THE NITROGEN CYCLE

Nitrogen gas constitutes 78% of the earth's atmosphere, but the total amount of fixed nitrogen in the soil, oceans, and the bodies of organisms is only about 0.03% of dial figure. The nitrogen cycles between reservoirs and organisms via the nitrogen cycle. In this process, relatively few kinds of organisms-all of them bade-ria-can convert, or fix, atmospheric nitrogen into forms that can be used for biological processes. The triple bond that links together the two atoms that make up diatomic atmospheric nitrogen (N2) makes it a very stable molecule. In living systems I the cleavage of atmospheric nitrogen is catalyzed by three proteins-ferredoxin, nitrogen reductase, and nitrogenase. This process uses ATP as a source of energy, electrons derived from photosynthesis or respiration, and a powerful reducing agent. Tht overall reaction can be written this way:
N2 + 3H2 -> 2NH3
All living organisms depend on nitrogen fixation to synthesize proteins, nuddt acids, and other necessary nitrogen-containing compounds.
Dozens of genera of bacteria, most of them free-living, have the ability to fix atmospheric nitrogen. Among these, some are free-living and some form symbiotic relationships with the roots of legumes (plants of the pea family, Fabaceae) and other plants. Only the latter usually fix enough nitrogen to be of major significance in nitrogen production. Plants that have symbiotic associations with nitrogen-fixing bacteria can grow in soils that have such low amounts of available nitrogen that they are unsuitable for most other plants. Because of the activities of such organisms in the past, however, there now exists a large reservoir of ammonia and nitrates in ecosystems;
this is the immediate source of much of the nitrogen used by organisms.
Although nitrogen gas constitutes about 78% of the earth's atmosphere, it becomes available to organisms almost entirely through the metabolic activities of a few genera of bacteria, some of which are free-living and others of which live symbiotically on the roots of legumes and other plants.
Bacteria of the genus Rhizobium, which inhabit nodules that they form on the roots of legumes, fix greater amounts of atmospheric nitrogen than do any other organisms. Bacteria of this genus make possible the growth of many legumes, a huge family of plants with about 18,000 species, in soils that are poor in nitrogen. Because of the presence of Rhisobium, legumes not only fix enough nitrogen for their own use, but also release excess fixed nitrogen into the soil, where it can be used by other plants. Nitrogen fixation by Rhizobium often exceeds 100 kilograms of nitrogen per hectare per year provided directly to the legumes. When legumes such as alfalfa are grown to enrich the soil, they are often plowed under so that all of the nitrogen they have fixed is available for future crops.
Specific strains of Rhizobium that will form nodules on the roots of different crop legumes, such as soybeans, alfalfa, and clover, are now available commercially. The individuals of Rhizobium enter the root hairs of their host legumes when the plants are still seedlings and move into the outer layers of the root tissue. There they stimulate repeated division of the outer cells of the root to form tumorlike nodules.
The roots of a few other kinds of plants form associations with nitrogen-fixing bacteria of the group known as actinomycetes. These bacteria constitute the genus Frankia. Some of the plants involved are alders (Ainus), sweet gale (Myrica), and mountain lilac (Ceanoihw].
In the flooded paddies of China and Southeast Asia, rice is cultivated continuously. The nitrogen that the rice plants require is supplied by cyanobacteria of the genus Anabaena, which occur symbiotically between the leaves of the small floating water fern Azolla. In these regions, people place the Azolla plants out among the rice crop; this mode of biological fertilization, therefore, can be practiced only where labor costs are relatively low.
It is estimated that the activities of all free-living bacteria, including cyanobacts-ria, may add, on the average, about 7 kilograms of fixed nitrogen per hectare of sal per year-about the same amount of fixed nitrogen that is added with the rain, whicli dissolves ammonia that reaches the atmosphere from various sources. In contrast,) crop of the legume alfalfa that is plowed back into the soil may add as much as 300 it, 500 kilograms of nitrogen per hectare per year. These figures clearly indicate [hi overwhelming importance of symbiotic associations in nitrogen fixation.
Chemoautrophic bacteria also contribute to the conversion of some ions to esse& tial amino acids needed by animals. For example, ammonia, in the presence ofwaMi is in equilibrium with the ammonium ion (NH4+). The chemoautotrophic nitrilyii bacterium Nitrosomonas is of primary importance for the oxidation of ammonia, whn is called nitrification. This reaction releases energy:
Another genus of bacteria, Nitrobacler, in turn oxidizes the nitrite ion (NO2-), which is toxic to plants, to nitrate (NO3), in which form it is absorbed by plants and is directly available to them:
2NO2 + 02 -> 2N03-
This reaction, too, yields energy; Nitrobacter, like Nitrosomonas, is chemoautotrophic.
Inside the plant cells, the nitrate ions are reduced back to ammonium ions, an energy-requiring process, and then are transferred to amino acids and other nitrogen-containing molecules. Some amino acids are formed directly; others are formed by the transfer of the amino group (-NH2) from one amino acid to another suitable molecule. Plants can produce all of the 20 amino acids they require. Most animals, however, can produce only 8; they obtain the other 12 from plants they consume directly or from animals that have eaten the plants.
Nitrogen-containing compounds, such as proteins, are decomposed rapidly by certain bacteria and fungi. These bacteria and fungi use the amino acids they obtain in this way to synthesize their own proteins and to release excess nitrogen in the form of ammonium ions (NH4+). This process, known as ammonification, occurs in most other kinds of organisms, as well. The ammonium ions can be converted to nitrites and nitrates by certain kinds of organisms and then be absorbed by plants.