nitrogen-fixing bacteria: see nitrogen cycle.

nitrogen mustard, any of various poisonous compounds originally developed for military use (see poison gas). Like mustard gas and lewisite, it is a vesicant (blistering agent). In the form of its crystalline hydrochloride it is used as a drug in the treatment of Hodgkin's disease, non-Hodgkin's lymphomas, and brain tumors. Nitrogen mustards cause mutations in the genetic material of cells, thereby disrupting mitosis, or cell division. Cells vary in their susceptibility to nitrogen mustards, with rapidly proliferating tumor and cancer cells most sensitive; bone marrow, which produces red blood cells, is also sensitive, and depression of red blood cell production is a frequent side effect of nitrogen mustard therapy. The nitrogen mustards also suppress the immune response (see immunity).

nitrogen cycle, the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay), nitrification, and denitrification. Nitrogen is vital to all living matter, both plant and animal; it is an essential constituent of amino acids, which form proteins of nucleic acids, and of many other organic materials.

Nitrogen Fixation

Although the earth's atmosphere is 78% nitrogen, free gaseous nitrogen cannot be utilized by animals or by higher plants. They depend instead on nitrogen that is present in the soil. To enter living systems, nitrogen must be "fixed" (combined with oxygen or hydrogen) into compounds that plants can utilize, such as nitrates or ammonia. A certain amount of atmospheric nitrogen is fixed by lightning and by some cyanobacteria (blue-green algae). But the great bulk of nitrogen fixation is performed by soil bacteria of two kinds: those that live free in the soil and those that live enclosed in nodules in the roots of certain leguminous plants (e.g., alfalfa, peas, beans, clover, soybeans, and peanuts). Among the free-living forms are species of Clostridium, discovered c.1893 by Sergei Winogradsky, and Azotobacter, discovered c.1901 by M. W. Beijerinck. Both Clostridium and Azotobacter are generally present in agricultural soils, and both are saprophytes, i.e., they use the energy from decaying organic matter in the soil to fuel soil processes, including nitrogen fixation.

Bacteria that live in the roots of legumes are of the genus Rhizobium, first isolated c.1888 by Beijerinck. These rod-shaped bacteria enter the roots chiefly through the root hairs and then work their way to the inner root tissues. There they stimulate the growth of tumorlike nodules. Within the nodules the bacteria develop into forms called bacteroids, which live in a symbiotic (mutually beneficial) relationship with the green plant. The bacteroids take carbohydrates from the plant for energy to fix nitrogen and synthesize amino acids; the plants take the amino acids elaborated in the nodule to build plant tissue. Animals in turn consume the plants and convert plant protein into animal protein. Rhizobia can be found free-living in the soil, but they cannot fix nitrogen in the free state, nor can the legume root fix nitrogen without Rhizobia.

The exact biochemistry of nitrogen fixation within the nodule is not yet understood. It is estimated that more than 300 lbs of nitrogen per acre (340 kg per hectare) can be fixed by fields of alfalfa and other legumes. After a harvest legume roots left in the soil decay, returning organic nitrogen compounds to the soil for uptake by the next generation of plants. For this reason crop rotation in which a leguminous crop is rotated with a nonleguminous one is a common practice for maintaining soil fertility.

Other Aspects of the Nitrogen Cycle

Decomposing animal remains and animal wastes also return organic nitrogen to the soil as ammonia. Many different kinds of decay microorganisms participate in ammonification. The nitrifying bacteria of the genus Nitrosomonas oxidize the ammonia to nitrites, and Nitrobacter oxidize the nitrites to nitrates. The nitrates can then be taken up again by the green plant. The cycle of fixation-decay-nitrification-fixation can proceed indefinitely without any nitrogen being returned to a gaseous state. But still another group of microorganisms, the denitrifying bacteria, can reduce nitrates all the way to molecular nitrogen. Denitrification occurs only in the absence of oxygen and is not common in well-cultivated soils.

