نیتروژن یا ازت یکی از عناصر شیمیایی در جدول تناوبی است که نماد آن N و عدد اتمی آن ۷ است. نیتروژن معمولاً به صورت یک گاز، غیر فلز، دو اتمی بی اثر، بیرنگ، بیمزه و بیبو است که ۷۸٪ جو زمین را دربر گرفته و عنصر اصلی در بافتهای زنده است. نیتروژن ترکیبات مهمی مانند آمونیاک اسید نیتریک و سیانیدها را شکل میدهد.
ویژگیهای درخور نگرش[ویرایش]
نوشتار اصلی: چرخه نیتروژن
نیتروژن از گروه غیرفلزات بوده و دارای بار الکترون منفی ۳٫۰ میباشد. نیتروژن پنج الکترون در پوسته خود داشته و در نتیجه در اکثر ترکیبات سهظرفیتی است. نیتروژن خالص یک گاز بیاثر و بیرنگ است و ۷۸٪ جو زمین را به خود اختصاص داده است. در ۶۳K منجمد شده و در ۷۷K به صورت مایع، به ماده سرمایشی معروف سرمازا (Cryogen) تبدیل میشود.
مهمترین کاربرد اقتصادی نیتروژن برای ساخت آمونیاک از طریق فرایند هابر (Haber) است. آمونیاک معمولاً برای تولید کود و مواد تقویتی و اسید نیتریک استفاده میشود. نیتروژن همچنین بهعنوان پرکننده بی اثر، در مخزنهای بزرگ برای نگهداری مایعات قابل انفجار، در هنگام ساخت قطعات الکترونیک مانند ترانزیستور، دیود و مدار یکپارچه و همچنین برای ساخت فلزات ضد زنگ استفاده میشود. نیتروژن همچنین به صورت ماده خنککننده، برای هم منجمد کردن غذا و هم ترابری آن، نگهداری اجساد و یاختههای تناسلی (اسپرم و تخم مرغ)، و در زیستشناسی برای نگهداری پایدار از نمونههای زیستی کاربرد دارد.
نمک اسید نیتریک شامل ترکیبات مهمی مانند نیترات پتاسیوم و سدیوم و نیترات آمونیوم است؛ که اولی برای تولید باروت و دومی برای تولید کود به کار میرود. ترکیبات نیترات شده مانند نیتروگلیسرین و تری نیترو تولوئن (تیانتی) معمولاً منفجر شونده هستند.
اسید نیتریک به عنوان ماده اکسید کننده در مایع سوخت موشکها استفاده میشود. هیدرازین و مشتقات آن نیز در سوخت موشکها بکار میروند. نیتروژن اغلب در سرمازاها (Cryogens)، به صورت مایع (معمولاً LN2) استفاده میشود. نیتروژن مایع با عمل تقطیر هوا به دست میآید. در فشار جو، نیتروژن در دمای -۱۹۵٫۸ درجه سانتیگراد (-۳۲۰٫۴ درجه فارنهایت) مایع میشود.
نیتروژن بیشترین عنصر جو کره زمین از نظر حجم است. (۷۸٫۱٪ حجمی) و برای اهداف صنعتی با عمل تقطیر هوای مایع بدست میآید. ترکیباتی که حاوی این عنصر هستند در فضای بیرونی نیز مشاهده شدهاند. نیتروژن -۱۴ در اثر عمل همجوشی هستهای در ستارگان، تولید میگردد. نیتروژن از ترکیبات عمده ضایعات حیوانی (مانند چلغوز(:فضلهٔ مرغان و پرندگان) یا کود) بوده و معمولاً به صورت اوره، اسید اوریک و ترکیباتی از محصولات نیتروژنی یافت میشود.
