Nitrogen fixation

Nitrogen fixation is the process by which nitrogen is taken from its natural, relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds (such as ammonia, nitrate and nitrogen dioxide).

Nitrogen fixation is performed naturally by a number of different prokaryotes, including bacteria, actinobacteria, and certain types of anaerobic bacteria. Microorganisms that fix nitrogen are called diazotrophs. Some higher plants, and some animals (termites), have formed associations with diazotrophs.

Nitrogen fixation also occurs as a result of non-biological processes. These include lightning, industrially through the Haber-Bosch Process, and combustion.

Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.

Biological Nitrogen Fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a pair of bacterial enzymes called nitrogenase. The formula for BNF is:

N2 + 8H+ + 8e− + 16 ATP → 2NH3 + H2 + 16 ADP + 16 Pi

Although ammonia (NH3) is the direct product of this reaction, it is quickly protonated into ammonium (NH4+). In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.

In most bacteria, the nitrogenase enzymes are very susceptible to destruction by oxygen (and many bacteria cease production of the enzyme in the presence of oxygen). Low oxygen tension is achieved by different bacteria by: living in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as Leghemoglobin - also spelled leghaemoglobin..

The best-known plants which contribute to nitrogen fixation in nature, are in the legume family - Fabaceae, which includes such  as clover, beans, alfalfa, lupines and peanuts. They contain symbiotic bacteria called rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (family Polygonaceae), which were often referred to as "green manure", since the other natural way of adding nitrogen to the soil is via animal waste products. The entire plant is often ploughed back into the field, thus not only adding more nitrogen, but also improving the soil's organic content and volume.

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