nitrogen cycle

Definition

Continuous natural cycle of biological, chemical, and geological processes by which nitrogen is circulated in the Earth's environment. Although the most plentiful element in atmosphere (78 percent by volume, compared to 21 percent of oxygen), nitrogen cannot be used directly by most living things unless it is converted ('fixed,' see nitrogen fixation) into nitrogen-compounds by microorganism such as the bacteria present in the root-nodules of legumes, blue-green algae, and by the action of lightning. These compounds are taken up by plants as nutrients from the soil and converted into plant proteins (amino acids). Plant proteins become animal proteins when eaten and metabolized by herbivorous animals, and when carnivorous animals eat the herbivorous. These proteins return to the soil through animal excrement and the decomposition of dead animals and plants, and are converted into carbon dioxide, water, and ammonia (gaseous compound of nitrogen and hydrogen) by a set of bacteria in the soil. A portion of this ammonia is converted into soil nitrogen (fixed nitrogen) by another set of bacteria and the balance is released into the atmosphere as free nitrogen (N2).