Effects of Nitrogen Inputs and Watershed Characteristics on Summer Stream Nitrogen Concentrations: A National-Scale Analysis

Abstract:

Nitrogen (N) inputs to the landscape have been linked previously to N loads exported from watersheds at the national scale; however, stream N concentration is arguably more relevant than N load for drinking water quality, freshwater biological responses and establishment of nutrient criteria. In this study, we combine national-scale anthropogenic N input data, including synthetic fertilizer, crop biological N fixation, manure applied to farmland, atmospheric N deposition, and point source inputs, with data from the 2008-09 National Rivers and Streams Assessment to quantify the relationship between N inputs and in-stream concentrations of total N (TN), dissolved inorganic N (DIN), and total organic N (TON) (calculated as TN – DIN). In conjunction with simple linear regression, we use multiple regression to understand how watershed and stream reach attributes modify the effect of N inputs on N concentrations. Median TN was 0.50 mg N L^-1 with a maximum of 25.8 mg N L^-1. Total N inputs ranged from less than 1 to 196 kg N ha^-1 y^-1, with a median of 14.4 kg N ha^-1 y^-1. Atmospheric N deposition was the single largest anthropogenic N source in the majority of sites, but agricultural sources generally dominate total N inputs in sites with elevated N concentrations. The sum of all N inputs were positively correlated with concentrations of all forms of N [r² = 0.44, 0.43, and 0.18 for TN, DIN, and TON, respectively (all log-transformed), n = 1112], indicating that watershed N inputs are strongly related to stream N concentrations during the summer, despite this being a biologically active and N-retentive period. Additionally, model results suggest that watershed characteristics like wetland area, riparian disturbance and forest cover moderate the effects of watershed N loading on in-stream N concentrations, and different forms of N are likely to respond differently to increasing agricultural and atmospheric N inputs depending on local watershed characteristics.