What Is the Process of Nitrogen Fixation?

Nitrogen is essential for plant growth because it is required to produce proteins, enzymes, chlorophyll and nucleic acids. Although approximately 78% of the atmosphere consists of nitrogen gas, most plants cannot use atmospheric nitrogen directly. The two nitrogen atoms in an N₂ molecule are joined by an exceptionally stable triple bond, making the molecule chemically resistant.

Nitrogen fixation is the process that converts atmospheric nitrogen into a chemically reactive form that can enter biological systems. In agriculture, the most important natural pathway is biological nitrogen fixation, which is performed by specialised bacteria and archaea collectively known as diazotrophs.

What Is Biological Nitrogen Fixation?

Biological nitrogen fixation is the microbial conversion of atmospheric nitrogen gas into ammonia. The reaction is catalysed by an enzyme complex called nitrogenase.

A simplified representation of the reaction is:

N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pi

This reaction requires a considerable amount of energy. The microorganism must therefore obtain sufficient carbon, electrons and ATP before nitrogen fixation can proceed efficiently.

In agricultural environments, diazotrophs may live freely in the soil, associate with plant roots, colonise internal plant tissues or establish specialised symbiotic relationships with particular host plants.

How Does Nitrogen Fixation Occur?

1. Nitrogen-fixing microorganisms become established

The process begins when suitable nitrogen-fixing bacteria become active in the soil, rhizosphere, plant tissues or root nodules.

Different organisms form different types of relationships with plants:

• Symbiotic diazotrophs, such as rhizobia, form nodules on compatible legumes.

• Associative diazotrophs, including selected Azospirillum strains, live closely around root surfaces.

• Endophytic diazotrophs, such as selected strains of Gluconacetobacter diazotrophicus, can colonise internal plant tissues.

• Free-living diazotrophs, including Azotobacter vinelandii, obtain energy independently from organic compounds in their environment.

• Certain cyanobacteria also perform nitrogen fixation in aquatic and terrestrial ecosystems.

These relationships are not interchangeable. Rhizobial symbioses are generally host-specific, while the establishment and activity of associative or endophytic bacteria depend on the microbial strain, crop and growing conditions.

2. The microorganism obtains energy and electrons

Breaking the triple bond in atmospheric nitrogen requires substantial energy. Free-living bacteria obtain this energy by metabolising organic carbon in the soil. Root-associated microorganisms can use carbon compounds released through root exudates.

In a legume–rhizobium symbiosis, the plant provides carbohydrates produced through photosynthesis. The bacteria use these carbon compounds to generate the ATP and reducing power needed by nitrogenase.

3. Nitrogenase reduces atmospheric nitrogen

The nitrogenase complex usually consists of two principal components: an iron protein that transfers electrons using energy from ATP, and a catalytic protein containing metal cofactors where N₂ reduction occurs.

Electrons are transferred repeatedly until atmospheric nitrogen is reduced to ammonia. Molybdenum-dependent nitrogenase is the most widespread form, although alternative vanadium- and iron-dependent nitrogenases occur in some microorganisms.

Nitrogenase is highly sensitive to oxygen. However, this does not mean that all nitrogen fixation occurs only under completely anaerobic conditions. Diazotrophs use different protective mechanisms. Legume nodules, for example, contain leghemoglobin, which regulates oxygen availability so that bacterial respiration can continue without exposing nitrogenase to damaging oxygen concentrations. Free-living aerobic bacteria may use rapid respiration or other protective systems.

How Does Symbiotic Nitrogen Fixation Develop?

In legumes, roots release chemical signals, including flavonoids, that can be recognised by compatible rhizobia. In response, the bacteria produce signalling molecules called Nod factors.

These signals initiate root-hair deformation, bacterial entry and nodule development. Inside mature nodules, rhizobia differentiate into specialised nitrogen-fixing forms known as bacteroids.

The plant supplies the bacteroids with carbon and maintains a low-oxygen environment. In return, the bacteria convert N₂ into ammonia. This ammonia is rapidly incorporated into amino compounds, principally through the glutamine synthetase–glutamate synthase pathway, and used by the plant to produce proteins and other nitrogen-containing molecules.

Compatibility is essential. A bacterium capable of nodulating one legume may not be effective on another. Even compatible strains can differ substantially in nodulation competitiveness and nitrogen-fixation efficiency, as described in this review of rhizobium–legume symbioses.

What Happens to Nitrogen Fixed by Free-Living Bacteria?

Nitrogen fixed by free-living or associative bacteria is not necessarily transferred immediately to a plant. Some remains within microbial cells and enters the wider soil nitrogen cycle after excretion, microbial turnover, grazing or decomposition.

Consequently, detecting nitrogenase activity in a laboratory does not by itself prove that a microorganism will supply a meaningful quantity of nitrogen to a crop under field conditions.

Factors Affecting Nitrogen Fixation

Biological nitrogen fixation is influenced by:

• Microbial strain and host compatibility

• Soil pH, moisture and temperature

• Available carbon and energy

• Oxygen concentration

• Phosphorus, iron, sulfur and molybdenum availability

• Salinity and other environmental stresses

• Competition with native microorganisms

• Inoculant viability and formulation

• Existing soil mineral nitrogen

High concentrations of available mineral nitrogen can reduce nodulation or suppress nitrogenase activity because biological nitrogen fixation is energetically expensive.

Agricultural Importance

Biological nitrogen fixation is a fundamental component of the global nitrogen cycle and an important source of nitrogen in agricultural systems. Effective legume–rhizobium symbioses can supply a substantial proportion of a legume crop’s nitrogen requirement and contribute nitrogen-containing residues to subsequent crops.

Associative, endophytic and free-living diazotrophs are also being investigated for cereals and other non-legume crops. However, their contribution is generally more variable and must be evaluated at the strain, crop, formulation and field level.

Nitrogen-fixing microorganisms should therefore be integrated with soil testing and responsible nutrient management. Their presence does not justify a universal reduction in nitrogen fertilizer without locally relevant field evidence.

Conclusion

Nitrogen fixation converts inert atmospheric nitrogen into ammonia through the activity of nitrogenase-producing microorganisms. The process requires substantial energy, a supply of electrons and protection of nitrogenase from oxygen.

Its agricultural contribution depends on the type of plant–microbe relationship. Symbiotic fixation in compatible legumes provides the clearest route for transferring fixed nitrogen to plants, while nitrogen fixed by associative and free-living microorganisms may enter plant nutrition through more variable pathways. Understanding these differences is essential for the scientifically responsible use of nitrogen-fixing microorganisms in agriculture.

The article distinguishes symbiotic, associative, endophytic and free-living fixation so it does not imply that all microbial nitrogen fixation results in immediate nitrogen delivery to crops.