Citation:

Lassiter, Meredith, J. Lin, J. Compton, R. Sabo, J. Phelan, J. Stoddard, AND T. Greaver. Temporal and Spatial Shifts in the Dominant Form of Nitrogen Deposition to Watersheds of the Continental United States. Association of the Sciences of Limnology and Oceanography-Society of Freshwater Sciences Joint Summer Meeting, Madison, Wisconsin, June 07 - 12, 2020.

Impact/Purpose:

The purpose of this presentation is to communicate findings on a project examining nitrogen deposition trends to watersheds, and subsequent stream chemistry response, at the Association for the Sciences of Limnology and Oceanography and Society for Freshwater Sciences joint meeting to be held June 7-12, 2020. This project is significant in that it examines shifts in the form of nitrogen across temporal and spatial scales using data from the EPA National Aquatic Resource Surveys (for rivers and streams) to elucidate deposition trends impacting waterbodies. The session will likely have a broad impact in terms of consideration of the effects of changing deposition over time in aquatic ecosystems and stream chemistry response to these long-term changes in nitrogen inputs.

Description:

Atmospheric deposition represents a significant source of nitrogen (N) to many watersheds. Across the United States, air quality regulations effectively decreased the deposition of oxidized N over the past several decades. In contrast, ammonia emissions from intensive agriculture and motor vehicles increased over time, such that reduced (organic + ammonia) deposition is currently equivalent to oxidized N deposition for the entire nation. In some areas, reduced nitrogen deposition now exceeds oxidized nitrogen. We coupled estimates of N deposition form with stream measurements from the U.S. EPA National Aquatic Resources Surveys (three rivers and streams surveys between 2000 to 2014) to evaluate the influence of changing watershed deposition on associated stream chemistry. We also used a recent input inventory to focus on watersheds where atmospheric deposition is the dominant N input source. There is a clear temporal shift from a greater proportion of watersheds dominated by oxidized forms of N deposition to a greater proportion of watersheds dominated by reduced deposition. This pattern varies spatially, with watersheds with the highest proportion of reduced N deposition inputs corresponding to regions where agriculture is prevalent. In the subset of streams where atmospheric sources are the dominant source of nitrogen, responses of stream nitrate, total N and dissolved organic carbon suggest that the ongoing shifts in the form of nitrogen deposition may impact water chemistry.

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