The nitrogen cycle is a natural process by which nitrogen in the air moves into the soil, is utilized by living organisms, and returns to the air. Organisms need nitrogen to make macromolecules, including amino acids and nucleic acids. Air is 79 percent nitrogen gas (N2).N2 has a triple bond, making it relatively inert and unable to be used by most organisms.

Nitrogen fixation, converting N2 to NH4+ (an ammonium ion), is carried out by Rhizobium bacteria, which live in root nodules of host plants of the legume family such as peas, beans, and clover. Bacteria get carbohydrates from the plant, and the plant uses some of the ammonium the bacteria fix. Free-living cyanobacteria also fix nitrogen.

Lightning fixes smaller amounts of nitrogen. Plants absorb nitrate or ammonium ions from the soil through root hairs. Ammonium is converted into organic nitrogen compounds by bacteria and by plants. Other organisms get organic nitrogen from what they eat. Organic nitrogen is converted to ammonia as microorganisms decompose dead matter (Botterill, 2003).

Soil bacteria, Nitrosomona, perform nitrification where NH4 is converted to NO2 - (nitrite) (Axelrod, 2004). NO2 - is converted to NO3 - (nitrate) by Nitrobacter. Completing the nitrogen cycle, nitrites are converted to N2 and N2O (nitrous oxide) in anaerobic conditions by bacteria Pseudomonas and Clostridium.

Humans have more than doubled the annual transfer of nitrogen gas into biologically available forms of nitrogen. This has occurred through burning of fossil fuels, manufacture of synthetic nitrogen fertilizers, and widespread cultivation of legumes (soy, alfalfa, and clover). Burning fossil fuels causes formation of oxides of carbon (carbon monoxide, carbon dioxide) (Axelrod, 2004). The burning of such fuels occurs at elevated temperatures that cause nitrogen and oxygen molecules in air to react to form oxides of nitrogen (NO, N2O, and NO2). NO2 forms smog and mixed with water forms nitric acid (HNO3), contributing to acid rain.

The Haber-Bosch process fixes N2 using hydrogen, high temperature, and pressure to form ammonia. Synthetic nitrogen fertilizers (ammonia, ammonium nitrate, and urea), applied directly to the soil, have led to a huge increase in agricultural productivity. The applied fertilizer not utilized by plants leaches out of soil and accumulates in water. N2O and CO2 are greenhouse gases (GHGs) that contribute to global warming by absorbing energy from the Earth's surface, stopping the loss of this energy, and raising the Earth's temperature. NO2 is not as abundant as CO2, yet it is an important, stable GHG that absorbs infrared energy about 270 times more strongly than does CO2. Nitrogen cycle influence on global warming is complex, as an increase in biologically active nitrogen stimulates plant growth, which increases uptake of CO2.

References

1. Axelrod, Regina S., David Leonard Downie, and Norman J. Vig, eds. The Global Environment: Institutions, Law, and Policy. 2d ed. Washington, D.C.: CQ Press, 2004.

2. Botterill, Linda C., and Melanie Fisher, eds. Beyond Drought: People, Policy, and Perspectives. Collingwood, Vic.: CSIRO, 2003.

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