Nitrogen cycle

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 fixation

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 form—as 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, it prompts a reaction between nitrogen and oxygen; oxides of nitrogen—primarily nitric oxide (NO) and nitrogen dioxide (NO₂)—are formed. Both oxides then combine with water vapor in the atmosphere to form nitric acid (HNO₃). Nitric acid is carried to the ground in rain and snow, where it is converted to nitrites and nitrates. Nitrites and nitrates are both compounds of nitrogen and oxygen, the latter containing more oxygen than the former. Naturally occurring minerals such as saltpeter (potassium nitrate; KNO₃) and Chile saltpeter (sodium nitrate; NaNO₃) are the most common nitrates found in Earth's crust.

Certain types of bacteria also have the ability to convert elemental dinitrogen to nitrates. Probably the best known of these bacteria are the rhizobium, which live in nodules on the roots of leguminous plants such as peas, beans, clover, and the soya plant.

Finally, dinitrogen is now converted to nitrates on very large scales by human processes. In the Haber process, for example, nitrogen and hydrogen are combined to form ammonia, which is then used in the manufacture of synthetic fertilizers, most of which contain nitrates.

Words to Know

Ammonification: The conversion of nitrogen compounds from plants and animals to ammonia and ammonium; this conversion occurs in soil or water and is carried out by bacteria.

Denitrification: The conversion of nitrates to dinitrogen (or nitrous oxide) by bacteria.

Dinitrogen fixation (nitrogen fixation): The conversion of elemental dinitrogen (N2) in the atmosphere to a compound of nitrogen deposited on Earth's surface.

Nitrification: The process by which bacteria oxidize ammonia and ammonium compounds to nitrites and nitrates.

Ammonification, nitrification, and denitrification

Nitrogen that has been fixed by one of the mechanisms described above can then be taken in by plants through their roots and used to build new stems, leaves, flowers, and other structures. Almost all animals obtain the nitrogen they require, in turn, by eating plants and taking in the plant's organic forms of nitrogen.

The nitrogen stored in plants and animals is eventually returned to Earth by one of two processes: elimination (in the case of animals) or death (in the case of both animals and plants). In whatever form the nitrogen occurs in the dead plant or animal, it is eventually converted to ammonia (NH₃) or one of its compounds. Compounds formed from ammonia are known as ammonium compounds. This process of ammonification is carried out (as the plant or animal decays) by a number of different microorganisms that occur naturally in the soil.

Ammonia and ammonium compounds, in their turn, are then converted to yet another form, first to nitrites and then to nitrates. The transformation of ammonia and ammonium to nitrite and nitrate is an oxidation process that takes place through the action of various bacteria such as those in the genus Nitrosomonas and Nitrobacter. The conversion of ammonia and ammonium compounds to nitrites and nitrates is called nitrification.

In the final stage of the nitrogen cycle, oxygen is removed from nitrates by bacteria in a process known as denitrification. Denitrification converts nitrogen from its compound form to its original elemental form as dinitrogen, and the cycle is ready to begin once again.

nitrogen cycle the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay), nitrification, and denitrification. Nitrogen is vital to all living matter, both plant and animal; it is an essential constituent of amino acids, which form proteins of nucleic acids , and of many other organic materials.

Nitrogen Fixation

Although the earth's atmosphere is 78% nitrogen, free gaseous nitrogen cannot be utilized by animals or by higher plants. They depend instead on nitrogen that is present in the soil. To enter living systems, nitrogen must be "fixed" (combined with oxygen or hydrogen) into compounds that plants can utilize, such as nitrates or ammonia. A certain amount of atmospheric nitrogen is fixed by lightning and by some cyanobacteria (blue-green algae). But the great bulk of nitrogen fixation is performed by soil bacteria of two kinds: those that live free in the soil and those that live enclosed in nodules in the roots of certain leguminous plants (e.g., alfalfa, peas, beans, clover, soybeans, and peanuts). Among the free-living forms are species of Clostridium, discovered c.1893 by Sergei Winogradsky, and Azotobacter, discovered c.1901 by M. W. Beijerinck. Both Clostridium and Azotobacter are generally present in agricultural soils, and both are saprophytes, i.e., they use the energy from decaying organic matter in the soil to fuel soil processes, including nitrogen fixation.

Bacteria that live in the roots of legumes are of the genus Rhizobium, first isolated c.1888 by Beijerinck. These rod-shaped bacteria enter the roots chiefly through the root hairs and then work their way to the inner root tissues. There they stimulate the growth of tumorlike nodules. Within the nodules the bacteria develop into forms called bacteroids, which live in a symbiotic (mutually beneficial) relationship with the green plant. The bacteroids take carbohydrates from the plant for energy to fix nitrogen and synthesize amino acids; the plants take the amino acids elaborated in the nodule to build plant tissue. Animals in turn consume the plants and convert plant protein into animal protein. Rhizobia can be found free-living in the soil, but they cannot fix nitrogen in the free state, nor can the legume root fix nitrogen without Rhizobia.

The exact biochemistry of nitrogen fixation within the nodule is not yet understood. It is estimated that more than 300 lbs of nitrogen per acre (340 kg per hectare) can be fixed by fields of alfalfa and other legumes. After a harvest legume roots left in the soil decay, returning organic nitrogen compounds to the soil for uptake by the next generation of plants. For this reason crop rotation in which a leguminous crop is rotated with a nonleguminous one is a common practice for maintaining soil fertility.

