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 WordNet 3.0 & Open English WordNet

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. Wikipedia

The complex set of processes by which crops acquire the large amount of nitrogen they need to make proteins, nucleic acids and other biochemicals of which they are composed, and how the nitrogen returns to the atmosphere. OmegaWiki

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. Wiktionary

Natural circulation of nitrogen. Wiktionary (translation)