اصلیترین هیدرید نیتروژن، آمونیاک است (NH3)، البته هیدرازین (N2H4) نیز مشهور است. ترکیب آمونیاک سادهتر از آب بوده و در محلول یونهای آمونیاک (NH4+) را تشکیل میدهد. آمونیاک مایع در حقیقت کمی آمفیروتیک بوده و آمونیاک و یونهای آمینه (NH2-) را بوجود میآورد؛ که البته هر دو نمک آمیدها و نیترید شناخته شدهاند، ولی در آب تجزیه میشوند. ترکیبات جانشین آمونیاک به تنهایی یا باهم، آمینها نامیده میشوند. زنجیرهها، حلقهها و ساختارهای بزرگتر هیدریدهای نیتروژنی نیز شناخته شدهاند، ولی در واقع ناپایدار هستند.
گروههای دیگر آنیونهای نیتروژن، آزیدینها (N3-) هستند، که خطی بوده و نسبت به دی اکسید کربن ایزو الکتریک هستند. مولکول دیگر با ساختار مشابه، مونوکسید دی نیتروژن N2O یا گاز خنده است، و یکی از اکسیدهای گوناگون بوده و برجسته تر از مونوکسید نیتروژن (NO) و دی اکسید نیتروژن (NO2) است، که هر دوی آنها الکترون غیر زوج دارند؛ که دومی تمایلی را به دوپارشدن نشان داده و از اجزای تشکیل دهنده هوای آلوده است.
اکسیدهای استاندارد بیشتری مانند تری اکسید دی نیتروژن (N2O3) و پنتاکسید دی نیتروژن (N2O5) معمولاً تا حدی نا پایدار و قابل انفجار هستند. اسیدهای متناظر آنها نیتروس (HNO2) و اسید نیتریک (HNO3) بوده با نمکهای متناظر که نیتریتها و نیتراتها نامیده میشوند. اسید نیتریک یکی از چند اسیدی است که از هیدرونیوم قوی تر است.
نیتروژن عنصر اصلی اسیدهای آمینه و اسیدهای هستهای که نیتروژن را مادهای حیاتی برای ادامه زندگی میکنند، است. لوبیا مانند اکثر گیاهانی که دانههای سبوسی دارند میتواند عمل بازیافت نیتروژن را به طور مستقیم از هوا انجام دهد، چراکه ریشههای آنها دارای برآمدگیهایی، برای نگهداری میکروبهایی است که عمل تبدیل به آمونیاک را فرایندی به نام تثبیت نیتروژن انجام میدهد، میباشد. این گیاهان آمونیاک را به اکسیدهای نیتروژن و آمینو اسید تبدیل کرده و پروتئین میسازند.
نیتروژن دو ایزوتوپ پایدار دارد: (N-14 , N-15). که مهمترین آن دو N-14 (99.634%) است که در چرخه CNO در ستارگان تولید میشود. مابقی، ایزوتوپ N-15 است. یکی از ده ایزوتوپی که به صورت مصنوعی تولید میشوند دارای نیمه عمر نه دقیقهای داشته و ایزوتوپهای دیگر نیمه عمر چند ثانیه یا کمتر دارند.
واکنشهای زیستشناختی-واسطهای (مانند همانند سازی، جذب و ترکیب نیترات سازی) و ... پویایی نیتروژن در خاک را به شدت کنترل میکنند. این ترکیبات معمولاً باعث عمل غنی سازی N-15 لایه زیرین و تخلیه محصول میشود. البته این فرایند سریع اغلب مقادیری از آمونیام و نیترات نیز دربردارد، زیرا آمونیوم بصورت ترجیحی بهوسیله سایبان جو نیترات، نکهداری میشود. خاک نیتراتی نسبت به خاک آمونیومی، توسط ریشه درختان بهتر جذب و ترکیب میشود.
پیوند به بیرون[ویرایش]
Nitrogen is a chemical element with symbol N and atomic number 7. It is the lightest pnictogen, and at room temperature it is a transparent, odorless diatomic gas. Nitrogen is a common element in the universe, estimated at about seventh in total abundance in the Milky Way and the Solar System. On Earth, the element forms about 78% of Earth's atmosphere and is the most abundant uncombined element. The element nitrogen was discovered as a separable component of air by Scottish physician Daniel Rutherford in 1772.
Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The extremely strong triple bond in elemental nitrogen (N≡N) dominates nitrogen chemistry, causing difficulty for both organisms and industry in converting the N2 into useful compounds, but at the same time causing release of large amounts of often useful energy when the compounds burn, explode, or decay back into nitrogen gas. Synthetically produced ammonia and nitrates are key industrial fertilizers, and fertilizer nitrates are key pollutants in the eutrophication of water systems.
Apart from its use in fertilizers and energy-stores, nitrogen is a constituent of organic compounds as diverse as Kevlar fabric and cyanoacrylate "super" glue. Nitrogen is a constituent of every major pharmacological drug class, including antibiotics. Many drugs are mimics or prodrugs of natural nitrogen-containing signal molecules: for example, the organic nitrates nitroglycerin and nitroprusside control blood pressure by metabolizing into nitric oxide. Plant alkaloids (often defense chemicals) contain nitrogen by definition, and many notable nitrogen-containing drugs, such as caffeine and morphine, are either alkaloids or synthetic mimics that act (as many plant alkaloids do) on receptors of animal neurotransmitters (for example, synthetic amphetamines).
Nitrogen occurs in all organisms, primarily in amino acids (and thus proteins), in the nucleic acids (DNA and RNA) and in the energy transfer molecule adenosine triphosphate. The human body contains about 3% by mass of nitrogen, the fourth most abundant element in the body after oxygen, carbon, and hydrogen. The nitrogen cycle describes movement of the element from the air, into the biosphere and organic compounds, then back into the atmosphere.
History and etymology
The existence of nitrogen is formally considered to have been discovered by Scottish physician Daniel Rutherford in 1772, who called it noxious air. Though he did not recognize it as an entirely different chemical substance, he clearly distinguished it from Joseph Black's "fixed air", or carbon dioxide. The fact that there was a component of air that does not support combustion was clear to Rutherford. Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as "mephitic air" or azote, from the Greek word ἄζωτος azotos, "lifeless". In an atmosphere of pure nitrogen, animals died and flames were extinguished. This "mephitic air" consisted mostly of N2, but might have included more than 1% argon.
Lavoisier's name for nitrogen is used in many languages (French, Italian, Portuguese, Polish, Russian, Albanian, Turkish, etc.) and still remains in English in the common names of many compounds, such as hydrazine and compounds of the azide ion. The English word nitrogen (1794) entered the language from the French nitrogène, coined in 1790 by French chemist Jean-Antoine Chaptal (1756–1832), from the French nitre (potassium nitrate, also called saltpeter) and the French -gène, "producing", from the Greek -γενής (-genes, "producer, begetter"). Chaptal's meaning was that nitrogen gas is the essential part of nitric acid, which in turn was produced from niter. In earlier times, niter had been confused with Egyptian "natron" (sodium carbonate) — called nitron (νίτρον) in Greek — which contained no nitrate.
Nitrogen compounds were well known by the Middle Ages. Alchemists knew nitric acid as aqua fortis (strong water). The mixture of nitric and hydrochloric acids was known as aqua regia (royal water), celebrated for its ability to dissolve gold (the king of metals). The earliest military, industrial, and agricultural applications of nitrogen compounds used saltpeter (sodium nitrate or potassium nitrate), most notably in gunpowder, and later as fertilizer. In 1910, Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced "active nitrogen", a monatomic allotrope of nitrogen. The "whirling cloud of brilliant yellow light" produced by his apparatus reacted with quicksilver to produce explosive mercury nitride.
