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Nitrogen Cycle

Nitrogen cycle

Nitrogen is a macronutrient essential to all living organisms. It is an integral component of amino acids which are the building blocks of proteins; it forms part of the nitrogenous bases common to DNA and RNA; it helps make up ATP, and it is a major component of the chlorophyll molecule in plants. In essence, life as we know it cannot exist without nitrogen.

Although nitrogen is readily abundant as a gas (it comprises 79 percent of atmospheric gases by volume), most organisms cannot use it in this state. It must be converted to a chemically usable form such as ammonia (NH3) or nitrate (NO3) for most plants, and amino acids for all animals. The processes involved in the conversion of nitrogen to its various forms comprise the nitrogen cycle. Of all the nutrient cycles, this is considered the most complex and least well understood scientifically. The processes that make up the nitrogen cycle include nitrogen fixation , ammonification, nitrification , and denitrification .

Nitrogen fixation refers to the conversion of atmospheric nitrogen gas (N2) to ammonia (NH3) or nitrate (NO3). The latter is formed when lightning or sometimes cosmic radiation causes oxygen and nitrogen to react in the atmosphere . Farmers are usually delighted when electrical storms move through their areas because it supplies "free" nitrogen to their crops, thus saving money on fertilizer . Ammonia is produced from N2 by a special group of microbes in a process called biological fixation, which accounts for about 90 percent of all fixed N2 each year worldwide. This process is accomplished by a relatively small number of species of bacteria and blue-green algae, or blue-green bacteria. The most well known of these nitrogen-fixing organisms are the bacteria in the genus Rhizobium which are associated with the root nodules of legumes. The legumes attract these bacteria by secreting a chemical into the soil that stimulates the bacteria to multiply and enter the root hair tips. The resultant swellings contain millions of bacteria and are called root nodules, and here, near the soil's surface, they actively convert atmospheric N2 to NH3, which is taken up by the plant. This is an example of a symbiotic relationship, where both organisms benefit. The bacteria benefit from the physical location in which to grow, and they also utilize sugars supplied by the plant photosynthate to reduce the N2 to NH3. The legumes, in turn, benefit from the NH3 produced. The energetic cost for this nutrient is quite high, however, and legumes typically take their nitrogen directly from the soil rather than from their bacteria when the soil is fertilized.

Although nitrogen fixation by legume bacteria is the major source of biological fixation, other species of bacteria are also involved in this process. Some are associated with non-legume plants such as toyon (Ceanothus ), silverberry (Elaeagnus ), water fern (Azolla ), and alder (Alnus ). With the exception of the water fern, these plants are typically pioneer species growing in low-nitrogen soil. Other nitrogen-fixing bacteria live as free-living species in the soil. These include microbes in the genera Azotobacter and Clostridium, which live in aerobic and anaerobic sediments, respectively.

Blue-green algae are the other major group of living organisms which fix atmospheric N2. They include approximately forty species in such genera as Aphanizomenon, Anabaena, Calothrix, Gloeotrichia, and Nostoc. They inhabit both soil and freshwater and can tolerate adverse and even extreme conditions. For example, some species grow in hot springs where the water is 212°F (100°C), whereas other species inhabit glaciers where the temperature is 32°F (0°C). The characteristic bluish-green coloration is a telltale sign of their presence. Some blue-green algae are found as pioneer species invading barren soil devoid of nutrients, particularly nitrogen, either as solitary individuals or associated with other organisms such as lichens . Flooded rice fields are another prime location for nitrogen-fixing blue-green algae.

Perhaps the most common environments where blue-green algae are found are lakes and ponds, particularly when the body of water is eutrophiccontaining high concentrations of nutrients, especially phosphorus . Algae can reach bloom proportions during the warm summer months and are often considered a nuisance because they float on the surface, forming dense scum. The resultant odor following decomposition is usually pungent, and fish such as catfish often acquire an off-flavor taste from ingesting these algae.

The next major component of the nitrogen cycle is ammonification. It involves the breakdown of organic matter (in this case amino acids) by decomposer organisms to NH3, yielding energy. It is therefore the reverse reaction of amino acid synthesis. Dead plant and animal tissues and waste materials are broken down to amino acids and eventually NH3 by the saprophagous bacteria and fungi in both soil and water.

Nitrification is a biological process where NH3 is oxidized in two steps, first to NO3 and next to nitrate (NO2). It is accomplished by two genera of bacteria, Nitrosomonas and Nitrobacter in the soil, and Nitrosococcus and Nitrococcus in salt water. Since nitrification is an oxidation reaction, it requires oxygenated environments.

Dentrification is the reverse reaction of nitrification and occurs under anaerobic conditions. It involves the breakdown of nitrates and nitrites into gaseous N2 by microorganisms and fungi. Bacteria in the genus Pseudomonas (e.g., P. dentrificans ) reduce NO3 in the soil.

