Air is nearly 80% nitrogen, yet plants can’t use it in that form and have to get their nitrogen from the soil. A small group of bacteria can pull it from the air and convert it into a form roots can take up, doing a job that would otherwise take fertilizer. Some of these bacteria live in partnership inside plant roots while others live freely in the soil, and that difference shapes how much they help and how easy they are to spot.
How Bacteria Turn Air Into Plant Food
These bacteria carry an enzyme that breaks apart nitrogen gas and turns it into ammonia.1PubMed Central. Structural Enzymology of Nitrogenase Enzymes The two nitrogen atoms in the gas are held together by one of the strongest bonds in nature, so splitting them takes a lot of energy. The bacteria therefore need a steady supply of sugar or other carbon fuel to keep fixing nitrogen.2CGSpace. Energy cost of biological nitrogen fixation Plants that host them pay for their nitrogen by handing over sugars from photosynthesis. Living things fix about 80% of the nitrogen drawn from the air naturally.3Carbon. Interaction between graphene oxide and nitrogen-fixing bacterium Azotobacter chroococcum: Transformation, toxicity and nitrogen fixation Ammonia made by bacteria quickly becomes ammonium, which roots can absorb along with nitrate dissolved in soil water. Lightning and factories also fix nitrogen, but bacteria supply most of the natural fixation.
Oxygen is the enzyme’s weak spot. It damages the enzyme, yet most of these bacteria need oxygen to make energy.4PubMed. Mechanisms for protecting nitrogenase from inactivation by oxygen So they protect the enzyme by living where oxygen is scarce, building barriers, burning off oxygen nearby, or making new enzyme as fast as the old breaks down.5PubMed. Mechanisms for protecting nitrogenase from inactivation by oxygen Some bacteria also lock the enzyme into a protected, temporarily switched-off state when oxygen suddenly rises.6PubMed Central. Unraveling the molecular mechanisms of nitrogenase conformational protection against oxygen in diazotrophic bacteria
Legumes use the same logic inside their root swellings, called nodules. Healthy, working nodules usually look pink or red when cut open. The color comes from a protein that grips oxygen. It keeps oxygen low around the enzyme while still supplying the bacteria. White, green, or brown nodules usually aren’t fixing much.
The Main Types of Nitrogen-Fixing Bacteria
Rhizobia are a group of bacteria that partner with legumes such as peas, beans, clover, and soybeans.7PubMed Central. Rhizobium Nod Factor Perception and Signalling When soil is low in nitrogen, legume roots form small swellings where the bacteria live inside plant cells and turn air nitrogen into ammonia for the plant.8PubMed Central. Rhizobium Nod Factor Perception and Signalling9PubMed Central. Rhizobium Nod Factor Perception and Signalling
The pairing is picky. Each legume works with particular rhizobia, so bacteria that suit soybeans won’t nodulate clover. Plant and bacteria swap chemical signals before a nodule forms, and soil that has never grown a given crop may lack its matching rhizobia. That’s why seed packets often sell crop-specific inoculants.
Legumes aren’t the only nodule-formers. More than 200 flowering plants in 25 groups, such as alders and sea buckthorn, form root nodules with soil bacteria called Frankia.10PubMed. Actinorhizal symbioses and their N(2) fixation Some trees and shrubs can therefore enrich poor soil on their own.
A third group lives on or inside roots without forming nodules. Azospirillum is the best-studied of the bacteria that live closely around grass roots and help plants grow.11PubMed. Azospirillum, a free-living nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects Others include Herbaspirillum, Azoarcus, and a bacterium found in sugarcane now called Gluconacetobacter.12PubMed. Azospirillum, a free-living nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects Some tropical grasses, especially sugarcane, flooded rice, and kallar grass, can get at least part of their nitrogen this way.13Field Crops Research. Nitrogen fixation in endophytic and associative symbiosis These loose partnerships usually supply far less than legume nodules, so the crops still need other nitrogen.
Some bacteria fix nitrogen on their own in the soil, with Azotobacter the best-known example.14PubMed Central. Characterization of free nitrogen fixing bacteria of the genus Azotobacter in organic vegetable-grown Colombian soils In farm soils Azotobacter also makes plant growth hormones and breaks down some toxic compounds.15PubMed Central. Characterization of free nitrogen fixing bacteria of the genus Azotobacter in organic vegetable-grown Colombian soils16Wikipedia. Nitrogen fixation
Nodule partnerships deliver the most nitrogen because the plant feeds the bacteria directly. Free-living bacteria must find their own fuel in soil organic matter, so they add small, steady amounts rather than a crop’s worth.
Cyanobacteria, once called blue-green algae, are another group. They fix nitrogen using sunlight for energy, matter in flooded rice paddies, and form crusts on dry soils. Some partner with the water fern Azolla, long used as a rice fertilizer.
