Urea Fluctuations in Stream Baseflow across Land Cover Gradients and Seasons in a Coastal Plain River System

Urea-N is a component of bioavailable dissolved organic nitrogen (DON) that contributes to coastal eutrophication. In this study, we assessed urea-N in baseflow across land cover gradients and seasons in the Manokin River Basin on the Delmarva Peninsula. From March 2010 to June 2011, we conducted monthly sampling of 11 streams (4 tidal and 7 nontidal), 2 wastewater treatment plants, an agricultural drainage ditch, and groundwater underlying a cropped field. At each site, we measured urea-N, DON, dissolved organic carbon (DOC), total dissolved nitrogen (TDN), NO ₃ ⁻ -N, and NH ₄ ⁺ -N. In general, urea-N comprised between 1% and 6% of TDN, with the highest urea-N levels in drainage ditches (0.054 mg N/L) and wetland-dominated streams (0.035–0.045 mg N/L). While urea-N did not vary seasonally in tidal rivers, nontidal streams saw distinct urea-N peaks in summer (0.038 mg N/L) that occurred several months after cropland fertilization in spring. Notably, the proportion of wetlands explained 78% of the variance in baseflow urea-N levels across the Manokin watershed. In wetland-dominated basins, we found urea-N was positively related to water temperature and negatively related to DOC:DON ratios, indicating short-term urea-N dynamics at baseflow were more likely influenced by instream and wetland-driven processes than by recent agricultural urea-N inputs. Findings demonstrate important controls of wetlands on baseflow urea-N concentrations in mixed land-use basins.