For a long time, sources of nitrogen compounds were limited. Natural sources originated either from biology or deposits of nitrates produced by atmospheric reactions. Nitrogen fixation by industrial processes like the Frank–Caro process (1895–1899) and Haber–Bosch process (1908–1913) eased this shortage of nitrogen compounds, to the extent that half of global food production (see Applications) now relies on synthetic nitrogen fertilizers. At the same time, use of the Ostwald process (1902) to produce nitrates from industrial nitrogen fixation allowed the large-scale industrial production of nitrates as feedstock in the manufacture of explosives in the World Wars of the 20th century.
Further information: Air separation
Nitrogen gas is an industrial gas produced by the fractional distillation of liquid air, or by mechanical means using gaseous air (i.e., pressurized reverse osmosis membrane or pressure swing adsorption). Nitrogen gas generators using membranes or PSA are typically more cost and energy efficient than bulk delivered nitrogen. Commercial nitrogen is often a byproduct of air-processing for industrial concentration of oxygen for steelmaking and other purposes. When supplied compressed in cylinders it is often called OFN (oxygen-free nitrogen).
Small amounts of impurities NO and HNO3 are also formed in this reaction. The impurities can be removed by passing the gas through aqueous sulfuric acid containing potassium dichromate. Very pure nitrogen can be prepared by the thermal decomposition of barium azide or sodium azide.
Nitrogen is a nonmetal, with an electronegativity of 3.04. It has five electrons in its outer shell and is, therefore, trivalent in most compounds. The triple bond in molecular nitrogen (N
At atmospheric pressure, molecular nitrogen condenses (liquefies) at 77 K (−195.79 °C) and freezes at 63 K (−210.01 °C) into the beta hexagonal close-packed crystal allotropic form. Below 35.4 K (−237.6 °C) nitrogen assumes the cubic crystal allotropic form (called the alpha phase). Liquid nitrogen, a fluid resembling water in appearance, but with 80.8% of the density (the density of liquid nitrogen at its boiling point is 0.808 g/mL), is a common cryogen.
Unstable allotropes of nitrogen consisting of more than two nitrogen atoms, such as N
Other (theoretical but not yet synthesized) allotropes include hexazine (N
See also: Isotopes of nitrogen
There are two stable isotopes of nitrogen: 14N and 15N. By far the most common is 14N (99.634%), which is produced in the CNO cycle in stars. Of the ten isotopes produced synthetically, 13N has a half-life of ten minutes and the remaining isotopes have half-lives on the order of seconds or less.
Biologically mediated reactions (e.g., assimilation, nitrification, and denitrification) strongly control nitrogen dynamics in the soil. These reactions typically result in 15N enrichment of the substrate and depletion of the product.
The radioisotope 16N is the dominant radionuclide in the coolant of pressurized water reactors or boiling water reactors during normal operation. It is produced from 16O (in water) via (n,p) reaction. It has a short half-life of about 7.1 s, but during its decay back to 16O produces high-energy gamma radiation (5 to 7 MeV).
Because of this, access to the primary coolant piping in a pressurized water reactor must be restricted during reactor power operation. 16N is a sensitive and immediate indicator of leaks from the primary coolant system to the secondary steam cycle, and is the primary means of detection for such leaks.
Similarly, access to any of the steam cycle components in a boiling water reactor nuclear power plant must be restricted during operation. Condensate is typically retained for 10 minutes to allow for decay of 16N to radioactive depletion.
Molecular nitrogen (14N2) is largely transparent to infrared and visible radiation because it is a homonuclear molecule and has no dipole moment to couple to electromagnetic radiation at these wavelengths. Significant absorption occurs at extreme ultraviolet wavelengths, beginning around 100 nanometres. This is associated with electronic transitions in the molecule to states in which charge is not distributed evenly between nitrogen atoms. Nitrogen absorption leads to significant absorption of ultraviolet radiation in the Earth's upper atmosphere and the atmospheres of other planetary bodies. For similar reasons, pure molecular nitrogen lasers typically emit light in the ultraviolet range.
Nitrogen also makes a contribution to visible air glow from the Earth's upper atmosphere, through electron impact excitation followed by emission. This visible blue air glow (seen in the polar aurora and in the re-entry glow of returning spacecraft) typically results not from molecular nitrogen but rather from free nitrogen atoms combining with oxygen to form nitric oxide (NO).
Nitrogen gas also exhibits scintillation.
In general, nitrogen is unreactive at standard temperature and pressure. N2 reacts spontaneously with few reagents, being resilient to acids and bases as well as oxidants and most reductants. When nitrogen reacts spontaneously with a reagent, the net transformation is often called nitrogen fixation.
N2 forms a variety of adducts with transition metals. The first example of a dinitrogen complex is [Ru(NH3)5(N2)]2+ (see figure at right). Notably, N2 ligand was obtained by the decomposition of hydrazine, and not coordination of free dinitrogen. Many such compounds are now known, including IrCl(N2)(PPh3)2, W(N2)2(Ph2PCH2CH2PPh2)2, and [(η5-C5Me4H)2Zr]2(μ2, η2,η2-N2). These complexes illustrate how N2 might bind to the metal(s) in nitrogenase and the catalyst for the Haber process. A catalytic process to reduce N2 to ammonia with the use of a molybdenum complex in the presence of a proton source was published in 2005.
The starting point for industrial production of nitrogen compounds is the Haber process, in which nitrogen is fixed by reacting N
Nitrogen gas (N2) is the largest constituent of Earth's atmosphere (78.082% by volume of dry air, 75.3% by weight in dry air). However, this high concentration does not reflect nitrogen's overall low abundance in the makeup of the Earth, from which most of the element escaped by solar evaporation, early in the planet's formation.
Nitrogen is a common element in the universe, and is considered the seventh most abundant chemical element by mass in the universe, the Milky Way, and the Solar System. According to physical cosmology theory, it was originally created by fusion processes from carbon and hydrogen in supernovas. Molecular nitrogen and nitrogen compounds have been detected in interstellar space by astronomers using the Far Ultraviolet Spectroscopic Explorer.
Due to the volatility of elemental nitrogen and its common compounds with hydrogen and oxygen, nitrogen was driven out of the planetesimals in the early Solar System by the heat of the Sun and lost to the rocky planets of the inner Solar System. Nitrogen is consequently a relatively rare element on these inner planets, including Earth. Neon, which occurs in equal abundance in the universe, is rare in the inner Solar System for the same reason. Nitrogen is estimated at 31st element in crustal abundance. Nitrogen also occurs in some relatively uncommon minerals, such as saltpeter (potassium nitrate), Chile saltpeter (sodium nitrate) and sal ammoniac (ammonium chloride). Because of the ready solubility of naturally-occurring nitrogen compounds, these minerals were concentrated from the crust by the leaching of rivers and collected in ancient seas which then evaporated, leaving the deposits of present day. A similar process occurred with the water-soluble compounds of the uncommon light element boron.
Nitrogen and its compounds occur far more commonly as gases in the atmospheres of planets and moons that are large enough to have atmospheres.[a] Molecular nitrogen is a major constituent of not only Earth's atmosphere, but also the Saturnian moon Titan's thick atmosphere. Also, held by gravity at colder temperatures, nitrogen and its compounds occur in variable amounts in the planetary atmospheres of the gas giant planets.
Nitrogen is present in all known living organisms, in proteins, nucleic acids, and other molecules. It typically makes up around 4% of the dry weight of plant matter, and around 3% of the weight of the human body. It is a large component of animal waste (for example, guano), usually in the form of urea, uric acid, ammonium compounds, and derivatives of these nitrogenous products, which are essential nutrients for all plants that cannot fix atmospheric nitrogen.
See also: Category:Nitrogen compounds.
The main neutral hydride of nitrogen is ammonia (NH
Other classes of nitrogen anions (negatively charged ions) are the poisonous azides (N−
The higher oxides dinitrogen trioxide N
Nitrogen is notable for the range of explosively unstable compounds. Nitrogen triiodide NI
Nitrogen can also be found in many organic compounds. Common nitrogen functional groups include: amines, amides, nitro groups, imines, and enamines. The amount of nitrogen in a chemical substance can be determined by the Kjeldahl method.
