Different patterns in long-term forested watershed nitrate export in the central Appalachian Mountains

Markedly different trends in nitrate export across forested watersheds of similar composition and age are difficult to replicate in ecohydrological models. Forested watersheds in the central Appalachian mountains of Pennsylvania have relatively large inputs of nitrogen from atmospheric deposition with only a small percentage exported in streamflow. Yet the area weighted fluxes and long-term trends vary dramatically despite catchment similarities. Furthermore, long-term stream export from Young Womans Creek, PA does not correlate with trends in atmospheric deposition. We hypothesize that hot spots and hot moments can vary greatly enough over decadal time scales to control stream nitrate export. Recent work has sought to quantify denitrification rates in discrete locations of the landscape (hot spots) during relative brief periods of time (hot moments) in an effort to upscale to watershed annual budgets and infer controls on nitrate export. While historic data on these nitrogen control points are unavailable, we identify opportunities for new data collection to help explain differential trends in nitrate export across forested watersheds. These control points in space and time serve as conceptual framework to better resolve watershed scale budgets. Much of the research on denitrification control points has focused on riparian zones and groundwater losses. For example, recent work in the Leading Ridge Experimental Watershed has shown the potential for stream network dynamics to control nitrate export. Coherent datasets that measure hot spots and hot moments in a way that can be upscaled and incorporated into ecohydrological models is important. This work could help understand climate change impacts on nitrogen yields from forested watersheds, which comprise more than 50% of the Chesapeake Bay watershed and contribute nearly 20% to the total nitrogen load.