nitrogen cycle n (Biology) 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 the animals feeding...
1.(Biology) the conversion of atmospheric nitrogen into nitrogen compounds by certain bacteria, such asRhizobiumin the root nodules of legumes 2.(Chemistry) a process, such as the Haber process, in which atmospheric nitrogen is converted into a nitrogen compound, used esp for the manufacture of...