Nitrogen (N) is so important to life that most forms of N released by one organism as "waste" usually gets taken up by another organism as food long before such "waste" N can get the chance to participate in the full set of oxidation/reduction reactions (nitrification/denitrification reactions) which identify the simplified N cycle. In terrestrial systems, an important product (and thus store) of this constant and dynamic cycling of organic "waste" is humus. This is reflected in the chemical composition of humus, which on the basis of C:N ratio is intermediate between that typical of plants and animals/microorganisms.
N fixation is the process in which atmospheric N2 is converted to compounds that can be utilized by plants. It is both a natural process that is mediated by certain microorganisms and an industrial process that requires large amounts of energy. Although all organisms require N compounds, very few are able to utilize N2, the most abundant readily-available form of the element. Most organisms require fixed forms of N in the form of NH3, NO3-, NO2-, or organic-N. The amount of fixed N may be a key limiting factor in the earth's ability to sustain life.
The N2 molecule is relatively inert and unreactive. It combines with other elements only at high temperature and pressure or in the presence of a catalyst. N can be fixed in the form of several oxides. Two sources of N oxides are internal combustion engines and lightning. Internal combustion engines produce NO and NO2 because the high internal temperatures and pressures cause atmospheric N2 and O2 to react. High-voltage electrical discharges, such as lightning, can oxidize N2. Cyanobacteria (blue-green algae) and bacteria associated with legumes can fix N2 by reducing it to ammoniacal (ammonia-like) N, mostly in the form of amino acids. N2 is reduced to NH3 industrially using high temperature, pressure, and a catalyst. NH3 production is the second-largest chemical industry in the United States, with an annual production of 17.2 million metric tons (14.2 million metric tons of N).
In pre-industrial times, the natural fixation of atmospheric N2 gas is estimated to be 40 to 200 million metric tons N/yr by marine ecosytems and 90 to 130 million metric tons N/yr by terrestrial ecosystems (Galloway, 1998). Lightning produces approximately 9.4 million metric tons of N oxides each year.
Estimates of global anthropogenic fixation of N2 include 80 million metric tons/yr of chemical N fertilizer, 20 million metric tons of N/yr released by combustion, 25 million metric tons of N/yr fixed by agricultural plantings of N-fixing legumes, and 25 million tons of N recycled in poorly defined anthropogenic wastes (Kinzig and Socolow, 1994; Galloway et al., 1995; Galloway, 1998; Smil, 1999). U. S. production in 1999 included approximately 17 million metric tons of anhydrous NH3, 7 million metric tons of ammonium nitrate, 3 milion metric tons of ammonium sulfate, and 8 million metric tons of nitric acid (Anon. 2000). The combustion estimate includes oxidation of the organic-N in coal and oil to NO. Although this is not fixation of N2, some researchers feel it is equivalent to N2 fixation because the organic N would otherwise remain unavailable for cycling in the biosphere (Vitousek et al., 1997).
It is interesting to note that the amount of N2 released by combustion and fertilizer manufacturing is of the same order of magnitude as estimated terrestrial biological of N fixation.
Biological Processes Involving N
N compounds are essential to all forms of life. Most important biomolecules contain N in a form similar to ammonia (-3 oxidation state). Almost all such N is called ammoniacal N - one of the hydrogen atoms combined with the N atom of ammonia being replaced by a carbon atom, e.g., C-NH2.. N is a vital component of proteins, peptides, enzymes, energy-transfer molecules (ATP, ADP), and genetic material (RNA and DNA) - substances that are vital to all organisms.
Whereas the amount of N needed by animals, microorganisms, and plants varies considerably, the
amounts of N required are always great enough to make N fall into the category of being an
essential macronutrient (needed in large amounts relative to other important essential nutrients such as:
calcium, phosphorus, potassium, sulfur, and magnesium). In all cases, the nutritional requirements for
N are exceeded only by those of carbon (C), hydrogen (H), and oxygen (O).
Certain bacteria also use N compounds in respiration (energy production). In all organisms, respiration is an oxidation-reduction (redox) reaction involving an oxidant (electron acceptor) and a reductant (electron donor). Aerobes (including humans) use oxygen as the electron acceptor. The energy available from various reactions that are mediated by organisms is given in the first seven rows of the following table, where "CH2O" (the generic formula for carbohydrate) indicates organic matter. (The reactions shown in the table are the net result of complex multi-step biochemical processes.)
