Microplastics are increasingly found in freshwater, estuarine and marine sediments. The bacterial communities within these sediments regulate key biogeochemical cycles, such as nitrogen and carbon. Previous studies have reported that polymer type can influence the composition of microbial communities thriving on floating plastics. Here, we show that microplastics of diverse polymer types differentially influenced sediment bacterial communities, as well as the biogeochemical cycles they mediate. With the two-fold goal to address microbial community structure and nitrogen cycling, we established a microcosm experiment with microplastics (53-300 mm) of four different polymer types: polyethylene (PE), polyvinyl chloride (PVC), polyurethane foam (PUF) and polylactic acid (PLA; a bio-polymer). After 7 and 16 days of incubation, sediment aliquots were sampled for 16S rRNA gene sequencing with Illumina MiSeq platform and qPCR of the genes responsible for nitrification (amoA) and denitrification (nirS and nirK). Nitrogen cycling processes were examined by measuring dissolved inorganic nitrogen concentrations in the overlying water and potential denitrification rates using a sediment slurry experiment with 15NO3- tracer. We observed that bacterial community compositions differed significantly between treatments with the most divergent in PVC-amended sediment. Bacterial amoA gene abundances and dissolved inorganic nitrogen fluxes revealed that nitrification was lowest in PVC treatments, and highest in PLA and PUF. Denitrification rates were correspondingly suppressed in PVC, but enhanced in PLA and PUF compared to the non-amended control. This suggests that both PLA and PUF may be serving as a carbon source to support denitrifiers in sediments. These results illustrate that microplastic interactions with bacteria extends beyond just the biofilm on the surface of the plastic, and can influence adjacent sediment bacterial communities mediating the critical biogeochemical cycles.