Wetlands are used as management tools to combat the problem of excess nitrogen in surface waters of the United States. This is particularly true in urban or urbanizing watersheds. However, due to hypothesized higher rates of atmospheric nitrogen deposition and altered hydrology in the urban context, urban wetlands may actually act as sources of nitrate to receiving waters. Inorganic nitrogen inputs and outputs were characterized in nine palustrine, forested wetlands located along a gradient from very urban to less urban conditions in northeastern New Jersey. Previous research indicated that some sites have low water tables and low denitrification rates associated with urbanization. Three throughfall collectors and three tension lysimeters at 50 cm depth were sampled weekly at each site. Throughfall and leachate samples were analyzed for nitrate and ammonium concentrations and natural abundances of 15N and 18O isotopes to distinguish between atmospheric versus nitrification sources of nitrate in soil leachate. Rates of atmospheric N deposition were higher in wetlands located in more intensely urban sub-watersheds, as defined by higher population densities, road densities, impervious surface coverage, and urban land use. Nitrate losses through leaching were generally low and did not correlate with landscape-level descriptors of urban intensity. Preliminary results from the dual isotope analysis indicate that nitrate leaching from urban wetlands may have an atmospheric signature during the winter. We conclude that nitrogen inputs are sensitive to the intensity of surrounding urban land use, but solution nitrate losses are determined by within-site characteristics that are independent of local land use.
