ment. Atmospheric gaseous nitrogen can be used directly only by certain nitrogen-fixing bacteria (e.g. Clostridium, Nostoc, Rhizobium). They convert nitrogen to ammonia, nitrites, and nitrates, which are released into the soil by excretion and decay. Some are free-living, whereas others form symbiotic associations with plants (see nitrogen fixation). Another method by which atmospheric nitrogen is fixed is by lightning, which causes nitrogen and oxygen to combine. The oxides so produced dissolve in rain to form nitrous and nitric acids; in the soil these acids combine with mineral salts to form nitrites and nitrates. This is an insignificant process when compared with microbial nitrogen fixation. When plants and animals die, the organic nitrogen they contain is converted back into nitrate in the process termed nitrification. Apart from uptake by plants, nitrate may also be lost from the soil by denitrifica-tion and by leaching. The use of nitrogen fertilizers in agriculture and the emission of nitrogen oxides in car exhaust fumes have influenced the nitrogen cycle and contributed to urban air pollution (see photochemical smog) and acid precipitation.
nitrogen fixation The formation of nitrogenous compounds from atmospheric nitrogen. In nature this may be achieved by electric discharge in the atmosphere or by the activities of certain microorganisms. For example, symbiotic bacteria of the genus Rhizobium are associated with leguminous plants, causing the root cortex to form nodules, which house the bacteria. These bacteria contain the nitrogenase enzyme that catalyzes the fixation of molecular nitrogen to ammonium ions, which the plant can assimilate. In return the legume supplies the bacteria with carbohydrate. Many other nitrogen-fixing symbioses are known, e.g. the nitrogen-fixing root nodules of bog myrtle (Myrica gale) and alder (Alnus glutinosa) appear to contain unicellular protoctists called plasmodiophorans, while water ferns (Azolla) have cyanobac-teria in their roots. Free-living bacteria that can fix nitrogen include members of the genera Azotobacter, Klebsiella, and Clostridium, some sulfur bacteria (e.g.
Chlorobium), and most cyanobacteria (e.g. Anabaena). Some yeast fungi have also been shown to fix nitrogen. Industrially, nitrogen fixation is mainly achieved using the haber process. See nitrogen cycle; symbiosis.
nitrogenous wastes Organic wastes that contain significant amounts of nitrogen.
noise pollution Harmful or unwanted sounds in the environment.
nomenclature An internationally recognized system of naming of living organisms and fossils that forms the basis of their scientific classification. See classification; International Codes of Nomenclature; taxonomy.
nominal scale See measurement scales.
nonequilibrium models Models of population dynamics that take into account spatial and temporal variation to explain the behavior of populations that are not in a state of equilibrium.
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