nitrogen

The nitrogen cycle is one of a number of cycles during which the chemicals necessary for life are recycled. The carbon, sulphur, and phosphorus cycles are others. Since there is only a limited amount of these chemicals in the Earth and its atmosphere, the chemicals must be continuously recycled if life is to go on.

Colourless, odourless, tasteless, gaseous, non-metallic element, atomic number 7, relative atomic mass 14.0067. It forms almost 80% of the Earth's atmosphere by volume and is a constituent of all plant and animal tissues (in proteins and nucleic acids). Nitrogen is obtained for industrial use by the liquefaction and fractional distillation of air. Its compounds are used in the manufacture of foods, drugs, fertilizers, dyes, and explosives.

Nitrogen has been recognized as a plant nutrient, found in manures and other organic matter, from early times, long before the complex cycle of nitrogen fixation was understood. It was isolated in 1772 by the English chemist Daniel Rutherford (1749–1819) and named in 1790 by the French chemist Jean Chaptal (1756–1832).

On Earth, nitrogen atoms are mainly found bound in pairs to form molecules of nitrogen gas. Nitrogen atoms are needed by living organisms to make protein. However, only a few organisms are capable of using the nitrogen gas in the air. These organisms are said to be nitrogen-fixing because they can convert nitrogen gas to nitrogen compounds that they and other organisms can easily use. The nitrogen compounds that plants can use are chemicals such as ammonia and nitrates, which are often used as fertilizers. Animals can only use nitrogen when it is combined to form compounds like proteins. The cycling of nitrogen compounds is described in the nitrogen cycle.