Atmospheric nitrogen deposition in the Chesapeake Bay watershed: A history of change: Fid-Bau Portal

Atmospheric nitrogen deposition in the Chesapeake Bay watershed: A history of change

Burns, Douglas A. / Bhatt, Gopal / Linker, Lewis C. / Bash, Jesse O. / Capel, Paul D. / Shenk, Gary W.

Abstract The Chesapeake Bay watershed has been the focus of pioneering studies of the role of atmospheric nitrogen (N) deposition as a nutrient source and driver of estuarine trophic status. Here, we review the history and evolution of scientific investigations of the role of atmospheric N deposition, examine trends from wet and dry deposition networks, and present century-long (1950–2050) atmospheric N deposition estimates. Early investigations demonstrated the importance of atmospheric deposition as an N source to the Bay, providing 25%–40% among all major N sources. These early studies led to the unprecedented inclusion of targeted decreases in atmospheric N deposition as part of the multi-stakeholder effort to reduce N loads to the Bay. Emissions of nitrogen oxides (NOx) and deposition of wet nitrate, oxidized dry N, and dry ammonium (NH4 ⁺) sharply and synchronously declined by 60%–73% during 1995–2019. These decreases largely resulted from implementation of Title IV of the 1990 Clean Air Act Amendments, which began in 1995. Wet NH4 ⁺ deposition shows no significant trend during this period. The century-long atmospheric N deposition estimates indicate an increase in total atmospheric N deposition in the Chesapeake watershed from 1950 to a peak of ~15 kg N/ha/yr in 1979, trailed by a slight decline of <10% through the mid-1990s, and followed by a sharp decline of about 40% thereafter through 2019. An additional 21% decline in atmospheric N deposition is projected from 2015 to 2050. A comparison of the Potomac River and James River watersheds indicates higher atmospheric N deposition in the Potomac, likely resulting from greater emissions from higher proportions of agricultural and urban land in this basin. Atmospheric N deposition rose from 30% among all N sources to the Chesapeake Bay watershed in 1950 to a peak of 40% in 1973, and a decline to 28% by 2015. These data highlight the important role of atmospheric N deposition in the Chesapeake Bay watershed and present a potential opportunity for decreases in deposition to contribute to further reducing N loads and improving the trophic status of tidal waters.

Highlights • Atmospheric deposition is important nitrogen source to the Chesapeake Bay watershed and was dominant source from about 1960 to 2000. • Implementation of Title IV of 1990 Clean Air Act Amendments has resulted in decreases of 60%–73% in wet and dry deposition of oxidized nitrogen and dry deposition of reduced nitrogen during 1995 to 2019. • Despite these declines, atmospheric nitrogen deposition still contributes about 25% of the annual nitrogen load to the Chesapeake Bay watershed. • A future projection based on planned implementation of Clean Air Act rules suggests atmospheric nitrogen deposition is likely to decline (from 2015) by an additional 21% by 2050.