Coastal salt marshes are some of the most productive ecosystems on Earth, providing numerous services such as soil carbon storage, flood protection and nutrient filtering, several of which are mediated by the sediment microbiome associated with marsh vegetation. Here, nutrient filtering (nitrate removal through denitrification) was examined by determining microbial (bacteria and archaea) community structure (16S rRNA gene iTag sequencing), diversity, denitrification rates and metabolic potential (assembled metagenomic sequences) in collocated patches of Spartina alterniflora (Spartina) and Juncus roemerianus (Juncus) sediments. The iTag data showed that diversity and richness in Spartina and Juncus sediment microbial communities were highly similar. However, microbial community evenness differed significantly, with the most even communities observed in Juncus sediments. Further, denitrification rates were significantly higher in Juncus compared to Spartina, suggesting oscillations in microbial abundances and in particular the core microbiome identified herein, along with plant diversity influence marsh nitrogen (N) removal. Amplicon and assembled metagenome sequences pointed to a potentially important, yet unappreciated Planctomycetes role in N removal in the salt marsh. Thus, ecosystem perturbations that alter marsh vegetation distribution could impact microbial diversity and may ultimately influence the ecologically important ecosystem functions the marsh sediment microbiome provides.