Effects of Artificial Fixation

Nitrogen fixation can also be accomplished artificially by various methods (see nitrogen). Humans annually fix vast amounts of nitrogen for industrial purposes and for use as fertilizer. Unfortunately, large-scale legume cultivation and artificial fixation may be upsetting the natural nitrogen cycle in the biosphere. There is some question whether natural denitrification can keep pace with fixation. For one thing, run-off of nitrate fertilizer can cause eutrophication of lakes and streams (see water pollution) and can foul drinking supplies. Another environmental problem is that inorganic fertilizers tend to depress legume fixation. As a consequence, root tissue remaining after harvest is poorer, and thus more fertilizer must be applied the following year.

nitrogen, gaseous chemical element; symbol N; at. no. 7; at. wt. 14.0067; m.p. -209.86°C;; b.p. -195.8°C;; density 1.25 grams per liter at STP; valence principally -3, +3, or +5. Nitrogen is a colorless, odorless, tasteless diatomic gas. It is found in Group 15 of the periodic table. It does not burn, does not support combustion, and is only slightly soluble in water. It is relatively inactive chemically, but many of its compounds display marked reactivity. At high temperatures it reacts with some of the other elements to form nitrides.

Nitrogen has several oxides. Nitrous oxide, N₂O, is a gas used as an anesthetic; it is often called laughing gas. Nitric oxide, NO, is a gas used in the manufacture of sulfuric acid; in air it forms nitrogen dioxide, NO₂, a poisonous reddish brown gas. Nitrogen trioxide, N₂O₃, is unstable at ordinary temperatures. Nitrogen pentoxide, N₂O₅, forms nitric acid when dissolved in water. Important compounds of nitrogen include nitric acid, ammonia, many explosives, cyanides, fertilizers, and the proteins. Many organic compounds contain nitrogen.

Nitrogen for industrial use is produced largely by the fractional distillation of liquid air. Nitrogen is used to some extent for filling light bulbs, in thermometers, and generally anywhere a relatively inert atmosphere is needed, as in the production of electronic parts such as transistors, diodes, and integrated circuits, and in food storage packaging to prevent spoilage. It is used in the manufacture of stainless steel and as a coolant for the immersion freezing of food products, for the transportation of foods, for the preservation of bodies and reproductive cells (sperm and eggs), and for the storage of biological samples. However, the chief importance of the element lies in its compounds, among them ammonia, nitric acid, and cyanide.

The expression "nitrogen fixation" refers to the extraction of the element from the atmosphere by its combination with other elements to form compounds. This is accomplished commercially in several ways. In the Haber process, nitrogen is reacted with hydrogen to form ammonia; in the cyanamide process, nitrogen is reacted with calcium carbide at high temperatures to form calcium cyanamide; in the arc process, nitrogen is reacted with oxygen in an electric arc to form nitrogen oxides.

Nitrogen is abundant in the atmosphere; it is about 78% (by volume) of dry air. Nitrogen is present in living things; it and its compounds are necessary for the continuation of life (see nitrogen cycle). Nitrogen also is found in foods and is important in the human diet.

Nitrogen compounds were known to alchemists as early as the Middle Ages, but nitrogen is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or phlogisticated air (air from which the oxygen had been removed, usually by combustion). 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 dephlogisticated air. It was well known to these late 18th century chemists that there was a fraction of air that did not support combustion. Antoine Lavoisier was the first to treat oxygenless air as a separate element, which he called azote, meaning without life. The term nitrogen was first used by J. A. Chaptal in 1790. This early "nitrogen" was later shown by John Strutt (Lord Rayleigh), and William Ramsay to contain argon; Henry Cavendish had shown in 1785 that there was an unreactive gas other than nitrogen present in air.

carbon-nitrogen-oxygen cycle: see nucleosynthesis.

Atomic Number:	Atomic Number: 7
Atomic Symbol:	Atomic Symbol: N
	Name of Element: Nitrogen
Atomic Weight:	Atomic Weight: 14.0067
Electron Configuration:	Electron Configuration: 2 · 5

or nitrogen euphoria or raptures of the deep

Effects of breathing nitrogen under increased pressure. In divers breathing compressed air, nitrogen saturates the nervous system, causing an intoxicating light-headed, numb feeling, then slowed reasoning and dexterity, and then emotional instability and irrationality. Severe cases progress to convulsions and blackout. Susceptibility varies, and severity increases with depth, but there are no aftereffects. Physical function remains normal, and divers may be unaware of the growing irrationality that can cause them to rise too fast (see decompression sickness) or let their air supply run out. Helium, which dissolves less easily in body tissues, is substituted for nitrogen for deep dives.

