Watershed Vulnerability to Invasive N2-Fixing Autumn Olive and Consequences for Stream Nitrogen Concentrations

Autumn olive ( Thunb.) is an invasive and exotic N-fixing plant species found throughout the United States. Proliferation and spread of autumn olive have displaced native plants and raised concerns about the effects of N fixation and cycling on water quality in invaded areas. This study investigated the relationship between autumn olive cover and stream N concentrations. Twelve forested watersheds were selected and classified into edge, mid-distance, and interior-of-the-forest watersheds based on autumn olive density and distance from the permanent edge of invasion point along a major road corridor. For the 2012 vegetation survey, autumn olive cover in edge, mid, and interior watersheds ranged from 37 to 61%, 18 to 37%, and 4 to 10%, respectively. From 2006 to 2012, mean stream water NO-N concentration in the edge watersheds was significantly higher (1.39 mg L, < 0.0001) than mid (0.37 mg L) and interior (0.27 mg L) watersheds. A linear relationship was found between NO-N concentration and autumn olive cover ( = 0.72, = 0.0001). Mean stream water NH-N, specific conductivity, and pH were significantly less in the interior watersheds than in the edge watersheds. Additionally, peak specific conductivity and NO-N from edge watersheds coincided with peak stage for these watersheds, demonstrating that N flushing events were driven by surface and shallow subsurface flow pathways proximal to the stream. Results from this study demonstrate how encroachment of autumn olive can influence water quality and transform biogeochemical cycles in natural systems, which points to the need for effective management of autumn olive in the edge watersheds and riparian zones that are vulnerable to invasion and increased N export.