Not every lump on a root is a nodule. Real rhizobia nodules sit on the outside of the root and can be rubbed off, leaving a small scar, and they are usually rounded and scattered or clustered along the roots. Swellings caused by root-knot nematodes are part of the root tissue and can’t be detached; they distort the root into a bumpy thickening along its length. Cutting one open sorts them out: a working nodule is pink or red inside, while a gall is plain and pale.
What Nitrogen-Fixing Bacteria Do for Soil
Planting nitrogen-fixing species raises the carbon stored in soil by about 16% compared with other plants.17PubMed. Meta-analysis shows that planting nitrogen-fixing species increases soil organic carbon stock Extra nitrogen lets soil microbes grow, and their dead remains bind to mineral particles as long-lasting organic matter.18Geoderma. Planting of nitrogen-fixing shrubs promote soil carbon sequestration by increasing mineral-associated organic fraction The gain builds the longer the plants stand and reaches into deeper soil.19Geoderma. Planting of nitrogen-fixing shrubs promote soil carbon sequestration by increasing mineral-associated organic fraction Carbon and nitrogen climb together, with about 7.8 grams of carbon gained for each gram of extra soil nitrogen.20PubMed. Meta-analysis shows that planting nitrogen-fixing species increases soil organic carbon stock
That carbon change is slow, so a single season of clover won’t noticeably change a garden bed. The nitrogen benefit to the next crop shows up much sooner, often within the following season. That benefit depends on how the nitrogen gets back to the soil. Most fixed nitrogen stays in the legume’s own leaves, stems, and seeds, and returns when roots and residues rot after harvest or mowing. Cover crops turned in before seeding pass on the most. A grain legume can take out nearly as much as it fixed, because harvesting the beans removes much of it. The gain is real mainly when green growth goes back into the ground.
Organic matter also feeds the free-living fixers. Returning crop straw to soil for years raised the number of nitrogen-fixing bacteria about 3 to 18 times.21Plant and Soil. Long-term straw return increases biological nitrogen fixation by increasing soil organic carbon and decreasing available nitrogen in rice–rape rotation Straw adds carbon fuel while lowering readily available nitrogen, and both favor fixers.22Plant and Soil. Long-term straw return increases biological nitrogen fixation by increasing soil organic carbon and decreasing available nitrogen in rice–rape rotation The extra nitrogen fixed this way was small, though, under about half a kilogram per hectare a year.23Plant and Soil. Long-term straw return increases biological nitrogen fixation by increasing soil organic carbon and decreasing available nitrogen in rice–rape rotation
What Slows Nitrogen Fixation Down
Nitrogen fertilizer cut fixation by about 43% in nodule partnerships and 53% in free-living bacteria.24Applied Soil Ecology. Fertilization effects on symbiotic and free-living biological nitrogen fixations: Similar effects but different mechanisms Fertilized legumes grow fewer and smaller nodules, while free-living bacteria simply use the easy soil nitrogen instead.25Applied Soil Ecology. Fertilization effects on symbiotic and free-living biological nitrogen fixations: Similar effects but different mechanisms Phosphorus, potassium, and organic fertilizers, by contrast, tended to boost fixation.26Applied Soil Ecology. Fertilization effects on symbiotic and free-living biological nitrogen fixations: Similar effects but different mechanisms A small starter dose is sometimes used anyway, since young seedlings have no nodules for the first couple of weeks. Larger doses delay nodulation and cut later fixation.
You can often see when fixation has been switched down. A legume growing in nitrogen-rich soil, such as a bed heavily fed with manure, often shows few or small nodules when dug up. If the leaves stay dark green anyway, the plant is taking nitrogen from the soil instead of the bacteria. Pale yellowing leaves together with few nodules point to a different problem, such as missing rhizobia or stress, since the plant is hungry and not being supplied.
Stress matters too. Salt, drought, acid or alkaline soil, nutrient shortages, heavy metals, and pesticides all suppress most rhizobia, though some strains tolerate these stresses.27PubMed. Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate Drought is one of the biggest threats, starving nodules of sugar and oxygen and letting fixed nitrogen back up.28Indian Journal of Experimental Biology. Effects of drought stress on legume symbiotic nitrogen fixation: Physiological mechanisms
A few habits keep fixers working. Water legumes evenly during flowering and pod fill, when demand peaks. Correct very acid soil with lime, since rhizobia struggle there. Feed phosphorus and potassium if a soil test shows they’re low, and use little or no nitrogen fertilizer on established legumes.
How to Tell if Nitrogen-Fixing Bacteria Are Working
The simplest check is to dig up a legume carefully and look for nodules on the roots. Slice a few open: pink or red inside means they’re actively fixing nitrogen. This works for legumes and Frankia plants but not for free-living or root-surface bacteria, which leave no visible sign.
Check legumes around early flowering, when nodules are largest, and dig rather than pull, since nodules snap off easily. No nodules after several weeks suggests missing rhizobia or excess nitrogen.
Read the result honestly. A pink center confirms fixation at the time of digging, but not how much nitrogen is supplied over the season. A plant with only a few small pink nodules can still be short of nitrogen, so size and number matter alongside color. White nodules may just be young and not yet working, so one early dig can mislead. Check several plants from different spots in the bed rather than a single one.
