Editing Nitrogen (section)

==Occurrence==
[[File:Nitrogen Cycle.svg|thumb|right|upright=1.8|Schematic representation of the [[Nitrogen cycle|flow of nitrogen compounds]] through a land environment]]
{{see also|Nitrogen cycle}}

Nitrogen is the most common pure element in the earth, making up 78.1% of the volume of the [[Atmosphere of Earth|atmosphere]]<ref name="Greenwood406" /> (75.5% by mass), around 3.89 million [[gigatonnes]] ({{val|3.89e18|u=kg}}). Despite this, it is [[Abundance of elements in Earth's crust|not very abundant]] in Earth's crust, making up somewhere around 19&nbsp;[[parts per million]] of this, on par with [[niobium]], [[gallium]], and [[lithium]]. (This represents 300,000 to a million gigatonnes of nitrogen, depending on the mass of the crust.<ref>Depending on the average thickness which is somewhere between 10 and 30 km, the mass of the earth's crust is between {{val|15|e=18}} and {{val|45|e=18}} tonnes.</ref>) The only important nitrogen minerals are [[nitre]] ([[potassium nitrate]], saltpetre) and [[sodanitre|soda nitre]] ([[sodium nitrate]], Chilean saltpetre). However, these have not been an important source of nitrates since the 1920s, when the industrial synthesis of ammonia and nitric acid became common.<ref name="Greenwood407">Greenwood and Earnshaw, pp. 407–09</ref>

Nitrogen compounds constantly interchange between the atmosphere and living organisms. Nitrogen must first be processed, or "[[nitrogen fixation|fixed]]", into a plant-usable form, usually ammonia. Some nitrogen fixation is done by lightning strikes producing the nitrogen oxides, but most is done by [[diazotroph]]ic bacteria through enzymes known as [[nitrogenase]]s (although today industrial nitrogen fixation to ammonia is also significant). When the ammonia is taken up by plants, it is used to synthesise proteins. These plants are then digested by animals who use the nitrogen compounds to synthesise their proteins and excrete nitrogen-bearing waste. Finally, these organisms die and decompose, undergoing bacterial and environmental oxidation and [[denitrification]], returning free dinitrogen to the atmosphere. Industrial nitrogen fixation by the [[Haber process]] is mostly used as fertiliser, although excess nitrogen–bearing waste, when leached, leads to [[eutrophication]] of freshwater and the creation of marine [[Dead zone (ecology)|dead zones]], as nitrogen-driven bacterial growth depletes water oxygen to the point that all higher organisms die. Furthermore, nitrous oxide, which is produced during denitrification, attacks the atmospheric [[ozone layer]].<ref name="Greenwood407" />

Many saltwater fish manufacture large amounts of [[trimethylamine oxide]] to protect them from the high [[osmosis|osmotic]] effects of their environment; conversion of this compound to [[dimethylamine]] is responsible for the early odour in unfresh saltwater fish.<ref>{{Cite journal|pmid=15186102|date=Jun 2004|author=Nielsen, M. K.|author2=Jørgensen, B. M.|title=Quantitative relationship between trimethylamine oxide aldolase activity and formaldehyde accumulation in white muscle from gadiform fish during frozen storage|volume=52|issue=12|pages=3814–22|doi=10.1021/jf035169l|journal=Journal of Agricultural and Food Chemistry|bibcode=2004JAFC...52.3814N }}</ref> In animals, [[free radical]] [[nitric oxide]] (derived from an [[amino acid]]), serves as an important regulatory molecule for circulation.<ref name="ocean">{{Cite book|url=https://books.google.com/books?id=qPuCVvAv2GwC&pg=PA392|page=392|title=Biology of the Southern Ocean|author=Knox, G. A.|publisher=CRC Press|date=2007|isbn=978-0-8493-3394-1|access-date=2020-08-24|archive-date=2021-10-01|archive-url=https://web.archive.org/web/20211001035610/https://books.google.com/books?id=qPuCVvAv2GwC&pg=PA392|url-status=live}}</ref>

Nitric oxide's rapid reaction with water in animals results in the production of its metabolite [[nitrite]]. Animal [[metabolism]] of nitrogen in proteins, in general, results in the [[excretion]] of [[urea]], while animal metabolism of [[nucleic acids]] results in the excretion of [[urea]] and [[uric acid]]. The characteristic odour of animal flesh decay is caused by the creation of long-chain, nitrogen-containing [[amine]]s, such as [[putrescine]] and [[cadaverine]], which are breakdown products of the amino acids [[ornithine]] and [[lysine]], respectively, in decaying proteins.<ref>{{Cite book|url=https://books.google.com/books?id=RMPis9OnJxkC&pg=PT121|page=121|title=Digestion, diet, and disease: irritable bowel syndrome and gastrointestinal function|author=Vickerstaff Joneja|author2=Janice M.|publisher=Rutgers University Press|date=2004|isbn=978-0-8135-3387-2|access-date=2020-08-24|archive-date=2021-10-01|archive-url=https://web.archive.org/web/20211001035605/https://books.google.com/books?id=RMPis9OnJxkC&pg=PT121|url-status=live}}</ref>