Denitrification in tidal freshwater wetlands may reduce the amount of nitrogen delivered to coastal watersheds from streams and rivers. To maximize this important ecosystem function, the interactions between plant community composition and rates of denitrification of tidal freshwater marshes need to be better understood. We investigated whether marsh surface elevation, plant community composition (annuals vs. perennials), and organic matter quantity/quality, influenced denitrification rates in a tidal freshwater marsh of the Potomac River, U.S.A. We measured denitrification rates using both denitrification enzyme assays (DEA) with acetylene inhibition and direct N₂ flux measurements with membrane inlet mass spectrometry (MIMS). 
Results/Conclusions

Results showed that organic carbon content and nitrate availability in the soil increased with elevation, and annual plants were found at higher elevations than perennials; however, DEA rates did not differ across marsh surface elevation. Soil organic carbon was highest in plots dominated by perennial graminoids, but DEA rates did not differ across plant community types. DEA rates increased with soil ammonium, and total N was higher in June than October. MIMS rates did not differ across plant community types, but increased with soil organic nitrogen. Denitrification rates were within the same range using both techniques, and suggest that denitrification at the site was substantial. In addition, denitrification rates found in Dyke Marsh were higher than rates found in the adjacent Potomac River. Therefore, tidal freshwater marshes represent an important sink for nitrogen, and factors fostering denitrification should be considered when restoring coastal wetlands as they are faced with pressures from urbanization and sea level rise.