As every farmer knows, planting legumes adds nitrogen to the soil because microorganisms on the plants' roots fix nitrogen from the atmosphere. Until 1940, human industrial activities fixed almost zero nitrogen. Therefore biologically available nitrogen has increased very rapidly. Indeed, a study in 1990 found that half of all the nitrogen ever fixed by industrial processes has been produced after 1980. By doubling the amount of available nitrogen, and by increasing the movement of nitrogen from place to place, humans are disrupting ecosystems on a grand scale.
Ozone is the most harmful common air pollutant to humans and vegetation.
The final product of oxidizing NO is nitric acid, a key component of acid rain, which is damaging forests in Canada, the U.S. and Europe. As a result of all these contributions of fixed nitrogen to the atmosphere, fixed nitrogen is deposited back on land and oceans at an increased rate. In the midwestern and eastern U.S., nitrogen deposition from the atmosphere onto the land is more than 10 times as great as the natural rate. In parts of northern Europe nitrogen deposition is now more than 100 times as great as the natural rate. Nitrogen deposited on the land tends to move into nearby waters, carrying with it calcium and magnesium from the soil.