What Is the Process of Nitrogen Fixation by Bacteria?
Updated: Oct 5
What Is the Process of Nitrogen Fixation by Bacteria?
Nitrogen fixation by bacteria is the conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) using the enzyme nitrogenase.
The process requires energy, electrons and protection of the enzyme from oxygen. Fixed nitrogen is then incorporated into biological compounds and, in suitable plant–microbe associations, contributes to crop nutrition. [1, 2]
The agricultural importance lies in what happens after fixation. In a compatible legume–rhizobium partnership, bacteria supply fixed nitrogen to the host through specialized root nodules. In soil and other root-associated systems, fixed nitrogen can remain in microbial biomass before becoming available to plants. The presence of nitrogen-fixing bacteria therefore does not, by itself, establish how much nitrogen a crop receives. [3, 4]
For organism selection and commercial supply, explore IndoGulf BioAg’s nitrogen-fixing bacteria overview.
Why plants need nitrogen fixation
Nitrogen is essential for proteins, nucleic acids and other components of plant growth. However, crops cannot simply absorb nitrogen gas from the air and use it as a nutrient. The strong bond within the N₂ molecule must first be overcome through a fixation process. [1]
Biological fixation creates a new input of reactive nitrogen to an ecosystem. It differs from processes that recycle nitrogen already present in soil. For example, mineralization converts organic nitrogen into ammonium, while nitrification transforms ammonium into nitrite and nitrate. [5]
These distinctions matter when evaluating microbial products: a nitrogen-fixing organism, a decomposer and a nitrifier perform different functions.
How nitrogen fixation works: six steps
1. Atmospheric nitrogen reaches the bacterium
Nitrogen gas enters the environment surrounding an active diazotroph—a microorganism capable of fixing nitrogen—and becomes available to its nitrogenase system. Fixation may take place in a root nodule, in soil, near a root surface or within plant tissues, depending on the organism and association. [1, 3]
2. Metabolism supplies energy and electrons
The bacterium needs energy in the form of ATP and a supply of electrons to reduce N₂. Carbon metabolism supports these requirements. In legume nodules, the plant supplies carbon compounds to its bacterial partner; outside nodules, the availability of usable carbon can constrain activity. [1, 4]
Nitrogen fixation is therefore an energy-demanding metabolic process, rather than an unlimited nutrient source that operates independently of the crop and environment.
3. The bacterium protects nitrogenase from oxygen
Nitrogenase is oxygen-sensitive, but nitrogen-fixing bacteria do not all require the same oxygen conditions. Different organisms protect the enzyme in different ways. Rhizobial nodules regulate oxygen availability; aerobic diazotrophs such as Azotobacter vinelandii can use respiratory protection. [1]
Low oxygen around nitrogenase should not be confused with a recommendation to waterlog soil. Crop roots and microbial energy metabolism also require suitable conditions.
4. Nitrogenase transfers electrons and reduces N₂
In the well-studied molybdenum nitrogenase system, the Fe protein interacts repeatedly with the catalytic MoFe protein. ATP-coupled electron transfer supplies the reducing equivalents needed to convert N₂ into ammonia. The reaction proceeds through multiple catalytic states and also produces hydrogen gas. [2]
5. Fixed nitrogen enters biological compounds
Ammonia and ammonium are chemically related forms whose balance depends on pH. Fixed nitrogen is assimilated into organic molecules. The glutamine synthetase–glutamate synthase pathway, commonly called GS/GOGAT, is an important route for incorporating ammonium into amino acids. These compounds support further nitrogen-containing molecules in cells. [6]
6. Nitrogen contributes to the plant or soil system
In effective legume nodules, bacterial fixation is coupled to nitrogen transfer and assimilation by the host. In non-symbiotic systems, some fixed nitrogen supports microbial growth. Its subsequent contribution to plant nutrition depends on release, microbial turnover and nutrient cycling, as well as root access and timing. [3, 4]
Fixation, nitrogen transfer and crop uptake are related but separate steps. A useful agricultural inoculant must be evaluated across the relevant steps, rather than only for nitrogenase activity.
What is nitrogenase?
Nitrogenase is the enzyme system responsible for biological N₂ reduction. The commonly studied molybdenum-dependent system has two cooperating components: [1, 2]
Component | Main role |
Fe protein, also called dinitrogenase reductase | Participates in ATP-coupled electron delivery to the catalytic component |
MoFe protein, also called dinitrogenase | Contains the metal clusters involved in electron transfer and substrate reduction |
Within the MoFe protein, the P-cluster participates in electron transfer and the FeMo cofactor is the catalytic site for substrate reduction. Some diazotrophs also possess alternative vanadium-dependent or iron-only nitrogenases. [2]
The nitrogen fixation equation
The conventional overall reaction for molybdenum nitrogenase is: [2]
N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pᵢ
Here, ATP supplies energy for the catalytic cycle, and Pᵢ represents inorganic phosphate. This biochemical equation does not predict a field nitrogen contribution or a fertilizer replacement rate.
How the legume–rhizobium partnership works
The best-known agricultural example is the association between compatible legumes and rhizobia. Members of genera such as Rhizobium and Bradyrhizobium include strains that establish nitrogen-fixing root-nodule partnerships. Host compatibility and the effectiveness of the strain are essential. [1, 7]
In a typical root-hair infection pathway:
The partners exchange signals. Compounds released by the legume root stimulate compatible bacteria; bacterial Nod factors help initiate the plant’s symbiotic response.
Bacteria enter the developing association. Root hairs respond and an infection thread can guide bacteria toward the developing nodule.
Nodule tissues develop. Bacteria become specialized bacteroids within plant-associated compartments.
The nodule supports fixation. The plant supplies carbon, while the nodule regulates oxygen conditions. Leghemoglobin helps support the balance between enzyme protection and respiration.
The host assimilates fixed nitrogen. Nitrogen supplied by the bacteroids enters plant metabolism through assimilation into organic compounds. [1, 6]
This describes a common pathway; not every legume–rhizobium association uses identical infection routes.
Rhizobial inoculants should be selected for the intended host. A product suitable for one legume cannot be assumed suitable for soybean, peas, lentils, clover and other legumes collectively. Even successful nodulation should be assessed alongside crop performance. [7]
Read more about Rhizobium species and plant nutrition.
Symbiotic, associative, endophytic and free-living fixation
These terms describe the relationship between a nitrogen-fixing microorganism and its environment. Some organisms can occupy more than one ecological niche. [1, 3]
Relationship | Illustrative example | Important distinction |
Symbiotic | Compatible rhizobia in legume nodules | Specialized structures support an organized exchange between host and bacterium |
Associative | Selected Azospirillum brasilense strains near cereal and grass roots | Close root association does not establish a legume-like nodule partnership |
Endophytic | Gluconacetobacter diazotrophicus, studied in association with sugarcane | Colonization of internal tissues must be distinguished from the quantity of nitrogen supplied |
Free-living | Azotobacter vinelandii in soil | Fixed nitrogen can first support microbial biomass rather than immediate crop uptake |
For a wider comparison, see the different types of nitrogen-fixing bacteria. For organism-specific context, explore our Azospirillum brasilense guide and Azotobacter vinelandii page.
What affects nitrogen fixation in agricultural soils?
Biological capability is only one part of field performance. Consider the conditions that allow the organism to survive, establish and express the intended function:
Crop and strain compatibility: especially important where nodulation is the intended mechanism.
Available nitrogen: readily available nitrogen can influence whether a bacterium invests energy in fixation, and can affect nodule formation and activity.
Carbon supply: fixation depends on energy-generating metabolism.
Moisture, temperature and soil conditions: conditions supporting activity differ among organisms and associations.
Nutrient supply: the process depends on the wider nutritional environment, including nutrients involved in metabolism and enzyme function.
Competition and establishment: an introduced organism must function within a resident microbial community. [1, 4, 7]
These factors explain why neither a universal soil target nor a fixed number of kilograms of nitrogen can be assigned to every inoculant.
Does fixed nitrogen become immediately available to crops?
Not necessarily. A nitrogen-fixing cell uses nitrogen for its own metabolism and growth. An effective nodule partnership organizes transfer to the host, while other associations have different nitrogen-transfer pathways and limitations. [3]
Nitrogen incorporated into microbial biomass may become available later as organisms turn over and organic material is mineralized. Soil microbes can also temporarily immobilize mineral nitrogen. Once nitrogen enters soil nutrient pools, crop uptake competes with retention and potential loss pathways. [5]
This makes the timing and location of nitrogen availability important. Detecting a diazotroph or observing an increase in bacterial numbers is insufficient evidence that the crop received a commercially useful quantity of nitrogen.
How should nitrogen-fixing inoculants be used in the field?
Start with the intended crop, microbial strain and formulation. Delivery routes can include seed treatment or placement near developing roots, where supported by the product’s validated use. Seed coating can help position beneficial microorganisms near emerging seedlings, but survival through formulation, treatment and storage remains important. [8]
For practical development and evaluation:
Define the objective. Decide whether the program aims to support nodulation, contribute nitrogen, improve establishment or evaluate a combination of functions.
Select the organism for that objective. Check host compatibility and the evidence relevant to the intended crop.
Protect viability. Follow formulation-specific handling, storage and application instructions. Verify compatibility before combining microbial products with seed treatments or other inputs.
Measure performance against a suitable comparator. Assess relevant outcomes such as nodulation, plant nitrogen uptake, yield and economics.
Validate fertilizer changes separately. If a lower nitrogen rate is proposed, test it within a suitable fertilizer-response trial rather than inferring a reduction from inoculation alone.
A universal dose in grams per plant or kilograms per hectare is inappropriate across different organisms, viable concentrations and formulations.
Does better growth prove that bacteria fixed more nitrogen?
No. Selected plant growth-promoting bacteria can influence root development and nutrient acquisition through mechanisms involving plant signaling. A larger root system or greater biomass may therefore result without demonstrating a particular nitrogen contribution from fixation. [9]
Different measurements answer different questions. Nitrogenase assays indicate enzyme activity under the test conditions; nitrogen-source measurements and appropriately designed crop trials are needed to evaluate nitrogen contribution. Isotope-based methods can help investigate the source of plant nitrogen, with suitable experimental design and interpretation. [4]
Ask what was measured, which strain and crop were tested, and whether the result came from a laboratory, greenhouse or field trial.
How does fixation relate to nitrogen use efficiency?
Nitrogen fixation adds nitrogen; nitrogen use efficiency evaluates how nitrogen supply translates into useful crop production or recovery. Neither a species name nor a visible growth response provides an NUE value. [10]
The practical goal is a nutrient program that supports crop production with an appropriate nitrogen supply. Read our nitrogen use efficiency guide for the main calculations, microbial mechanisms and field evaluation considerations.
Develop a nitrogen-fixing microbial program with IndoGulf BioAg
IndoGulf BioAg works with distributors and formulators seeking bulk microbial ingredients, custom blends and private label supply. Explore our nitrogen-fixing bacteria category to discuss organism selection for your intended program.
For a commercial enquiry, share your company, target market, intended crops, application route, formulation requirements and anticipated purchase volumes. Contact our team or explore private label services.
Frequently asked questions
What is the main product of bacterial nitrogen fixation?
Nitrogenase reduces atmospheric nitrogen to ammonia. Ammonia and ammonium are related through a pH-dependent equilibrium; fixed nitrogen can then be incorporated into amino acids and other biological compounds. [2, 6]
Do nitrogen-fixing bacteria produce nitrate directly?
The nitrogenase reaction produces ammonia, not nitrate. Nitrification is a separate process that converts ammonium through nitrite to nitrate. [2, 5]
Why does nitrogen fixation require ATP?
Nitrogenase relies on ATP-coupled electron transfer during its catalytic cycle. Fixation consequently depends on the organism’s energy metabolism and access to suitable resources. [1, 2]
Can nitrogen fixation happen in oxygenated soil?
Yes. Oxygen-sensitive nitrogenase must be protected, but some nitrogen-fixing organisms are aerobic. Azotobacter vinelandii, for example, can use respiratory protection. The conditions required depend on the organism and association. [1]
Do cereals form the same nitrogen-fixing nodules as legumes?
