The nitrogen delivered from both nonpoint sources and through the boundaries also has an atmospheric origin that must be considered. While the origin of the boundary load would require extensive monitoring and research to fully understand, nonpoint sources in the drainage basin are becoming better understood as related to atmospheric contribution of nitrogen. How much of that nitrogen is of human origin and how much of it is delivered to Long Island Sound from each of the three land cover categories are questions that are not fully answered, but evaluations strongly suggest the atmospheric contribution is high and of management interest. If estimates for boundary and tributary import were available, the percent of enrichment due to the atmosphere would be higher. It is other forms of nitrogen, such as ammonia and oxides of nitrogen, that are of concern.
Ammonia is a mineralized form of nitrogen and is readily usable by plants as a nutrient.
A small amount of organic nitrogen is also found in atmospheric deposition, which has not been fully characterized but is likely to be small, airborne particles of organic matter such as pollen or plant and animal fragments. While there are natural sources of ammonia and nitrogen oxides, human activity tends to increase the levels of both in the atmosphere, particularly the nitrogen oxides. Ammonia enrichment results from industrial sources and agricultural activities that concentrate animal wastes where volatilization into the atmosphere occurs. Oxides of nitrogen, are a byproduct of most combustion processes. Hence, our use of fossil fuels for energy production, home heating, and to power automobiles and other machinery has greatly increased the amount of nitrogen oxides in the atmosphere.