nitrogen cycle 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 N 2 ) unavailable to most organisms. Nitrogen fixa¬ tion 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 nitro¬ gen goes through the cycle repeatedly without returning to the gaseous state.
nitrogen fixation 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 90% of all nitrogen fixation. Though nitrogen is part of all proteins and essential in both plant and ani¬ mal metabolism, plants and animals cannot use elemental nitrogen such as the nitrogen gas (N 2 ) that forms 80% 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 bac¬ teria 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.
nitrogen narcosis 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.
nitroglycerin \ 1 nI-tro- , gli-s9-r9n\ or glyceryl trinitrate Organic compound, powerful explosive and ingredient of most forms of dynamite. It is a colourless, oily, somewhat toxic liquid with a sweet, burning taste. Its safe use as a blasting explosive became possible after Alfred P. Nobel developed dynamite in the 1860s with an inert porous material (modera¬ tor) such as charcoal or diatomaceous earth. Nitroglycerin is also used in a mixture in rocket propellants. In medicine, it is used to dilate blood ves¬ sels, especially to ease angina pectoris.
nitrous oxide Vnl-tr9s\ or laughing gas Inorganic compound, one of the oxides of nitrogen. A colourless gas with a pleasantly sweetish odour and taste, it has an analgesic effect when inhaled; it is used as an anes¬ thetic (often called just “gas”) in dentistry and surgery. This effect is pre¬ ceded by mild hysteria, sometimes with laughter, hence the name laughing gas. It is also used as a propellant in food aerosols and as a leak detector.
Nivernais \,ne-v3r-'na\ Historical region, central France. Originally part of Burgundy, it became a county c. 10th century. Francis I of France made it a duchy in 1539 for Francis of Cleves. In 1659 it was sold to Cardinal Mazarin. His descendants possessed it until the French Revolu¬ tion, when it was the last great fief to be reunited to the French crown. During the ancien regime it was administered from Nevers.
Nixon, Richard M(ilhous) (b. Jan. 9, 1913, Yorba Linda, Calif., U.S.—d. April 22,1994, New York, N. Y.) 37th president of the U.S. (1969— 74). He studied law at Duke University and practiced in California (1937- 42). After serving in World War II, he was elected to the U.S. House of Representatives (1946). As a member of the House Un-American Activi¬ ties Committee, he received national attention for his hostile questioning of Alger Hiss. In 1950 he was elected to the Senate following a bitter campaign in which he unfairly portrayed his opponent as a communist sympathizer; the epithet “Tricky Dick” dates from this period. He won the vice presi¬ dency in 1952 as the running mate of Republican Dwight D. Eisenhower. During the campaign he delivered a nationally televised address, the “Checkers” speech (named for the dog he admitted receiving as a political gift), to rebut charges of financial misconduct. He and Eisenhower were reelected easily in 1956. As the Republican presidential candidate in 1960, he lost narrowly to John F. Kennedy. After failing to win the 1962 California gubernatorial race, he announced his retirement from politics and criticized the press, declaring that it would not “have Dick Nixon to kick around any¬ more.” He moved to New York to practice law. He reentered politics by running for president in 1968, narrowly defeating Hubert H. Humphrey with his “southern strategy” of seeking votes from southern and western con¬ servatives in both parties. As president, he began to withdraw U.S. military forces from South Vietnam while resuming the bombing of North Vietnam. His expansion of the Vietnam War to Cambodia and Laos in 1970 provoked widespread protests in the U.S. He established direct relations with China and made a state visit there in 1972, the first by a U.S. president. On a visit to the Soviet Union later that year, he signed agreements resulting from the Strategic Arms Limitation Talks between the U.S. and the Soviet Union held between 1969 and 1972, known as SALT I. In domestic affairs, Nixon responded to persistent inflation and increasing unemployment by devalu¬ ing the dollar and imposing unprecedented peacetime controls on wages and prices. His administration increased funding for many federal civil- rights agencies and proposed legislation that created the Occupational Safety and Health Administration (OSHA) and the Environmental Protec¬ tion Agency (EPA). In 1972 he won reelection with a landslide victory over George McGovern. Assisted by Henry A. Kissinger, he concluded a peace agreement with North Vietnam (1973), though the war did not come to an end until 1975. His administration helped to undermine the coalition gov¬ ernment of Chile’s Marxist Pres. Salvador Allende, leading to Allende’s overthrow in a military coup in 1973. Nixon’s second term was overshad¬ owed by the Watergate scandal, which stemmed from illegal activities by Nixon and others related to the burglary and wiretapping of the headquar¬ ters of the Democratic Party. After lengthy congressional investigations and facing near-certain impeachment, Nixon resigned the presidency on Aug. 8,1974, the first president to do so. Though never convicted of wrong¬ doing, he was pardoned by his successor, Gerald Ford. In retirement, he wrote his memoirs and several books on foreign policy, which modestly rehabilitated his reputation and earned him a role as an elder statesman and foreign-policy expert.