Nitrogen cycle

Nitrogen cycle, cycle of chemical changes that keep nitrogen flowing through the biosphere, in air and soil. Nitrogen is a fundamental part of living protoplasm. In order to be absorbed by living things, however, it has to be combined into hydrogen and oxygen compounds that plants can use. This process, called nitrogen fixation, is carried on by bacteria in the soil that produce ammonia and nitrates. Plants then absorb these from the soil and use them to make protein. When animals eat plants, they convert the plant protein into animal protein. Meanwhile, some of the nitrates present in soils seeps into groundwater and rivers, and the remainder undergoes denitrification, a process that breaks nitrates down into nitrogen and nitrous oxide, which is then released into the atmosphere, soil, and water. When remains of dead animals and wastes decay, nitrogen is put back into the soil in the form of ammonia, which bacteria oxidize to nitrites and then to nitrates, thus beginning the cycle again.