Stream nitrogen response to Clean Air Act policies in the US: An application of National Nutrient Inventory and the National Rivers and Streams Assessment

Air quality regulations have decreased nitrogen (N) and sulfur (S) deposition to the United States (US) landscape over the last several decades. To assess the stream response to declining deposition across the US, we combined the US EPA's National Nutrient Inventory with N chemistry in small streams (watershed areas < 1000 km2) between 2000 and 2014 from the EPA's National Rivers and Streams Assessment. We focus the analysis on watersheds where deposition was the largest N input to the watershed using the inventory data. Weighted change analysis quantified that deposition declined across most of the US watersheds, especially across the Eastern region. However, average growing season stream nitrate concentrations showed only a small and non-significant decline in these areas. This small trend in nitrate was offset by a substantial and significant gain in total organic N (TON) concentrations across most of the US. Instead of a net decline, total N concentrations increased in these small streams where atmospheric deposition is the largest source across all regions. Increases in TON concentration coincided with reductions in atmospheric deposition of N and S, pointing toward a recovery from acidification, and is similar to recent increases in dissolved organic matter or "browning" of streams and lakes across northern latitudes. The water quality benefits of reducing N deposition loads through air quality regulations are masked by increases in organic nitrogen mobilization in small streams across the US.

Disclaimer: The views expressed in this abstract are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency.