What Is the Role of Nitrogen Fixation in Agriculture?

Nitrogen fixation plays a fundamental role in agriculture by converting atmospheric nitrogen into forms that can enter biological and agricultural systems. Nitrogen is essential for chlorophyll, proteins, enzymes, amino acids and nucleic acids, yet most crops cannot use atmospheric nitrogen gas directly.

Biological nitrogen fixation provides a natural pathway through which specialised microorganisms introduce reactive nitrogen into soils and plants. Its agricultural importance is most clearly demonstrated in legumes, although associative, endophytic and free-living nitrogen-fixing microorganisms are also being investigated for cereals, grasses and other crops.

What Is Biological Nitrogen Fixation?

Biological nitrogen fixation is performed by selected bacteria and archaea known as diazotrophs. These microorganisms use the enzyme complex nitrogenase to convert atmospheric nitrogen gas into ammonia.

Nitrogenase requires considerable energy and is sensitive to oxygen. Nitrogen-fixing microorganisms have therefore developed different strategies to obtain energy and protect the enzyme. Symbiotic bacteria operate inside specialised root nodules, while free-living and associative organisms may use rapid respiration, protected microenvironments or other physiological mechanisms.

The resulting ammonia can be incorporated into amino acids and other nitrogen-containing compounds. However, the pathway by which this nitrogen becomes available to crops differs among types of nitrogen-fixing microorganisms.

Supplying Nitrogen to Legume Crops

The most established agricultural role of nitrogen fixation occurs in the symbiosis between legumes and compatible rhizobia. Important agricultural legumes include soybean, peas, beans, chickpeas, lentils, clover, alfalfa and groundnut.

Legume roots release chemical signals that attract compatible rhizobia. The bacteria respond by producing Nod factors, which initiate root infection and nodule formation. Inside mature nodules, the bacteria differentiate into nitrogen-fixing forms called bacteroids.

The plant supplies carbohydrates as an energy source, while the bacteroids reduce atmospheric nitrogen to ammonia. The nitrogen is then assimilated into compounds that support plant growth and protein formation.

This relationship can supply a substantial part of a well-nodulated legume crop’s nitrogen requirement. However, rhizobial compatibility is highly specific. A strain effective on one legume species or cultivar may not establish an effective symbiosis with another.

Supporting Crop Rotations

Nitrogen fixation also contributes to crop rotations. After a legume crop, some nitrogen remains in roots, nodules, fallen leaves and crop residues. As these materials decompose, part of the organic nitrogen may become available to subsequent crops.

This residual contribution can reduce the mineral nitrogen requirement of a following cereal or other non-legume crop. The actual nitrogen credit depends on the legume species, biomass production, harvested portion, residue management, soil conditions and timing of mineralisation.

A legume does not necessarily leave all biologically fixed nitrogen in the soil. Much of the nitrogen may be removed in harvested grain or forage. Fertilizer decisions for the following crop should therefore use locally validated previous-crop credits rather than assuming that every legume supplies the same amount.

Nitrogen Fixation in Non-Legume Crops

Selected associative and endophytic diazotrophs have been studied in crops such as wheat, maize, rice, sugarcane and other grasses. Examples include selected strains of Azospirillum, Gluconacetobacter, Herbaspirillum and Paenibacillus.

These microorganisms may live on root surfaces, in the rhizosphere or within plant tissues. Root exudates can provide carbon compounds that support microbial activity. In return, nitrogen fixed by the microorganisms may enter the plant–soil system.

However, this relationship differs from the direct nitrogen exchange occurring in an effective legume nodule. Nitrogen fixed by associative or free-living bacteria may remain in microbial biomass and only become available through microbial turnover, excretion or decomposition.

Consequently, laboratory detection of nitrogenase activity does not prove that a microorganism will deliver an agronomically significant quantity of nitrogen to a field-grown crop. Performance must be evaluated by strain, formulation, crop and environment.

Improving Nitrogen-Use Strategies

Where biological nitrogen fixation operates effectively, it can reduce dependence on externally supplied nitrogen, particularly in legume production. This can support more efficient nutrient-management programs and reduce the energy demand associated with producing synthetic nitrogen fertilizer.

However, biological nitrogen fixation should not be presented as an automatic replacement for mineral fertilizer. Its contribution varies considerably, especially in non-legume crops. Nitrogen supplied through fixation must be considered alongside soil nitrogen, manure, crop residues and applied fertilizer.

An integrated strategy may include:

• Soil and plant-tissue testing

• Crop-specific fertilizer recommendations

• Compatible and verified microbial inoculants

• Appropriate crop rotations

• Previous-crop and manure nitrogen credits

• Split fertilizer applications where nitrogen-loss risk is high

• Field monitoring of crop growth and nodulation

Factors Affecting Nitrogen Fixation

The effectiveness of biological nitrogen fixation depends on several interacting conditions:

• Microbial strain and host compatibility

• Soil pH, moisture and temperature

• Phosphorus, sulfur, iron and molybdenum availability

• Carbon and energy supply

• Salinity and other environmental stresses

• Competition with native microorganisms

• Inoculant viability, formulation and placement

• Seed-treatment compatibility

• Existing levels of mineral nitrogen

High concentrations of available soil nitrogen can suppress nodule development or nitrogenase activity because fixation requires substantial energy. Poor soil fertility can also restrict the process; for example, inadequate phosphorus may limit root development, nodule activity and energy transfer.

Research on rhizobial inoculants shows that field performance is influenced not only by nitrogen-fixation capacity but also by the strain’s ability to survive, colonise roots and compete with native microorganisms for nodule occupancy. These limitations are reviewed in Frontiers in Plant Science.

Agricultural Use of Nitrogen-Fixing Inoculants

Nitrogen-fixing bacteria may be delivered through seed treatment, in-furrow application or root-zone application, depending on the organism and formulation.

Successful use requires verified strain identity, viable-cell concentration, shelf-life stability, crop compatibility and suitable application conditions. Seed coating can be an effective delivery method, but microbial survival depends on the carrier, storage conditions and compatibility with chemical seed treatments, as discussed by Rocha et al.

Nitrogen fixation supports agriculture by introducing atmospheric nitrogen into plant and soil nutrient cycles. Its strongest and most predictable role is in compatible legume–rhizobium symbioses. In crop rotations, biologically fixed nitrogen can also contribute to the nutrition of subsequent crops through residue decomposition.

Associative, endophytic and free-living diazotrophs offer additional opportunities, particularly in non-legume crops, but their nitrogen contribution is more variable. For responsible agricultural use, nitrogen fixation should be integrated with soil testing, crop-specific nutrient planning and locally relevant field evidence rather than treated as a universal substitute for nitrogen fertilizer.