Background (Encyclopedia of Global Resources)
Nitrogen is in high demand in biological systems. However, most nitrogen is not readily available to plants and animals. Although the biosphere contains 300,000 terrograms (billion kilograms) of nitrogen, that amount is far less nitrogen than is in the hydrosphere (23 million terrograms) and much less nitrogen than is in the atmosphere (about 4 billion terrograms). Atmospheric nitrogen is almost all in the form of nitrogen gas (N2), which composes 78 percent of the atmophere by volume. The greatest reservoir of nitrogen on Earth is the lithosphere (164 billion terrograms). Here the nitrogen is bound up in rocks, minerals, and deep-ocean sediments.
Even though living things exist in a “sea” of nitrogen gas, it does them little good. The bond between the nitrogen atoms is so strong that nitrogen gas is relatively inert. For living things to use nitrogen gas, it must first be converted to an organic or an inorganic form. The nitrogen cycle is the collection of processes, most of them driven by microbial activity, that convert nitrogen gas into these usable forms and later return nitrogen gas back to the atmosphere. It is considered a cycle because every nitrogen atom can ultimately be converted by each process, though that conversion may take a long time. It is estimated, for example, that the average nitrogen molecule spends 625 years in the biosphere before returning to the atmosphere to complete the cycle.
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Nitrogen Fixation (Encyclopedia of Global Resources)
The first step in the nitrogen cycle is nitrogen fixation. nitrogen fixation is the conversion, by bacteria, of nitrogen gas into ammonium (NH4+) and then organic nitrogen (proteins, nucleic acids, and other nitrogen-containing compounds). It is estimated that biological nitrogen fixation adds about 160 billion kilograms of nitrogen to the biosphere each year. This represents about half of the nitrogen taken up by plants and animals. The microorganisms that carry out nitrogen fixation are highly specialized. Each one carries a special enzyme complex, called nitrogenase, that allows it to carry out fixation at temperatures and pressures capable of permitting life, something industrial nitrogen fixation does not allow.
Nitrogen-fixing microbes may either be free-living or grow in association with higher organisms such as legumes (in which case the process is called symbiotic nitrogen fixation). Symbiotic nitrogen fixation is an important process and is one reason legumes are so highly valued as a natural resource. Because they are able to form these symbiotic associations with nitrogen-fixing bacteria, legumes can produce seeds and leaves with more nitrogen than other plants. When they die, they return much of that nitrogen to soil, enriching it for future growth.
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Mineralization and Nitrification (Encyclopedia of Global Resources)
When plants and animals die they undergo a process called mineralization (also called ammonification). In this stage of the nitrogen cycle, the organic nitrogen in decomposing tissue is converted back into ammonium. Some of the ammonium is taken up by plants as they grow. This process is called assimilation or uptake. Some of the ammonium is taken up by microbes in the soil. In this case the nitrogen is not available for plant growth. If this happens, it is said that the nitrogen is immobilized. Some nitrogen is also incorporated into the clay minerals of soil. In this case it is said that the nitrogen is fixed—it is not immediately available for plant and microbial growth, but it may become available at a later date.
Ammonium has another potential fate, and this step in the nitrogen cycle is nitrification. In nitrification the ammonium in soil is oxidized by bacteria (and some fungi) to nitrate (NO3-) in a two-step process. First, ammonium is oxidized to nitrite. Next, nitrite is rapidly oxidized to nitrate. Nitrification requires oxygen, so it occurs only in well-aerated environments. The nitrate that forms during nitrification can also be taken up by plants and microbes. However, unlike ammonium, which is a cation and readily adsorbed by soil, nitrate is an anion and readily leaches or runs off of soil. Hence, nitrate is a serious water contaminant in areas where excessive fertilization or manure...
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Denitrification (Encyclopedia of Global Resources)
Obviouslysome process is responsible for returning nitrogen to the atmosphere; otherwise organic and inorganic nitrogen forms would accumulate in the environment. The process that completes the nitrogen cycle and replenishes the nitrogen gas is denitrification. Denitrification is a bacterial process that occurs in anaerobic or oxygen-limited environments (waterlogged soil or sediment, for example). Nitrate and nitrite are reduced by denitrifying bacteria which can use these nitrogen oxides in place of oxygen for their metabolism. Wetlands are particularly important in this process because at least half of the denitrification that occurs in the biosphere occurs in wetlands.
The major product of denitrification is nitrogen gas, which returns to the atmosphere and approximately balances the amount of nitrogen gas that is biologically fixed each year. In some cases, however, an intermediate gas, nitrous oxide (N2O), accumulates. Nitrous oxide has serious environmental consequences. Like carbon dioxide, it absorbs infrared radiation, so it contributes to global warming. More important, when nitrous oxide rises to the stratosphere, it contributes to the catalytic destruction of the ozone layer. Besides the potential for fertilizer nitrogen to contribute to nitrate contamination of groundwater, there is the concern that some of it can be denitrified and contribute to ozone destruction.
The nitrogen cycle is a global cycle...
