Nitrogen Retention in Salt Marsh Systems Across Nutrient-Enrichment, Elevation, and Precipitation Regimes: a Multiple-Stressor Experiment

In the Northeastern USA, multiple anthropogenic stressors, including changing nutrient loads, accelerated sea level rise, and altered climatic patterns, are co-occurring and are likely to influence salt marsh nitrogen (N) dynamics. We conducted a multiple-stressor mesocosm experiment to assess impacts of climate change and nutrient load on N uptake by the ecosystem dominant species. The New England salt marsh plant Spartina alterniflora was planted at mean high water (MHW) and 15 cm above and below MHW in tanks plumbed to mimic tides. The experiment consisted of two nutrient treatments (enriched, unenriched), three precipitation treatments (rain, storm, and no precipitation or control), and three elevations (low, mean, and high), with four replicate pots for each. A quarter of the way into the experiment (1 month), an N stable isotope tracer was added to a portion of the precipitation events received by the rain and storm treatments to assess how N is retained by the different components of each treatment. At the completion of the experiment, Spartina pots in the rain treatments retained far more tracer than the pots receiving the twice monthly storms, with the most tracer recovered at the highest elevation in all precipitation treatments as these pots received direct tracer input to stems and sediment surface. Experimental results suggest that the elevation of the marsh as well as the timing and delivery of rainfall may be important factors in how salt marshes intercept, retain, and transform N.