Nitrogen delivery from watersheds to the coast harms aquatic ecosystems, including the formation of dead zones, where oxygen concentrations drop below levels that can support much aquatic life. Salt marshes provide an important line of defense against nutrient pollution by intercepting watershed nitrogen before it enters estuaries. This nitrogen can enhance the growth of marsh plants, much as fertilizers enhance growth in gardens. However, some forms of nitrogen can also be used by microbes, who use it as a substitute for oxygen to decompose organic matter in low-oxygen sediments. Understanding which of these two outcomes is most likely to occur is important because, if marsh plants take up that nitrogen and grow more robustly, they will allow the marsh to gain elevation by trapping more sediment and adding more plant material to the sediments. This will increase salt marsh resilience to sea-level rise. However, if marsh microbes can outcompete marsh plants for the added nitrogen, then it could enhance sediment decomposition and reduce the ability of marshes to keep up with rising seas. The results of this research will help predict how marshes will respond to sea-level rise in coastal systems that experience large inputs of land-derived nitrogen, helping to create more resilient coastal communities.