User:Fuchs190/Ammonium

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The ammonium cation is a positively charged polyatomic ion with the chemical formula NH4+ or [NH4]+. It is formed by the protonation of ammonia. Ammonium is also a general name for positively charged, protonated amines and quaternary ammonium cations, where one or more hydrogen atoms are replaced by organic or other groups. Not only is ammonium a source of nitrogen and a key metabolite for many living organisms, but it is an integral part of the global nitrogen cycle. As such, the human impact in recent years could have an effect on the biological communities that depend on it.

Biology
Because nitrogen often limits net primary production due to its use in enzymes that mediate the biochemical reactions that are necessary for life, ammonium is utilized by some microbes and plants. For example, energy is released by the oxidation of ammonium in a process known as nitrification, which produces nitrate and nitrite. This process is a form of autotrophy that is common amongst Nitrosomonas, Nitrobacter, Nitrosolobus, and Nitrosospira, amongst others.

The amount of ammonium in soil that is available for nitrification by microbes varies depending on environmental conditions. For example, ammonium is deposited as a waste product from some animals, although it is converted into urea in mammals, sharks, and amphibians, and into uric acid in birds, reptiles, and terrestrial snails. Its availability in soils is also influenced by mineralization, which makes more ammonium available from organic nitrogen sources, and immobilization, which sequesters ammonium into organic nitrogen sources, both of which are mitigated by biological factors.

Conversely, nitrate and nitrite can be reduced to ammonium as a way for living organisms to access nitrogen for growth in a process known as assimilatory nitrate reduction. Once assimilated, it can be incorporated into proteins and DNA.

Ammonium can accumulate in soils where nitrification is slow or inhibited, which is common in hypoxic soils. For example, ammonium mobilization is one of the key factors for the symbiotic association between plants and fungi, called mycorrhizae. However, plants that consistently utilize ammonium as a nitrogen source often must invest into more extensive root systems due to ammonium's limited mobility in soils compared to other nitrogen sources.

Human Impact
Ammonium deposition from the atmosphere has increased in recent years due to volatilization from livestock waste and increased fertilizer use. Because net primary production is often limited by nitrogen, increased ammonium levels could impact the biological communities that rely on it. For example, increasing nitrogen content has been shown to increase plant growth, but aggravate soil phosphorus levels, which can impact microbial communities.