Atmospheric nitrogen, on the other hand, is derived from many elusive sources, many of which are not regulated under the Clean Air Act. Moreover, nitrogen levels appear to be increasing in the atmosphere. Studies indicate that atmospheric deposition of nitrogen poses great risk for the eutrophication of surface waters. Thus, the following discussion will focus primarily on the formation and survival of nitrogen in the atmosphere. The predominant natural source of nitrogen is the microbial decomposition of organic matter in soil and water.
Most anthropogenic nitrogen is emitted during the combustion of fossil fuels.
The production and application of fertilizers comprise a much smaller, albeit significant, pool of anthropogenic nitrogen emissions. Once emitted into the atmosphere, nitrogen may be deposited locally or may travel great distances before deposition. More than 3.2 million tons of atmospheric nitrogen is deposited on the United States' watersheds each year. In addition, a sizable amount of atmospheric nitrogen is deposited in the Atlantic ocean. Atmospheric nitrogen may be deposited in dry or wet form.