Other Aspects of the Nitrogen Cycle

Decomposing animal remains and animal wastes also return organic nitrogen to the soil as ammonia. Many different kinds of decay microorganisms participate in ammonification. The nitrifying bacteria of the genus Nitrosomonas oxidize the ammonia to nitrites, and Nitrobacter oxidize the nitrites to nitrates. The nitrates can then be taken up again by the green plant. The cycle of fixation-decay-nitrification-fixation can proceed indefinitely without any nitrogen being returned to a gaseous state. But still another group of microorganisms, the denitrifying bacteria, can reduce nitrates all the way to molecular nitrogen. Denitrification occurs only in the absence of oxygen and is not common in well-cultivated soils.

Effects of Artificial Fixation

Nitrogen fixation can also be accomplished artificially by various methods (see nitrogen ). Humans annually fix vast amounts of nitrogen for industrial purposes and for use as fertilizer. Unfortunately, large-scale legume cultivation and artificial fixation may be upsetting the natural nitrogen cycle in the biosphere. There is some question whether natural denitrification can keep pace with fixation. For one thing, run-off of nitrate fertilizer can cause eutrophication of lakes and streams (see water pollution ) and can foul drinking supplies. Another environmental problem is that inorganic fertilizers tend to depress legume fixation. As a consequence, root tissue remaining after harvest is poorer, and thus more fertilizer must be applied the following year.

Nitrogen Cycle

The nitrogen cycle is the series of biogeochemical transformations in which the element nitrogen is transferred among organisms and nonliving reservoirs such as the soil, the oceans, and the atmosphere. Nitrogen is an essential element for all living things because it is a principal component of proteins and nucleic acids. Whereas animals generally have access to abundant nitrogen, it is often in short supply for plants.

Ammonification

Most of the world's nitrogen is in the atmosphere, in the form of nitrogen gas (N₂), which is extremely unreactive. Atmospheric nitrogen is ammonified, or converted to ammonia (NH₃) or ammonium ion (NH₄⁺), in several ways. It occurs through enzymatic nitrogen fixation, which is carried out by either free-living or symbiotic bacteria; through lightning, volcanic eruptions, and other high-energy events in the atmosphere; and finally by industrial processes. Industrial ammonification, which requires large amounts of energy, is used to create ammonia and nitrate for use as agricultural fertilizer. Industrial processes convert approximately 80 million metric tons of nitrogen per year, whereas bacterial nitrogen fixation converts slightly more, with about half of that carried out by crop plants. Ammonia is also produced through the action of fungus and bacteria breaking down organic compounds in the soil.

Nitrification, Denitrification, and Assimilation

Aerobic soil bacteria convert ammonia and ammonium to nitrate (NO₃⁻), which can be absorbed by plants. This process, called nitrification, is counterbalanced by denitrification, which forms N₂ and N₂O, carried out by anaerobic bacteria. Nitrate is assimilated, or absorbed by plants, through their roots. Within the plant, nitrate is reconverted to ammonium for use in building organic compounds. Nitrogen moves through the food chain in these compounds and is eventually returned to the environment through urine, feces, or the decomposition of the organism.

Human nitrogen use has had a major impact on the nitrogen cycle. The agricultural use and overuse of nitrogen fertilizer has caused pollution of water bodies both near farms and more distantly. Nitrate in the soil is easily washed out and can become a pollutant of both groundwater and surface water. Nitrogen is usually a limiting nutrient in aquatic ecosystems , and therefore its runoff often produces overgrowth, or "eutrophication." Chronic eutrophication can change species composition of lakes, streams, and rivers.

see also Biogeochemical Cycles; Cyanobacteria; Ecology; Eubacteria; Nitrogen Fixation; Pollution and Bioremediation

Richard Robinson

Bibliography

Berner, Elizabeth Kay, and Robert A. Berner. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall, 1996.

Human Alterations of the Global Nitrogen Cycle. <http://esa.sdsc.edu/tilman.htm>.

nitrogen cycle One of the major cycles of chemical elements in the environment (see biogeochemical cycle). Nitrates in the soil are taken up by plant roots and may then pass along food chains into animals. Decomposing bacteria convert nitrogen-containing compounds (especially ammonia) in plant and animal wastes and dead remains back into nitrates, which are released into the soil and can again be taken up by plants (see nitrification). Though nitrogen is essential to all forms of life, the huge amount present in the atmosphere is not directly available to most organisms (compare carbon cycle). It can, however, be assimilated by some specialized bacteria (see nitrogen fixation) and is thus made available to other organisms indirectly. Lightning flashes also make some nitrogen available to plants by causing the combination of atmospheric nitrogen and oxygen to form oxides of nitrogen, which enter the soil and form nitrates. Some nitrogen is returned from the soil to the atmosphere by denitrifying bacteria (see denitrification). See illustration.

nitrogen cycle Circulation of nitrogen through plants and animals in the biosphere. Plants obtain nitrogen compounds for producing essential proteins through assimilation. Nitrogen-fixing bacteria in the soil or plant root nodules take free nitrogen from the soil and air to form the nitrogen compounds (nitrates) used by plants in assimilation (see nitrogen fixation). Herbivores obtain their nitrogen from the plants, and carnivores obtain nitrogen by eating herbivores (see food chain). Saprophytes decompose the tissue of all the organisms concerned and the nitrogen is released back into the cycle.

nitrogen cycle A description of the balance, changes, and nature of the nitrogen-containing compounds circulating between the atmosphere, the soil, and living matter. For plants, nitrogen fixation by soil bacteria, which renders nitrogen readily available for assimilation by the plants, is an essential and crucial part of the nitrogen cycle.

nitrogen cycle A description of the balance, changes, and nature of the nitrogen-containing compounds circulating between the atmosphere, the soil, and living matter. For plants, nitrogen fixation by soil bacteria, which renders nitrogen readily available for assimilation by the plants, is an essential and crucial part of the nitrogen cycle.