Definition: 'Nitrogen Cycle'
1. the series of events in which the nitrogen of the atmosphere is fixed, thus made available for plant and animal life, and is then returned to the atmosphere: nitrifying bacteria convert N2 and O2 to NO2− and NO3−, the latter being absorbed by plants and converted to protein; if plants decay, the nitrogen is in part given up to the atmosphere and the remainder is converted by microorganisms to ammonia, nitrites, and nitrates; if the plants are eaten, the animals' excreta or bacterial decay return the nitrogen to the soil and air.
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