Nitrogen-cycle definitions

The circulation of nitrogen in nature, consisting of a cycle of chemical reactions in which nitrogen from the atmosphere is fixed in compounds in soil or water, assimilated by plants and animals, released to the soil and water through decomposition, and returned to the atmosphere through denitrification.
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The cycle of natural processes through which atmospheric nitrogen is converted by nitrogen fixation and nitrification into compounds used by plants and animals in the formation of proteins and is eventually returned by decay and denitrification to its original state.
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The definition of nitrogen cycle is the process in which nitrogen changes into various forms.

An example of nitrogen cycle is organic nitrogen being released when an animal dies and bacteria turning the nitrogen into ammonium.

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The circulation of nitrogen in nature, consisting of a cycle of chemical reactions in which nitrogen from the atmosphere is fixed in compounds in soil or water, assimilated by plants and animals, released to the soil and water through decomposition, and returned to the atmosphere through denitrification.
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The continuous process by which nitrogen is exchanged between organisms and the environment. Nitrogen is an essential nutrient, needed to make amino acids and other important organic compounds, but most organisms cannot use free nitrogen, which is abundant as a gas in the atmosphere. Gaseous nitrogen is broken apart and fixed (converted to stable, biologically assimilable inorganic compounds) in the process of nitrogen fixation . Some atmospheric nitrogen is fixed naturally during lightning strikes and some by industrial processes. Cyanobacteria and certain other species of bacteria, especially those living as symbionts in the roots of legumes, fix atmospheric nitrogen biologically in ammonium ions. Ammonia and ammonium ions are also produced by the ongoing decay of organic materials. Ammonia can be absorbed directly by plant cells, and certain bacteria living in soil and water convert ammonia and ammonium ions into nitrites and nitrates in the process known as nitrification . The nitrates are easily absorbed by plant roots. In this way, nitrogen is passed into the food chain and ultimately returned to the soil, water, and atmosphere by the metabolism and decay of plants and animals.
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The natural circulation of nitrogen, in which atmospheric nitrogen is converted to nitrogen oxides by lightning and deposited in the soil by rain where it is assimilated by plants and either eaten by animals (returned as faeces) or decomposed back to elemental nitrogen by bacteria.
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