Nitrogen is commonly used during sample preparation in chemical analysis. It is used to concentrate and reduce the volume of liquid samples. Directing a pressurized stream of nitrogen gas perpendicular to the surface of the liquid causes the solvent to evaporate while leaving the solute(s) and un-evaporated solvent behind.
Nitrogen can be used as a replacement, or in combination with, carbon dioxide to pressurize kegs of some beers, particularly stouts and British ales, due to the smaller bubbles it produces, which makes the dispensed beer smoother and headier. A pressure-sensitive nitrogen capsule known commonly as a "widget" allows nitrogen-charged beers to be packaged in cans and bottles.
Nitrogen gas has become the inert gas of choice for inert gas asphyxiation, and is under consideration as a replacement for lethal injection in Oklahoma. Nitrogen is promoted by euthanasia advocate Philip Nitschke to end life in a "peaceful, reliable [and] totally legal" manner.
Main article: Liquid nitrogen
Liquid nitrogen is a cryogenic liquid. At atmospheric pressure, it boils at −195.8 °C (−320.4 °F). When insulated in proper containers such as Dewar flasks, it can be transported without much evaporative loss.
Like dry ice, the main use of liquid nitrogen is as a refrigerant. Among other things, it is used in the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples and materials. It is used in the clinical setting in cryotherapy to remove cysts and warts on the skin. It is used in cold traps for certain laboratory equipment and to cool infrared detectors or X-ray detectors. It has also been used to cool central processing units and other devices in computers that are overclocked, and that produce more heat than during normal operation.
Main article: Solid nitrogen
The solid form of the element nitrogen forms a significant dynamic surface coverage on Pluto and outer moons of the Solar System such as Triton. Even at the low temperatures of solid nitrogen it is fairly volatile and can sublime to form an atmosphere, or condense back into nitrogen frost. It is very weak and flows in the form of glaciers and on Triton geysers of nitrogen gas come from the polar ice cap region.
There are a variety of crystalline forms of solid nitrogen known, not all with discrete dinitrogen molecular structures. Such non-molecular forms of solid nitrogen are of interest as they possess a higher energy density than any other non-nuclear material.
Molecular nitrogen (N2) in the atmosphere is relatively non-reactive due to its strong triple bond, N≡N, and molecular nitrogen plays an inert role in the human body, being neither produced nor destroyed. In nature, nitrogen is converted into biologically (and industrially) useful compounds by lightning, and by some living organisms, notably certain bacteria (i.e., nitrogen-fixing bacteria—see Biological role below). Molecular nitrogen is released into the atmosphere in the process of decay, in dead plant and animal tissues.
The ability to combine, or fix, molecular nitrogen is a key feature of modern industrial chemistry. Previously to the 20th century, access to nitrogen compounds for fertilizers and gunpowder had been through deposits of natural nitrates, such as Chilean saltpeter. However, first the Frank–Caro process for producing cyanamide, and then the Haber–Bosch process for producing ammonia from air and natural gas (developed just before the first world war) eased this shortage of nitrogen compounds, to the extent that half of global food production now relies on synthetic nitrogen fertilizers.
The Ostwald process, developed a few years before the Haber process, allowed large-scale production of nitric acid and nitrate from ammonia, thus freeing large-scale industrial production of nitrate explosives and weapons propellants from the need to mine nitrate salt deposits. The organic and inorganic salts of nitric acid have been important historically as convenient stores of chemical energy for warfare and rocket fuels. Historically, such compounds included important compounds such as potassium nitrate, used in gunpowder which was often produced by biological means (bacterial fermentation) before natural mineral sources were discovered. Later, all such sources were displaced by industrial production, in the early 1900s.
