Nitrate assimilation is essential for the
synthesis of organic matter
Living matter contains a large amount of nitrogen incorporated in pro-teins, nucleic acids, and many other biomolecules. This organic nitrogen is present in oxidation state III (as in NH3). During autotrophic growth the nitrogen demand for the formation of cellular matter is met by inorganic nitrogen in two alternative ways:
1. Fixation of molecular nitrogen from air; or
2. Assimilation of the nitrate or ammonia present in water or soil.
Only some bacteria, including cyanobacteria, are able to fix nitrogen (N2) from air. Some plants enter a symbiosis with N2-fixing bacteria, which supply them with organic bound nitrogen . However, about 99% of the organic nitrogen in the biosphere is derived from the assimila-tion of nitrate. NH4+ is formed as an end product of the degradation of organic matter, primarily by the metabolism of animals and bacteria, and is oxidized to nitrate again by nitrifying bacteria in the soil. Thus a con-tinuous cycle exists between the nitrate in the soil and the organic nitrogen in the plants. NH4+ accumulates only in poorly aerated soils with insuf-ficient drainage, where, due to lack of oxygen, nitrifying bacteria cannot grow. Mass animal production can lead to a high ammonia input into the soil, not only from manure but also from the air. If NH4+ instead of nitrate is available, many plants can utilize it as a nitrogen source.
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