the natural circulation of nitrogen by living organisms. Nitrates in the soil, derived from dead organic matter by bacterial action, are absorbed and synthesized into complex organic compounds by plants and reduced to nitrates again when the plants and theanimals feeding on them die and decay
nitrification, nitrogen fixation
nitrogen cycle in American English
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