Ammonium nitrate has been used as both fertilizer and explosive (see ANFO). Various other nitrated organic compounds, such as nitroglycerin, trinitrotoluene, and nitrocellulose, are used as explosives and propellants for modern firearms. Nitric acid is used as an oxidizing agent in liquid fueled rockets. Hydrazine and hydrazine derivatives find use as rocket fuels and monopropellants. In most of these compounds, the basic instability and tendency to burn or explode is derived from the fact that nitrogen is present as an oxide, and not as the far more stable nitrogen molecule (N2), which is a product of the compounds' thermal decomposition. When nitrates burn or explode, the formation of the powerful triple bond in the N2 produces most of the energy of the reaction.
Nitrogen is a constituent of molecules in every major drug class in pharmacology and medicine. Nitrous oxide (N2O) was discovered early in the 19th century to be a partial anesthetic, though it was not used as a surgical anesthetic until later. Called "laughing gas", it was found capable of inducing a state of social disinhibition resembling drunkenness. Other notable nitrogen-containing drugs are drugs derived from plant alkaloids, such as morphine (there exist many alkaloids known to have pharmacological effects; in some cases, they appear as natural chemical defenses of plants against predation). Drugs that contain nitrogen include all major classes of antibiotics and organic nitrate drugs like nitroglycerin and nitroprusside that regulate blood pressure and heart action by mimicking the action of nitric oxide.
Elemental nitrogen in the atmosphere cannot be used directly by either plants or animals, and must be converted to a reduced (or 'fixed') state to be useful for higher plants and animals. Precipitation often contains substantial quantities of ammonium and nitrate, thought to result from nitrogen fixation by lightning and other atmospheric electric phenomena. This was first proposed by Liebig in 1827 and later confirmed. However, because ammonium is preferentially retained by the forest canopy relative to atmospheric nitrate, most fixed nitrogen reaches the soil surface under trees as nitrate. Soil nitrate is preferentially assimilated by tree roots relative to soil ammonium, however this is not the case for all species.
Specific bacteria (e.g., Rhizobium trifolium) possess nitrogenase enzymes that can fix atmospheric nitrogen (see nitrogen fixation) into a form (ammonium ion) that is chemically useful to higher organisms. This process requires a large amount of energy and anoxic conditions. Such bacteria may live freely in soil (e.g., Azotobacter) but normally exist in a symbiotic relationship in the root nodules of leguminous plants (e.g. clover, Trifolium, or soybean plant, Glycine max) and fertilizer trees. Nitrogen-fixing bacteria are also symbiotic with a number of unrelated plant species such as alders (Alnus) spp., lichens, Casuarina, Myrica, liverworts, and Gunnera.
As part of the symbiotic relationship, the plant converts the 'fixed' ammonium ion to nitrogen oxides and amino acids to form proteins and other molecules, (e.g., alkaloids). In return for the 'fixed' nitrogen, the plant secretes sugars to the symbiotic bacteria. Legumes maintain an anaerobic (oxygen free) environment for their nitrogen-fixing bacteria.
Plants are able to assimilate nitrogen directly in the form of nitrates that may be present in soil from natural mineral deposits, artificial fertilizers, animal waste, or organic decay (as the product of bacteria, but not bacteria specifically associated with the plant). Nitrates absorbed in this fashion are converted to nitrites by the enzyme nitrate reductase, and then converted to ammonia by another enzyme called nitrite reductase.
Nitrogen compounds are basic building blocks in animal biology as well. Animals use nitrogen-containing amino acids from plant sources as starting materials for all nitrogen-compound animal biochemistry, including the manufacture of proteins and nucleic acids. Plant-feeding insects are dependent on nitrogen in their diet, such that varying the amount of nitrogen fertilizer applied to a plant can affect the reproduction rate of insects feeding on fertilized plants.
