nitrogen cycle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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 (and returned as faeces) or decomposed back to elemental nitrogen by bacteria.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the circulation of nitrogen; nitrates from the soil are absorbed by plants which are eaten by animals that die and decay returning the nitrogen back to the soil