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Further Reading (Encyclopedia of Global Resources)
Chapin, F. Stuart, III, Pamela A. Matson, and Harold A. Mooney. “Internal Cycling of Nitrogen.” In Principles of Terrestrial Ecosystem Ecology. New York: Springer, 2002.
Jacobson, Michael C., et al. Earth System Science: From Biogeochemical Cycles to Global Change. 2d ed. San Diego, Calif.: Academic Press, 2000.
Mosier, Arvin, J. Keith Syers, and John R. Freney, eds. Agriculture and the Nitrogen Cycle: Assessing the Impacts of Fertilizer Use on Food Production and the Environment. Washington, D.C.: Island Press, 2004.
Nieder, R., and D. K. Benbi. Carbon and Nitrogen in the Terrestrial Environment. Dordrecht, the Netherlands: Springer, 2008.
Schlesinger, William H. Biogeochemistry: An Analysis of Global Change. 2d ed. San Diego, Calif.: Academic Press, 1997.
Sigel, Astrid, Helmut Sigel, and Roland K. O. Sigel, eds. Biogeochemical Cycles of Elements. Boca Raton, Fla.: Taylor & Francis, 2005.
Sprent, Janet I. The Ecology of the Nitrogen Cycle. New York: Cambridge University Press, 1987.
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Definition (Encyclopedia of Global Warming)
The nitrogen cycle is a natural process by which nitrogen in the air moves into the soil, is utilized by living organisms, and returns to the air. Organisms need nitrogen to make macromolecules, including amino acids and nucleic acids. Air is 79 percent nitrogen gas (N2). N2 has a triple bond, making it relatively inert and unable to be used by most organisms. Nitrogen fixation, converting N2 to NH4+ (an ammonium ion), is carried out by Rhizobium bacteria, which live in root nodules of host plants of the legume family such as peas, beans, and clover. Bacteria get carbohydrates from the plant, and the plant uses some of the ammonium the bacteria fix.
Free-living cyanobacteria also fix nitrogen. Lightning fixes smaller amounts of nitrogen. Plants absorb nitrate or ammonium ions from the soil through root hairs. Ammonium is converted into organic nitrogen compounds by bacteria and by plants. Other organisms get organic nitrogen from what they eat. Organic nitrogen is converted to ammonia as microorganisms decompose dead matter. Soil bacteria, Nitrosomona, perform nitrification where NH4 is converted to NO2- (nitrite). NO2- is converted to NO3- (nitrate) by Nitrobacter. Completing the nitrogen cycle, nitrites are converted to N2 and N2O (nitrous oxide) in anaerobic conditions by bacteria Pseudomonas and Clostridium.
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Significance for Climate Change (Encyclopedia of Global Warming)
Humans have more than doubled the annual transfer of nitrogen gas into biologically available forms of nitrogen. This has occurred through burning of fossil fuels, manufacture of synthetic nitrogen fertilizers, and widespread cultivation of legumes (soy, alfalfa, and clover). Burning fossil fuels causes formation of oxides of carbon (carbon monoxide, carbon dioxide). The burning of such fuels occurs at elevated temperatures that cause nitrogen and oxygen molecules in air to react to form oxides of nitrogen (NO, N2O, and NO2). NO2 forms smog and mixed with water forms nitric acid (HNO3), contributing to acid rain.
The Haber-Bosch process fixes N2 using hydrogen, high temperature, and pressure to form ammonia. Synthetic nitrogen fertilizers (ammonia, ammonium nitrate, and urea), applied directly to the soil, have led to a huge increase in agricultural productivity. The applied fertilizer not utilized by plants leaches out of soil and accumulates in water. N2O and CO2 are greenhouse gases (GHGs) that contribute to global warming by absorbing energy from the Earth’s surface, stopping the loss of this energy, and raising the Earth’s temperature. NO2 is not as abundant as CO2, yet it is an important, stable GHG that absorbs infrared energy about 270 times more strongly than does CO2. Nitrogen cycle influence on global warming is complex, as an increase in biologically active nitrogen stimulates plant growth,...
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Nitrogen Cycle (Encyclopedia of Science)
The term nitrogen cycle refers to a series of reactions in which the element nitrogen and its compounds pass continuously through Earth's atmosphere, lithosphere (crust), and hydrosphere (water component). The major components of the nitrogen cycle are shown in the accompanying figure. In this diagram, elemental nitrogen is represented by the formula N2, indicating that each molecule of nitrogen consists of two nitrogen atoms. In this form, nitrogen is more correctly called dinitrogen.
Nitrogen is the most abundant single gas in Earth's atmosphere. It makes up about 80 percent of the atmosphere. This fact is important because plants require nitrogen for their growth and, in turn, animals depend on plants for their survival. The problem is, however, that plants are unable to use nitrogen in its elemental forms dinitrogen. Any process by which elemental dinitrogen is converted to a compound is known as nitrogen fixation.
Dinitrogen is converted from an element to a compound by a number of naturally occurring processes. When lightning passes through the atmosphere,...
(The entire section is 700 words.)