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Colourless, toxic gas (NO), formed from nitrogen and oxygen by the action of electric sparks or high temperatures or, more conveniently, by the action of dilute nitric acid on copper or mercury. First prepared circa 1620 by Jan B. Helmont, it was first studied in 1772 by Joseph Priestley, who called it “nitrous air.” An industrial procedure for the manufacture of hydroxylamine is based on the reaction of nitric oxide with hydrogen in the presence of a catalyst. The formation of nitric oxide from nitric acid and mercury is applied in a volumetric method of analysis for nitric acid or its salts. The gas is synthesized via enzyme-catalyzed reactions in humans and other animals, where it serves as a signaling molecule. Among its numerous biological roles, it causes dilation of blood vessels and as such is an important regulator of blood pressure. Nitric oxide is one of the components of air pollution generated by internal-combustion engines.

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Any natural or industrial process that causes free nitrogen in the air to combine chemically with other elements to form more reactive nitrogen compounds such as ammonia, nitrates, or nitrites. Soil microorganisms (e.g., Rhizobium bacteria living in root nodules of legumes) are responsible for more than 90percnt of all nitrogen fixation. Though nitrogen is part of all proteins and essential in both plant and animal metabolism, plants and animals cannot use elemental nitrogen such as the nitrogen gas (N₂) that forms 80percnt of the atmosphere. Symbiotic nitrogen-fixing bacteria invade the root hairs of host plants, where they multiply and stimulate the formation of root nodules, enlargements of plant cells and bacteria in close association. Within the nodules the bacteria convert free nitrogen to nitrates, which the host plant uses for its development. Nitrogen fixation by bacteria associated with legumes is of prime importance in agriculture. Before the use of synthetic fertilizers in the industrial countries, usable nitrogen was supplied as manure and by crop rotation that included a legume crop.

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Circulation of nitrogen in various forms throughout nature. Nitrogen is essential to life, but in the atmosphere it is in a form (the diatomic molecule N₂) unavailable to most organisms. Nitrogen fixation by microbes turns this nitrogen into nitrates and other compounds, which plants or algae assimilate into their tissues. Animals that eat plants in turn incorporate the compounds into their own tissues. Microbes decompose the remains and waste of all living things into ammonia (ammonification); the ammonia may leave the soil through vaporization into the air or leaching into water. Ammonia remaining in soil may be transformed by bacteria into nitrates (nitrification), which then can be reassimilated into living organisms, or into free nitrogen (denitrification), which reenters the atmosphere. Hence, once fixed from air, some nitrogen goes through the cycle repeatedly without returning to the gaseous state.

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Gaseous chemical element, chemical symbol N, atomic number 7. A colourless, odourless, tasteless gas, it makes up 78percnt of Earth's atmosphere and is a constituent of all living matter. As the nearly unreactive diatomic molecule N₂, it is useful as an inert atmosphere or to dilute other gases. Nitrogen is commercially produced by distillation of liquefied air. Nitrogen fixation, achieved naturally by soil microbes and industrially by the Haber-Bosch process, converts it to water-soluble compounds (including ammonia and nitrates). Industrially, ammonia is the starting material for most other nitrogen compounds (especially nitrates and nitrites), whose main uses are in agricultural fertilizers and explosives. In compounds, nitrogen usually has valence 3 or 5. It forms several oxides including nitrous oxide (N₂O; laughing gas), nitric oxide (NO), nitrogen dioxide (NO₂), and other forms (such as N₂O₃ and N₂O₅). Some of the nitrogen oxides, often referred to generically as NO_math.x, are notorious as contributors to urban air pollution. Other compounds include the nitrides, exceptionally hard materials made from nitrogen and a metal; cyanides; azides, used in detonators and percussion caps; and thousands of organic compounds containing nitrogen in functional groups or in a linear or ring structure (see heterocyclic compound). Seealso nitrogen cycle.

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