The Nitrogen Cycle
Nitrogen (N) is essential for plant growth and makes up 78% of the atmosphere. Only certain "nitrogen-fixing" bacteria (e.g. Rhizobium) can convert N2 gas to a form that plants and animals can use. The process requires energy so nitrogen-fixing bacteria often associate with plants to obtain energy-rich organic carbon compounds from them, supplying fixed N to the plant. This enters the N cycle hence supporting animals, plants and all other life in soil (nitrogen-fixing photosynthetic bacteria are important in water but less so in soil where light cannot penetrate beyond the surface). Otherwise the energy from lightning can generate oxides of nitrogen from the atmosphere, some is released when fossil fuels are burned and man-made fertilizers are an important source for agriculture.
Nitrogen compounds can be lost from soil as nitrates in water. Organic N is broken down to ammonia (which is not very mobile in soil) - this is converted to nitrate (which is more soluble) by the action of a limited group of nitrifying bacteria. Ammonia is oxidized to nitrite by the "ammonia oxidizers"; then to nitrate by the "nitrite oxidizers". Many groups of "denitrifying bacteria" in soil reduce nitrate to oxides of nitrogen (including N2O - a green house gas) and N2 gas which can result in significant gaseous losses. Denitrifying bacteria use nitrate as a terminal electron acceptor in place of oxygen in anaerobic conditions, for example where soil is waterlogged, warm and contains plenty of organic matter.
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