In the lab, a quick, cheap test exposes roots or bacteria to acetylene gas and measures how much the enzyme turns into ethylene.29PubMed. Acetylene Reduction Assay: A Measure of Nitrogenase Activity in Plants and Bacteria It shows whether fixation is happening right now, in nodules or free-living bacteria.30PubMed. Acetylene Reduction Assay: A Measure of Nitrogenase Activity in Plants and Bacteria
To measure how much of a plant’s nitrogen came from the air, scientists use tagged forms of nitrogen that can be told apart in the plant.31African Journal of Biotechnology. Appraisal of the 15N-isotope dilution and 15N natural abundance methods for quantifying nitrogen fixation by flood-tolerant green manure legumes Two such methods can disagree by about 20% on the same crop,32African Journal of Biotechnology. Appraisal of the 15N-isotope dilution and 15N natural abundance methods for quantifying nitrogen fixation by flood-tolerant green manure legumes so estimates of fixed nitrogen are approximate, even from good labs. This approach showed some maize varieties got meaningful nitrogen from Azospirillum bacteria.33Biology and Fertility of Soils. Biological nitrogen fixation in Azospirillum strain-maize genotype associations as evaluated by the 15N isotope dilution technique
Labs can also find fixers by reading soil DNA for a gene that fixers carry for part of their enzyme.34PubMed Central. Ecological Patterns of nifH Genes in Four Terrestrial Climatic Zones Explored with Targeted Metagenomics Using FrameBot, a New Informatics Tool That shows which fixers are present, not how much they’re fixing. Methods vary between labs and can miss some groups, so results aren’t always comparable.35PubMed Central. Global soil metagenomics reveals distribution and predominance of Deltaproteobacteria in nitrogen-fixing microbiome Newer sensors can detect a single specific rhizobia strain down to near one cell.36ACS Omega. Sensitive Detection of Nitrogen-Fixing Soil Bacteria by Impedimetric Immunosensing
Do Inoculants and Biofertilizers Work
An inoculant is a product of live bacteria added to seed or soil. Results with growth-promoting bacteria are often inconsistent and depend on local soil and plant conditions.37PubMed Central. Plant growth-promoting rhizobacterial biofertilizers for crop production: The past, present, and future In one field trial, a mixed biofertilizer of bacteria and root fungi raised cowpea yield and soil phosphorus and potassium.38Scientia Horticulturae. Effects of biofertilizer with diazotrophic bacteria and mycorrhizal fungi in soil attribute, cowpea nodulation yield and nutrient uptake in field conditions Inoculants are worth trying, but not a guaranteed fertilizer replacement.
For legumes, buy the strain labeled for your crop, such as pea and vetch or soybean. Check the expiry date, since the bacteria are alive and die over time. Keep the product cool and out of sun, and coat seed just before planting.
It pays off most on new ground that has never grown the legume, where there are no matching rhizobia and adding them can decide whether nodules form at all. Beds where the same legume grew well in recent years probably hold enough bacteria already. Very acid or waterlogged soil can kill the added bacteria, so fix those problems first. To check the value, dig a few plants after a few weeks and look for nodules.
Products for corn or grass rely on looser root bacteria. Gains tend to be modest and vary year to year. Compare a treated and untreated strip to judge results yourself.
Downsides and What’s Coming Next
Growing peas and lentils with fixation did not itself raise nitrous oxide, a potent greenhouse gas.39Soil Biology and Biochemistry. Nitrous oxide emissions associated with nitrogen fixation by grain legumes Nitrogen-fixing trees, however, more than double soil nitrous oxide compared with other trees.40PubMed. Nitrogen-fixing trees increase soil nitrous oxide emissions: a meta-analysis Planting them widely in reforestation could offset up to about 4.4% of its climate benefit.41PubMed. Nitrogen-fixing trees increase soil nitrous oxide emissions: a meta-analysis Rich legume residues like jack bean can also cause higher nitrous oxide as they rot.42Geoderma Regional. Biological nitrogen fixation and soil N2O emissions from legume residues in an Acrisol in SE Brazil Rotting residue releases nitrogen faster than plants can take it up, and soil microbes turn the surplus into nitrous oxide, especially in wet soil. Working residue in close to planting lets the next crop use it.
Researchers are also trying to build fixation into crops. They have modified soil bacteria to keep fixing nitrogen even in fertilized soil, aiming to replace some fertilizer in maize.43PubMed Central. Genetic remodeling of soil diazotrophs enables partial replacement of synthetic nitrogen fertilizer with biological nitrogen fixation in maize One early, not yet confirmed study put nitrogen-fixing genes into a lab plant and saw moderate fixation.44PubMed Central. Biological nitrogen fixation in cereal crops: Progress, strategies, and perspectives Cereals that fertilize themselves remain a long-term research goal, not something you can plant now. It’s hard because the enzyme needs many genes working together and must be shielded from oxygen inside plant cells. Plants would also spend sugars on it, possibly lowering yield.