Nitrogen dynamics in a forested exurban watershed

Small streams are important sites of nitrogen removal, yet many have been so heavily impacted by humans that they cannot appreciably reduce the downstream flux of N.  There is evidence that N processing is reduced in urban watersheds, yet we are just beginning to understand how urbanization impacts nutrient processing in exurban watersheds.  Here we examine N dynamics in a forested-exurban watershed within the Maryland Piedmont using a combination of water-quality and soil surveys, ecosystem manipulations, and microcosm studies.  We present evidence that the impacts of urbanization on ecosystem function are not limited to streams draining urban landscapes; with both N saturation and nutrient limitation occurring as a result of chronically elevated N concentrations.  Atmospheric deposition is the probable source of N; with delivery occurring primarily via groundwater and the greatest proportion of the in-stream load transported during baseflow.  We need to reconsider the impacts of urbanization the delivery of N to downstream water bodies; in particular, we need to focus on managing urban N sources, improving our understanding of the mechanisms underlying N saturation and our ability to manage and restore streams to process N, and preserving ecosystem function in forested watersheds given the greater context of landscape urbanization.