Cereals do not normally form the familiar rhizobial root-nodule partnerships found in compatible legumes. They can associate with diazotrophs, but those relationships and their crop nitrogen contributions should be evaluated separately. [3]
Can nitrogen-fixing bacteria replace all nitrogen fertilizer?
No universal replacement claim applies. Biological fixation can be important in suitable systems, especially effective legume symbioses. The value of a particular inoculant and any proposed fertilizer adjustment require crop-, strain- and environment-relevant evidence. [4]
Are nitrogen fixation and nitrogen use efficiency the same?
No. Fixation converts atmospheric nitrogen into reactive nitrogen. NUE measures production or nitrogen recovery relative to a defined nitrogen supply. See our nitrogen use efficiency article for the relevant metrics. [10]
Scientific references
Ahemad, M., & Kibret, M. (2014). Mechanisms and applications of plant growth promoting rhizobacteria: Current perspective. Journal of King Saud University – Science, 26(1), 1–20. https://doi.org/10.1016/j.jksus.2013.05.001. Supplied advance-publication PDF is dated 2013; final volume publication is 2014. Supports microbial relationships and broad fixation mechanisms; not IndoGulf-specific efficacy.
Einsle, O., & Rees, D. C. (2020). Structural Enzymology of Nitrogenase Enzymes. Chemical Reviews, 120, 4969–5004. https://doi.org/10.1021/acs.chemrev.0c00067. Accessible full text. Supports enzyme architecture, electron transfer and the conventional reaction; mechanistic evidence, not field nitrogen replacement.
Rosenblueth, M., et al. (2018). Nitrogen Fixation in Cereals. Frontiers in Microbiology, 9, 1794. https://doi.org/10.3389/fmicb.2018.01794. Supports cereal-associated diazotrophs and the distinction between fixation capability and crop nitrogen contribution.
Ladha, J. K., Peoples, M. B., Reddy, P. M., Biswas, J. C., Bennett, A., Jat, M. L., & Krupnik, T. J. (2022). Biological nitrogen fixation and prospects for ecological intensification in cereal-based cropping systems. Field Crops Research, 283, 108541. https://doi.org/10.1016/j.fcr.2022.108541. Accessible full text. Supports agronomic context, measurement and constraints; no universal inoculant outcome.
University of Minnesota Extension. Understanding nitrogen in soils. Official guidance. Supports mineralization, immobilization, nitrification and loss pathways.
Thepbandit, W., & Athinuwat, D. (2024). Rhizosphere Microorganisms Supply Availability of Soil Nutrients and Induce Plant Defense. Microorganisms, 12, 558. https://doi.org/10.3390/microorganisms12030558. Supplied review; used for nodule development, nitrogen assimilation and nutrient context, not disease-control or efficacy claims.
Porter, S. S., Dupin, S. E., Denison, R. F., Kiers, E. T., & Sachs, J. L. (2024). Host-imposed control mechanisms in legume–rhizobia symbiosis. Nature Microbiology, 9, 1929–1939. https://doi.org/10.1038/s41564-024-01762-2. Abstract and reference list accessible; used only for the stated variability in rhizobial benefit and host compatibility, not detailed mechanisms inferred from inaccessible full text.
Rocha, I., Ma, Y., Souza-Alonso, P., Vosátka, M., Freitas, H., & Oliveira, R. S. (2019). Seed Coating: A Tool for Delivering Beneficial Microbes to Agricultural Crops. Frontiers in Plant Science, 10, 1357. https://doi.org/10.3389/fpls.2019.01357. Supplied review; supports delivery and viability considerations, not a universal application rate.
Vacheron, J., et al. (2013). Plant growth-promoting rhizobacteria and root system functioning. Frontiers in Plant Science, 4, 356. https://doi.org/10.3389/fpls.2013.00356. Supports root-development mechanisms distinct from nitrogen fixation.
Congreves, K. A., et al. (2021). Nitrogen Use Efficiency Definitions of Today and Tomorrow. Frontiers in Plant Science, 12, 637108. https://doi.org/10.3389/fpls.2021.637108. Supports distinctions among NUE metrics.
Research on an organism or strain does not establish the performance of every commercial formulation. Application rates and fertilizer changes should follow formulation-specific information, local agronomic guidance and appropriate validation.
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