Nitrogen Cycle

One of the major influences of humans on the nitrogen cycle occurs through the use of nitrogen-containing fertilizers in agriculture. Under conditions in which agricultural plants have access to as much water as they require, their productivity is usually constrained by the rate at which they can obtain nitrogen in available forms, particularly nitrate, and sometimes ammonium. Under such conditions, farmers attempt to increase the availability of these nutrients, usually by applying fertilizers. In intensive agricultural systems, rates of fertilization often exceed 1,103 lb (500 kg) of N per hectare per year. This represents an enormously larger rate of nitrogen input than occurs naturally by atmospheric deposition and dinitrogen fixation, and it is also much larger than the rates in which ammonium and nitrate are made naturally available by ammonification and nitrification. Consequently, the ability of the crop and ecosystem to assimilate this much nitrogen input is satiated, and some of the input, especially nitrate, leaches from the site into groundwater and surface waters such as streams and rivers. This can cause a severe contamination of these waters in agricultural areas, which can lead to risks for human health through drinking nitrate-rich ground water, and contribute to increased productivity of surface waters through eutrophication. In addition, when soils with large nitrate concentrations become wet, the rate of denitrification is greatly increased. This represents a loss of fixed nitrogen capital, and the emitted nitrous oxide may contribute to an enhancement of Earth's greenhouse effect.

Humans also influence the nitrogen cycle by dumping sewage and other types of organic matter into water-bodies. There is a great deal of environmental damage associated with these practices, including lowered dissolved oxygen levels associated with microbial oxidation of the organic matter, and the presence of fecal pathogens and parasites. The accidental fertilization of waterbodies with large amounts of nitrogen contributes greatly to eutrophication.

Humans also affect the nitrogen cycle through the emissions of large quantities of NO_x gases to the atmosphere. The most important sources of emission are automobiles, power plants, home furnaces, and factories. The emitted NO_x is an important air pollutant, because it is critical in the photochemical oxidative reactions by which toxic ozone is formed, and because the NO_x is an important source of nitrate in acidic precipitation.

The nitrogen cycle is a critical biogeochemical process, and is the ultimate means of entry and exit of biologically available forms of nitrogen in ecosystems. However, there are many ways by which human activities can substantially change the rates of the processes in this cycle, causing a great deal of environmental damage. The key to sustainable management of the nitrogen cycle is the recognition of the minimum and maximum rates that represent a healthy degree of ecological function. Human influences must then be kept within those bounds of desirable rates of nitrogen cycling.