Biogeochemical processes are essential for ecosystem function through the transformation and recycling of nutrients. Benthic microbial communities facilitate biogeochemical processes that have global implications, including nitrification and methanogenesis, which are components of nitrogen and carbon cycling, respectively. Microplastics (plastic particles <5mm) are present in every ecosystem. Yet, we know very little about their influence on biogeochemical processes, particularly in freshwater habitats. We spiked naïve sediments in sealed microcosms over the course of 14 days to investigate the effects of 0.5% (w/w) polyvinylchloride (PVC) microparticles and polyester (PES) microfibers on NH4+ oxidation, NO3– reduction, and CH4 production in single and mixture exposure. Water was collected within the first 48 hours to measure nitrogen transformation, whereas gas samples were collected from day 1 – 3 and on day 14 to measure CH4. Pilot results showed over 48 hours, microplastic treatments had a mean nitrification rate three times higher (-4.1 to -4.3 µg N L-1 hr-1) than controls (-1.4 µg N L-1 hr-1), and a mean denitrification rate two-times higher (-19.6 to -20.3 µg N L-1 hr-1) compared to controls (-10.0 µg N L-1 hr-1). After 3 days CH4 production rates were lowest in PVC treatments (1.9 ppm day-1), and similar in PES (4.3 ppm day-1), PVC + PES (4.7 ppm day-1), and controls (3.5 ppm day-1); however, after 14 days PES rates were approximately 1.5-times higher (5.1 ppm day-1) than controls (3.5 ppm day-1). These preliminary results suggest microplastics may affect nitrogen and carbon reduction-oxidation reactions, presumably due to effects on microbial communities and expression of functional genes responsible for specific processes investigated.