Soluble nitrate is an important limiting factor in the growth of certain bacteria in ocean waters. In many places in the world, artificial fertilizers applied to crop-lands to increase yields result in run-off delivery of soluble nitrogen to oceans at river mouths. This process can result in eutrophication of the water, as nitrogen-driven bacterial growth depletes water oxygen to the point that all higher organisms die. Well-known "dead zone" areas in the U.S. Gulf Coast and the Black Sea are due to this important polluting process.
Many saltwater fish manufacture large amounts of trimethylamine oxide to protect them from the high osmotic effects of their environment; conversion of this compound to dimethylamine is responsible for the early odor in unfresh saltwater fish. In animals, free radical nitric oxide (NO) (derived from an amino acid), serves as an important regulatory molecule for circulation.
Nitric oxide's rapid reaction with water in animals results in production of its metabolite nitrite. Animal metabolism of nitrogen in proteins, in general, results in excretion of urea, while animal metabolism of nucleic acids results in excretion of urea and uric acid. The characteristic odor of animal flesh decay is caused by the creation of long-chain, nitrogen-containing amines, such as putrescine and cadaverine, which are breakdown products of the amino acids ornithine and lysine, respectively, in decaying proteins.
Decay of organisms and their waste products may produce small amounts of nitrate, but most decay eventually returns nitrogen content to the atmosphere, as molecular nitrogen. The circulation of nitrogen from atmosphere, to organic compounds, then back to the atmosphere, is referred to as the nitrogen cycle.
Although nitrogen is non-toxic, when released into an enclosed space it can displace oxygen, and therefore presents an asphyxiation hazard. This may happen with few warning symptoms, since the human carotid body is a relatively poor and slow low-oxygen (hypoxia) sensing system. An example occurred shortly before the launch of the first Space Shuttle mission in 1981, when two technicians died from asphyxiation after they walked into a space located in the Shuttle's Mobile Launcher Platform that was pressurized with pure nitrogen as a precaution against fire.
A closed space, initially filled with air, will ultimately contain only nitrogen, carbon dioxide, and trace gases if some oxidative process, such as the rusting of iron, consumes all the oxygen.
When inhaled at high partial pressures (more than about 4 bar, encountered at depths below about 30 m in scuba diving), nitrogen is an anesthetic agent, causing nitrogen narcosis, a temporary state of mental impairment similar to nitrous oxide intoxication.
Nitrogen dissolves in the blood and body fats. Rapid decompression (as when divers ascend too quickly or astronauts decompress too quickly from cabin pressure to spacesuit pressure) can lead to a potentially fatal condition called decompression sickness (formerly known as caisson sickness or the bends), when nitrogen bubbles form in the bloodstream, nerves, joints, and other sensitive or vital areas. Bubbles from other "inert" gases (gases other than carbon dioxide and oxygen) cause the same effects, so replacement of nitrogen in breathing gases may prevent nitrogen narcosis, but does not prevent decompression sickness.
Direct skin contact with liquid nitrogen will cause severe frostbite (cryogenic "burns"). This may happen almost instantly on contact, or after a second or more, depending on the form of liquid nitrogen. Bulk liquid nitrogen causes less rapid freezing than a spray of nitrogen mist (such as is used to freeze certain skin growths in the practice of dermatology). The extra surface area provided by nitrogen-soaked materials is also important, with soaked clothing or cotton causing far more rapid damage than a spill of direct liquid to skin. Full "contact" between naked skin and large collected-droplets or pools of liquid nitrogen may be prevented for a second or two, by a layer of insulating gas from the Leidenfrost effect. This may give the skin a second of protection from nitrogen bulk liquid. However, liquid nitrogen applied to skin in mists, and on fabrics, bypasses this effect, and causes local frostbite immediately.
Oxygen sensors are sometimes used as a safety precaution when working with liquid nitrogen to alert workers of gas spills into a confined space. Nitrogen that evaporates from liquid is still colder and heavier than (oxygenated) air, therefore any spill of liquid nitrogen tends to stay at the bottom of a closed space, expelling oxygen.