Download or read book Measuring Critical Loads of Nitrogen Deposition in the U.S. written by Rebecca G. Nisley and published by . This book was released on 2011 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: Air pollution is a serious environmental and public health problem resulting from emissions by power and industrial plants, motor vehicles, and agriculture. Sulfur (S) and nitrogen (N) emissions are two components of air pollution and cause wide-ranging, long-term threats to the health of forest, range, and wetland ecosystems. Sulfur and nitrogen gas emissions can combine with moisture in the air to become acidic molecules that fall to earth as acid deposition (commonly called acid rain, fog, or snow). Most pollution control efforts have focused on sulfur dioxide (SO2) emissions and these efforts have shown some positive results, with a measurable reduction in acidity of some lakes and wetlands. Negative impacts of nitrogen in deposition were overlooked because nitrogen is usually thought of as a fertilizer or a S2good thing.S3 This is not always the case: Elevated N availability can also result in nutrient imbalances that increase plant susceptibility to stressors such as cold, drought, or pests. For example, winter injury in high-elevation red spruce can be attributed to elevated N. The most serious effect of excess N, however, is the alteration of ecosystem diversity. In most terrestrial ecosystems in the United States, plant growth is limited by nitrogen availability. Thus, when additional nitrogen is added to an ecosystem, native plants (those that are original to a site) may lose their competitive edge. The excess nitrogen allows other plants (which often are nonnative invasive species that do not thrive in low nitrogen) to out-compete native plants and thus alter the species composition of an ecosystem. For example, in Joshua Tree National Park in southern California, N deposition favors the production of sufficient invasive grass biomass to sustain fires that threaten the survival of the namesake species.