The cycling of nitrogen in an ecosystem is obviously complex. In aquatic ecosystems, nitrogen can enter the food chain through various sources, primarily surface runoff into lakes or rivers, mixing of nutrient-rich bottom waters (normally only during spring and fall turnovers in north temperate lakes), and biological fixation of atmospheric nitrogen by blue-green algae. Phytoplankton (microscopic algae) then rapidly take up the available nitrogen in the form of NH3 or NO3 and assimilate it into their tissues, primarily as amino acids. Some nitrogen is released by leakage through cell membranes. Herbivorous zooplankton that ingest these algae convert their amino acids into different amino acids and excrete the rest. Carnivorous or omnivorous zooplankton and fish that eat the herbivores do the same. Excretion (usually as NH3 and urea) is thus a valuable nutrient recycling mechanism in aquatic ecosystems. Different species of phytoplankton actively compete for these nutrients when they are limited. Decomposing bacteria in the lake, particularly in the top layer of the bottom sediments, play an important role in the breakdown of dead organic matter which sinks to the bottom. The cycle is thus complete.

The cycling of nitrogen in marine ecosystems is similar to that in lakes, except that nitrogen lost to the sediments in the deep open water areas is essentially lost. Recycling only occurs in the nearshore regions, usually through a process called upwelling. Another difference is that marine phytoplankton prefer to take up nitrogen in the form of NH3 rather than NO3.

In terrestrial ecosystems, NH3 and NO3 in the soil is taken up by plants and assimilated into amino acids. As in aquatic habitats, the nitrogen is passed through the food chain/web from plants to herbivores to carnivores, which manufacture new amino acids. Upon death, decomposers begin the breakdown process, converting the organic nitrogen to inorganic NH3. Bacteria are the main decomposers of animal matter and fungi are the main group that break down plants. Shelf and bracket fungi, for example, grow rapidly on fallen trees in forests. The action of termites, bark beetles, and other insects that inhabit these trees greatly speed up the process of decomposition.

There are three major differences between nitrogen cycling in aquatic versus terrestrial ecosystems. First, the nitrogen reserves are usually much greater in terrestrial habitats because nutrients contained in the soil remain accessible, whereas nitrogen released in water and not taken up by phytoplankton sinks to the bottom where it can be lost or held for a long time. Secondly, nutrient recycling by herbivores is normally a more significant process in aquatic ecosystems. Thirdly, terrestrial plants prefer to take up nitrogen as NO3 and aquatic plants prefer NH3.

Forces in nature normally operate in a balance, and gains are offset by losses. So it is with the nitrogen cycle in freshwater and terrestrial ecosystems. Losses of nitrogen by detritrification, runoff, sedimentation , and other releases equal gains by fixation and other sources.

Humans, however, have an influence on the nitrogen cycle that can greatly change normal pathways. Fertilizers used in excess on residential lawns and agricultural fields add tremendous amounts of nitrogen (typically as urea or ammonium nitrate) to the target area. Some of the nitrogen is taken up by the vegetation, but most washes away as surface runoff, entering streams, ponds, lakes, and the ocean. This contributes to the accelerated eutrophication of these bodies of water. For example, periodic unexplained blooms of toxic dinoflagellates off the coast of southern Norway have been blamed on excess nutrients, particularly nitrogen, added to the ocean by the fertilizer runoff from agricultural fields in southern Sweden and northern Denmark. These algae have caused massive dieoffs of salmon in the mariculture pens popular along the coast, resulting in millions of dollars of damage. Similar circumstances have contributed to blooms of other species of dinoflagellates, creating what are known as red tides. When filter-feeding shellfish ingest these algae, they become toxic, both to other fishes and humans. Paralytic shellfish poisoning may result within thirty minutes, leading to impairment of normal nerve conduction, difficulty in breathing, and possible death. Saxotoxin, the toxin produced by the dinoflagellate Gonyaulax, is fifty times more lethal than strychnine and curare.

Other forms of human intrusion into the nitrogen cycle include harvesting crops, logging , sewage, animal wastes, and exhaust from automobiles and factories. Harvesting crops and logging remove nitrogen from the system. The other processes are point sources of excess nitrogen. Autos and factories produce nitrous oxides (NOx) such as nitrogen dioxide (NO2), a major air pollutant. NO2 contributes to the formation of smog , often irritating eyes and leading to breathing difficulty. It also reacts with water vapor in the atmosphere to form weak nitric acid (HNO3), one of the major components of acid rain .

See also Agricultural pollution; Air pollution; Aquatic weed control; Marine pollution; Nitrogen waste; Soil fertility; Urban runoff

[John Korstad ]


RESOURCES

BOOKS

Ehrlich, P. R., A. H. Ehrlich, and J. P. Holdren. Ecoscience: Population, Resources, Environment. New York: W. H. Freeman, 1977.

Ricklefs, R. E. Ecology. 3rd ed. New York: W. H. Freeman, 1990.

Smith, R. E. Ecology and Field Biology. 4th ed. New York: Harper and Row, 1990.

PERIODICALS

Brill, W. J. "Biological Nitrogen Fixation." Scientific American 236 (1977): 6881.

Delwiche, C. C. "The Nitrogen Cycle." Scientific American 223 (1970): 13646.

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