Temporal and Spatial Shifts in the Dominant Form of Nitrogen Deposition to Watersheds of the Continental United States
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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.