Collaborative Research: Quantifying the effects of different nitrogen forms on marsh resilience to environmental change

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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 most 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 plant organic matter in low-oxygen sediments. Understanding which of these two outcomes of nitrogen delivery is most likely to occur is important to the environment. If marsh plants take up that nitrogen and grow larger, this will cause the marsh to gain elevation by trapping more sediment and adding more plant material to the sediments. This process 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 sea levels. 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. This research also supports workforce development through training across multiple levels of education (high school, undergraduate, and graduate), in particular for people from historically marginalized groups. To understand how nitrogen speciation and plant genetic diversity affect the ability of salt marshes to keep pace with sea-level rise, this research has three goals: quantify how environmental nitrogen availability alters responses of the coupled plant-microbe system, determine how these responses vary under different flooding regimes, and assess how different populations of cordgrass respond to different nitrogen sources. To address our first goal, we are conducting paired plot-level nutrient enrichment experiments in two locations along the eastern US coast where nitrate and ammonium will be added at a range of concentrations for two years. We are measuring the effects of nitrogen addition on marsh plants, microbes, and carbon and nitrogen cycling. To address our second goal, we are performing experiments at both locations, where nitrogen form is crossed with elevation to assess how variation in elevation alters the responses of the plant and microbial communities to different forms of nitrogen. These experiments are purposefully designed so that results can be used in a new generation of the Marsh Equilibrium Model that incorporates how future nitrogen inputs will alter the capacity of marshes to keep pace with sea-level rise. Lastly, greenhouse nitrogen uptake experiments are providing essential data on population-specific uptake rates of both nitrogen forms in the absence of sediment microbes. Taken together, this research mechanistically addresses how different forms of nitrogen and plant genetic variation affect marsh ability to store carbon and keep pace with sea-level rise. This information is being incorporated into a modeling framework that allows resource managers to predict how marshes will respond to the combined effects of nutrient enrichment and sea-level rise.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.