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.