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 4 + production from the sediment was correlated with NO 3 − uptake, suggesting dissimilatory reduction to NH 4 + as a potentially important process. Denitrification rates were greatest in July, averaging 5.5 ± 0.6 mg N m −2 h −1 . Adjusted for tidal inundation using a refined digital elevation model, denitrification averaged 0.08, 0.5, and 0.2 g N m −2 mo −1 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.