plant community composition influences bacterial community structure at a very fine scale, but that these changes are not due to altered soil total nitrogen or carbon content. The dominant direct effect of smooth brome invasion on soil communities suggests non-edaphic, that is inter- and intratrophic, interactions among smooth brome and non-bacterial components of the soil ecosystem are key drivers of soil community structure.