The intimate associations between plants and microbes that develop in rhizosphere soils may alter the sensitivity of bacterial communities to environmental filters during the process of community assembly. We compared the sensitivity of rhizosphere and bulk soil communities to ecological variables in several New England salt marshes dominated by Spartina alterniflora (Smooth Cordgrass), characterizing community variation based on linear combinations of 16S rDNA fragments representing bacterial taxonomic units. Rhizosphere microbes showed significantly greater sensitivity to plant biomass and pH, perhaps because of their relationships with the quality of plant exudates and availability of nutrients, respectively, while bulk microbes showed significantly greater sensitivity to elevation and redox potential, perhaps through waterlogging and an absence of plants oxygenating the soil.