Eutrophication, the increased supply of organic matter to a system, is often attributed to excess nutrient inputs and can lead to detrimental effects such as low oxygen and habitat loss. However, coastal sediments harbor microbial communities capable of transforming bioavailable nitrogen into inert gas (i.e. denitrification), thus potentially mediating eutrophication. For example, filter-feeding bivalves have been recognized as important facilitators of nitrogen removal by enhancing denitrification in sediments. Similarly, tidal salt marshes can serve as important hotspots for nitrogen removal through denitrification. This talk will explore controls on this critical microbial metabolism using clam aquaculture and salt marsh habitats as model systems. Surprising results showed that, on a local scale, high densities of clams can be a source of nitrogen by facilitating nitrogen recycling and promoting dissimilatory nitrate reduction to ammonium (DNRA), a microbial pathway that competes with denitrification. Depending on the ultimate source of phytoplankton supporting the cultivated bivalves, these filter-feeders may serve as a noncanonical bottom-up control on primary production on a local scale. Since denitrification removes nitrogen while DNRA recycles it, understanding what controls the partitioning between these microbial pathways and how this dynamic may shift in response to changes such as organic matter input, nutrient addition, or salinity is essential to predicting how key ecosystem services will change over time. These environmental drivers fluctuate in salt marsh sediments, where denitrification and DNRA co-exist, making this habitat an ideal setting to investigate these pathways. Using high throughput sequencing in combination with biogeochemical rate measurements in controlled experiments we explore the microbial mechanisms that govern this competition between denitrification and DNRA.