Exchange of Nitrogen through an Urban Tidal Freshwater Wetland in Philadelphia, Pennsylvania

Tidal freshwater wetlands in urban settings can be subject to elevated N concentrations, which can promote the exchange of N between the marsh, water, and atmosphere, including denitrification. We used a multitiered approach consisting of direct measurements of N fluxes and denitrification, tidal hypsometry, and N load modeling to examine N exchanges in an urban tidal freshwater wetland of the Delaware River Estuary, Philadelphia, PA. Sediment cores and aboveground biomass were collected at 20 locations across a range of elevations and plant communities in April, July, and October 2010. Nitrate was taken up by the marsh during all seasons. In the spring, the high rate of NH production from the sediment was correlated with NO uptake, suggesting dissimilatory reduction to NH as a potentially important process. Denitrification rates were greatest in July, averaging 5.5 ± 0.6 mg N m h. Adjusted for tidal inundation using a refined digital elevation model, denitrification averaged 0.08, 0.5, and 0.2 g N m mo for April, July, and October, respectively. Less than 10% of the modeled N load was estimated to have been removed in the months measured. A combination of high N load, limited marsh area that represented ∼1% of the watershed area, and conservative extrapolation of denitrification rates contributed to the low estimate of the N load attenuated.