Nitrogen Fixation (redirected from Nitrogen fixation process)

nitrogen fixation

Any natural or industrial process that causes free nitrogen in the air to combine chemically with other elements to form more reactive nitrogen compounds such as ammonia, nitrates, or nitrites. Soil microorganisms (e.g., Rhizobium bacteria living in root nodules of legumes) are responsible for more than 90% of all nitrogen fixation. Though nitrogen is part of all proteins and essential in both plant and animal metabolism, plants and animals cannot use elemental nitrogen such as the nitrogen gas (N₂) that forms 80% of the atmosphere. Symbiotic nitrogen-fixing bacteria invade the root hairs of host plants, where they multiply and stimulate the formation of root nodules, enlargements of plant cells and bacteria in close association. Within the nodules the bacteria convert free nitrogen to nitrates, which the host plant uses for its development. Nitrogen fixation by bacteria associated with legumes is of prime importance in agriculture. Before the use of synthetic fertilizers in the industrial countries, usable nitrogen was supplied as manure and by crop rotation that included a legume crop.

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nitrogen fixation

1. the conversion of atmospheric nitrogen into nitrogen compounds by certain bacteria, such as Rhizobium in the root nodules of legumes

2. a process, such as the Haber process, in which atmospheric nitrogen is converted into a nitrogen compound, used esp for the manufacture of fertilizer

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nitrogen fixation [′nī·trə·jən ‚fik¦sā·shən]

(chemical engineering)

Conversion of atmospheric nitrogen into compounds such as ammonia, calcium cyanamide, or nitrogen oxides by chemical or electric-arc processes.

(microbiology)

Assimilation of atmospheric nitrogen by heterotrophic bacteria. Also known as dinitrogen fixation.

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Nitrogen Fixation 

the process of binding molecular atmospheric nitrogen (N₂) and converting it to nitrogenous compounds. Nitrogen fixation is accomplished by nitrogen-fixing microorganisms, including Rhizobia and other microorganisms (bacteria, actinomycetes, yeasts, fungi, and blue-green algae) that inhabit the soil, freshwater bodies, seas, and oceans.

Nitrogen fixation is a very important biological process, playing a large role in the nitrogen cycle in nature and enriching the soil and ponds with bound nitrogen. The atmosphere covering 1 hectare (ha) of ground contains over 70,000 tons of free nitrogen, and it is only as a result of nitrogen fixation that a portion of this becomes available to higher plants. Free-living nitrogen-fixing bacteria bind several tens of kilograms of nitrogen per ha per year. Blue-green algae in rice fields bind up to 200 kg per ha per year. The overall increase in nitrogen (in above-ground organs and postharvest residues) with legume culture is 57.5–335 kg per ha per year. The quantity of nitrogen carried into the soil by legumes because of the activity of Rhizobia amounts to 100–250 kg per ha in a season. Naturally, this process has great significance for the improvement of soil and for increasing the yield of agricultural crops. It is for this purpose that legume seeds are mixed with Rhizobium preparations before sowing, legumes are used as predecessors to cereal grains in crop rotation, corn is sown with clover, vetch is sown with oats, and so forth.

Research on the mechanism of nitrogen fixation is very important. As far back as 1894, S. N. Vinogradskii suggested that ammonia was formed as a result of nitrogen fixation. This proposition has been confirmed by contemporary research methods, including the use of a heavy isotope of nitrogen (N¹⁵). A. N. Bakh assumed in 1934 that nitrogen fixation is a result of the conjugative effect of oxidation-reduction enzymes. It has been established that the reduction of molecular nitrogen (N₂) to ammonia (NH₃) occurs with the participation of an enzyme system containing iron, molybdenum, and magnesium and functioning as a carrier of electrons to the N₂. Nitrogen-fixing enzyme systems catalyze the reduction of N₂ in the presence of an energy source—adenosine triphosphate (ATP)—and a reducing agent—for example, molecular hydrogen (H₂), or a hydrosulfite (Na₂S₂O₄). Thus, nitrogen fixation does not require oxygen in the proper sense and is a reductive process.

REFERENCES

Kretovich, V. L., and B. I. Liubimov. “Biokhimiia fiksatsii azota.” Priroda, 1964, no. 12. Pages 14–21.
Mishustin, E. N., and V. K. Shi’ nikova. Biologicheskaia fiksatsiia atmosfernogo azota. Moscow, 1968.

V. L. KREMOVICH and V. I. LIUBIMOV