The electron donor that yields the most energy usually determines the predominant type of
respiration in a particular environment. Therefore, when oxygen is present, aerobic respiration is the
predominant form of respiration. Denitrification is the second most energetic reaction in the table. Therefore,
when oxygen becomes depleted, nitrate becomes the preferred electron acceptor, followed by
manganese and iron oxides, and finally sulfate (SO42-). This sequence of redox reactions is often observed
in environments that are not in contact with the atmosphere, including sediments, flooded soils, and aquifer
systems. (See N Speciation Under Varying Oxidation-Reduction Conditions.) In nitrification (the last two
rows of the table) N species serve as electron donors.
Plants convert NO3- to organic N compounds, including amino acids, purines, and pyrimidines. NO3- (mostly oxidation state +5) is reduced to organic N(oxidation state -3). Plants can also utilize NH3 and organic N. The overall 8-electron reduction is described by the following equation.
The first step of assimilation is the reduction of NO3- to NO2-, which is catalyzed by the enzyme NO3- reductase. The subsequent reduction of NO2- to NH4+ is catalyzed by nitrite reductase. The process in which NH4+ is incorporated into amino acids is called amination.
NH3/NH4+ is released when organic matter is degraded. The oxidation state of the N is unchanged in this process. The process of breaking organic compounds down into inorganic compounds such as NH3 and CO2 is called mineralization.
Nitrosomonas and Nitrobacter bacteria use NH4+ and NO2-, respectively, as electron donors in respiration. Nitrification occurs under oxic conditions.
Denitrification (also called dissimilatory denitrification) completes the N cycle by reducing NO3- and NO2- to N2 and returning N to the atmosphere. "Some scientists believe that the reason N is the principal component of the earth's atmosphere is because of the continued activity of denitrifying microorganisms throughout geological history." (Stevenson 1972). Most denitrifying bacteria belong to the genera Psuedomonas, Micrococcus, Achromobacter, and Bacillus. Global estimates of terrestrial denitrification range from 13 to 233 million metric tons N/yr (Schlesinger, 1997, p. 389). Most denitrification occurs under anoxic conditions.
Denitrification is a form of respiration in which NO3- or NO2- is used as the electron acceptor. The net reaction is described by the following equation.
Denitrification consists of four sequential reactions. The first reaction, reduction of NO3- to NO2-, is catalyzed by the enzyme nitrate reductase. The second reaction, reduction of NO2- to NO, is catalyzed by nitrite reductase. The third reaction, reduction of NO to N2O, is catalyzed by nitric oxide reductase. The final reaction, reduction of N2O to N2, is catalyzed by nitrous oxide reductase.
NO appears to be produced during denitrification in acidic, anaerobic soils by chemical denitrification of nitrite (NO2-) with soil organic matter to NO, N2O and N2. Natural NO emissions may be relatively small compared to the other two N gases emitted by soils (N20 and N2) because NO2- rarely accumulates naturally in appreciable amounts and microbial denitrification of NO3- and NO2- to N2O and N2 is common. Most NO emissions are believed to be connected with NH4+ fertilization and subsequent restricted nitrification of the added fertilizer to NO2-. Emission estimates for NO vary widely. The IPCC (1992) estimates NO emissions for forested soils to be 3.6 million metric tons N/yr and the estimated total from all soils 10.8 million metric tons N/yr. However others consider soil NO emissions so small so as not to consider NO emissions at all. Others report emission values ranging from 6-45 million metric tons N/yr (National Research Council, 1977, p. 20; Galloway et al, 1995, p. 236).
The consensus is that most emissions of soil N2O are produced by biological denitrification. Bacteria capable of biological denitrification are essentially ubiquitous. Therefore, other factors control the rate and extent of denitrification, namely; the presence of oxidized forms of N to act as terminal electron acceptors, suitable electron donors (e.g., organic C compounds, reduced S compounds, H2), and the absence of oxygen gas (O2). The estimated flux of N2O from the soil to the atmosphere ranges from around 5-70 million metric tons N/yr. The contribution of agricultural soils is estimated to range from about 3-57 million metric tons N/yr (Soderlund and Svensson, 1997, p. 53; National Research Council, 1977, p. 20; Schlesinger, 1991, p. 395; Matthews, 1994, p. 412)
NO3- absorbs solar UV radiation (Gaffney et al., 1992) and is quite reactive in its excited states. While
photolysis is not a major sink for NO3-, it is one of the principal sources of hydroxyl radical in surface
waters and atmospheric water droplets (Zepp et al. 1987).
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