The changing nitrogen landscape of United States streams: Declining deposition and increasing organic nitrogen

Air quality regulations decreased nitrogen (N) and sulfur deposition across the conterminous United States (CONUS) during the last several decades. But it is unclear if declining deposition has also altered stream N at large scales. We compared watershed N inputs with N chemistry from over 2000 CONUS streams where deposition was the largest N input to the watershed. Change analysis showed deposition declined across most watersheds, especially in the Eastern CONUS. Nationally, declining N deposition was not associated with large-scale declines in stream nitrate concentration. Instead, significant increases in stream organic C and organic N were widespread across regions. Possible mechanisms behind these increases include declines in acidity and/or ionic strength drivers, changes in carbon availability, and/or climate variables. Our results illustrate the complexity of ecosystem nutrient cycling that links long term N deposition to water quality.  

Impact/Purpose

Nitrogen deposition to the landscape of the eastern US has declined substantially since the initial enactment of the US Clean Air Act Amendment regulations in 1990. Although one of the goals of these regulations is to protect aquatic ecosystems from acidification and eutrophication from added N and S, few studies assess whether these reductions in N and S deposition have led to widespread reductions in stream N and improvements in water quality. EPA scientists combined two EPA datasets: the National Nutrient Inventory and three National Rivers and Streams Assessment (NRSA) surveys that occurred approximately every 5 years from 2000-2014. Using a watershed approach, this EPA team documented changes in N inputs and stream N concentrations in areas where N deposition was the largest source of N inputs. While N deposition has declined across the US, only small and non-significant declines in nitrate were observed in the eastern US. Increases in organic N concentrations point toward a recovery from acidification, but overall total N increased across the country. Clean Air Act policies have reduced “acid rain” from sulfur and nitrogen deposited to the US landscape, but widespread recovery of stream water quality associated with nitrogen has not been documented. Recovery from acidification and declining deposition was accompanied by significant increases in organic nitrogen concentrations in streams, especially in the eastern US, impeding the recovery of water quality that would have resulted from declining air deposition alone. 

Citation

Lin, J., J. Compton, R. Sabo, A. Herlihy, R. Hill, M. Weber, J. Renee Brooks, S. Paulsen, AND J. Stoddard. The changing nitrogen landscape of United States streams: Declining deposition and increasing organic nitrogen. Oxford University Press, OXFORD, UKpgad362, (2024). [DOI: 10.1093/pnasnexus/pgad362]

DOI: The changing nitrogen landscape of United States streams: Declining deposition and increasing organic nitrogen