Deposition of Reduced Nitrogen (NHx) in California and Other Western Regions: Prevalence and Ecological Importance

Mark E. Fenn1, Andrzej Bytnerowicz1 and Stuart B. Weiss2

Recent data on atmospheric ammonia (NH3) concentrations and ammonium deposition fluxes in southern and central California suggest a greater prevalence of reduced N deposition than N emissions inventories in California indicate. According to the California Air Resources Board approximately 80% of NH3 emissions statewide are from livestock waste. However, other NH3 emissions sources such as motor vehicles equipped with three-way catalytic converters seem to be an underappreciated source of NH3 emissions. Large scale infrared satellite observations of atmospheric NH3 indicate that NH3 emissions are underestimated in much of the northern hemisphere, including California and much of the western U.S. As NOx emissions continue to decrease, reduced N is becoming proportionally more important as a driver of eutrophication and acidification effects to ecosystems and as a contributor to particulate matter. Ammonia is important ecologically because of its high deposition velocity, its ready biological availability, toxicity to lichens and bryophytes, and because of direct stomatal uptake by higher plants. In soils of the western U.S. ammonium is generally taken up by plants or microbes or nitrified rapidly when moisture is not limiting, which contributes to NO3 leaching losses and soil acidification. Even in forests downwind of major urban emissions sources in southern California with elevated photochemical oxidant exposure, 43% of throughfall deposition is in reduced form. At 19 sites in the Sierra Nevada Mountains 42-67% of the N deposited as throughfall was as ammonium. Atmospheric concentrations and deposition of reduced N is highly elevated adjacent to highways, thus contributing to eutrophication effects along transportation corridors. Exposure to NH3 can be far reaching, as demonstrated by two-week average concentrations in summer as high as 1-4 g m-3 in the eastern Sierra Nevada. Concentrations of NH3 have increased significantly in the eastern Sierra Nevada since the mid 1980s. Ammonia exposure has caused major changes in lichen communities throughout much of the Sierra Nevada in California and is a major component driving the N excess effects observed in a number of vegetation types in California and elsewhere and as recently reported in high elevation lakes in the eastern Sierra Nevada and in the Greater Yellowstone Ecosystem.

*Corresponding author: Tel. 951-680-1565
1U.S. Forest Service, PSW Research Station, Forest Fire Laboratory, 4955 Canyon Crest Drive, Riverside, CA 92507, USA
2Creekside Center for Earth Observations, Menlo Park, CA USA