Legumes are the most familiar nitrogen-fixing plants, but they are far from the only ones. Dozens of tree and shrub species outside the legume family also form root nodules with nitrogen-fixing bacteria, and looser partnerships between bacteria and non-nodulating plants contribute meaningful nitrogen in forests, grasslands, and agricultural fields. The full roster of nitrogen-fixing plants spans tropical tree legumes, temperate alders and bayberries, boreal mosses harboring cyanobacteria, and even sugarcane hosting bacteria inside its tissues. Understanding this range matters for farming, forestry, and ecological restoration, because each partnership works differently and thrives under different conditions.
How Nitrogen Fixation Actually Works in Plants
No plant fixes nitrogen on its own. The work is always done by microorganisms carrying an enzyme called nitrogenase, which converts atmospheric nitrogen gas into ammonia that living cells can use. Nitrogenase is the only enzyme in nature that performs this reaction, and it requires a large energy investment to do so.1PubMed Central. Biological nitrogen fixation in theory, practice, and reality: a perspective on the molybdenum nitrogenase system The enzyme also has a critical vulnerability: it is destroyed by oxygen. This creates an engineering problem for any plant that wants to host nitrogen-fixing bacteria inside its tissues, because the plant’s own metabolism needs oxygen for energy production.
Legumes solve this problem elegantly. Inside their root nodules, they produce a specialized protein called leghemoglobin, which works much like the hemoglobin in your blood. Leghemoglobin shuttles just enough oxygen to the bacterial cells for respiration while keeping the free oxygen concentration extremely low, protecting nitrogenase from damage.2PubMed. Symbiotic leghemoglobins are crucial for nitrogen fixation in legume root nodules but not for general plant growth and development When researchers knocked out leghemoglobin production in a model legume, nodule oxygen levels rose, the bacteria lost their nitrogenase protein entirely, and nitrogen fixation stopped. The plants grew normally when given fertilizer, proving that leghemoglobin’s role is specifically about protecting the fixation machinery rather than general plant health.3PubMed. Hemoglobins in the legume-Rhizobium symbiosis
Legumes and Their Bacterial Partners
The legume family (Fabaceae) contains roughly 20,000 species, and the majority of those tested can form nodules with nitrogen-fixing bacteria collectively known as rhizobia. The partnership begins with a chemical conversation. Plant roots release flavonoid compounds into the soil, which trigger nearby rhizobia to produce signaling molecules called Nod factors.4PubMed. Nod factor perception: an integrative view of molecular communication during legume symbiosis The plant’s root-hair cells recognize these Nod factors through specific receptors, and if the match is right, the root curls around the bacterium and builds an infection thread that guides it deep into the root tissue, where a nodule forms.5Glycobiology. Nod factor structures, responses, and perception during initiation of nodule development
The diversity of legume nitrogen fixers goes well beyond the familiar crop species like soybeans, peas, and clover. Many tropical and subtropical trees are legumes. In the Brazilian semi-arid caatinga, researchers identified multiple tree species as active nitrogen fixers, including several Mimosa species, Anadenanthera colubrina, and Piptadenia stipulacea, all confirmed through isotopic analysis that distinguished fixed nitrogen from soil nitrogen.6Journal of Arid Environments. Biological nitrogen fixation in tree legumes of the Brazilian semi-arid caatinga Other leguminous trees include acacias, mesquite, and Inga species used as shade trees in coffee plantations across Latin America.7Soil Biology and Biochemistry. An assessment of nodulation and nitrogen fixation in inoculated Inga oerstediana, a nitrogen-fixing tree shading organically grown coffee in Chiapas, Mexico
Not every legume fixes nitrogen under all conditions, though. The amount of nitrogen a legume actually fixes depends on soil fertility, the availability of compatible rhizobia, and the plant’s age. Young Inga shade trees in Mexican coffee plantations, for example, derived only about 20% of their nitrogen from fixation during their first few years, suggesting that supplemental nitrogen fertilizer is needed while the trees establish their symbiosis.7Soil Biology and Biochemistry. An assessment of nodulation and nitrogen fixation in inoculated Inga oerstediana, a nitrogen-fixing tree shading organically grown coffee in Chiapas, Mexico
Actinorhizal Plants, the Other Nodulators
A separate group of nitrogen-fixing plants, far less famous than legumes, forms root nodules with a completely different bacterium: Frankia, a filamentous soil organism in the actinobacteria group. These plants are called actinorhizal plants, and they include a surprisingly wide variety of woody shrubs and trees.8FEMS Microbiology Letters. Aspects of nitrogen-fixing Actinobacteria, in particular free-living and symbiotic Frankia Alders (Alnus species) are the best known, but the group also includes bayberry (Myrica), sweet fern (Comptonia), sea buckthorn (Hippophae), she-oaks (Casuarina), and ceanothus (Ceanothus), among others. In total, actinorhizal plants span eight different plant families and include over 200 species.
Alders are typical actinorhizal nitrogen-fixing trees. They thrive along streams, on landslide scars, and on recently exposed soils where nitrogen is scarce.9PubMed Central. Comparative analysis of nitrogen content and its influence on actinorhizal nodule and rhizospheric microorganism diversity in three Alnus species Casuarina trees, widely planted as windbreaks and for coastal stabilization in tropical regions, are another prominent example. These species perform the same fundamental trick as legumes, housing bacteria inside root nodules and feeding them carbon in exchange for ammonia, but using a different set of signaling pathways and a different microbial partner.
Despite their ecological importance, actinorhizal plants have received far less research attention than legumes. A recent review noted that while legume trees are frequently used for restoring depleted and contaminated soils, the potential of actinorhizal plants for the same purpose remains largely unexplored.10Trends in Plant Science. What Plants Fix Nitrogen? From Legumes to Trees This is a missed opportunity, given that actinorhizal species include cold-hardy trees and shrubs that can colonize environments where tropical legume trees would not survive.
Parasponia, the Rule-Breaking Outlier
Virtually every plant that nodulates with rhizobium is a legume, with one remarkable exception. Parasponia, a small group of tropical trees in the hemp family (Cannabaceae), can form nitrogen-fixing nodules with rhizobium bacteria despite having no close evolutionary relationship to legumes.11PubMed Central. Comparative genomics of the nonlegume Parasponia reveals insights into evolution of nitrogen-fixing rhizobium symbioses They are the only known non-legumes to pull off this particular trick. Parasponia trees are fast-growing pioneers in disturbed tropical landscapes, particularly in Southeast Asia, where they colonize volcanic soils and riverbanks.
The rhizobium that partners with Parasponia carries nodulation genes closely related to those found in typical legume-associated rhizobia, which was one of the earliest clues that the bacterial signaling machinery is largely shared across these partnerships.12Nucleic Acids Research. Conserved nodulation genes from the non-legume symbiont Bradyrhizobium sp. (Parasponia) From the plant side, genomic studies of Parasponia have revealed that some of the genes required for nodulation overlap with genes legumes use, suggesting the two groups may have drawn on the same ancestral genetic toolkit. This finding has made Parasponia a focus of research aimed at understanding what it takes for a non-legume to evolve nodulation capacity.
Why Only Some Plants Evolved Nodulation
All known nodulating plants, both legume and actinorhizal, belong to a single large clade of flowering plants sometimes called the nitrogen-fixing clade. This clade also includes many plant families that do not fix nitrogen at all, such as roses, elms, and beeches. The restriction of nodulation to one part of the plant family tree has puzzled biologists for decades.13PubMed. Phylogenomics reveals multiple losses of nitrogen-fixing root nodule symbiosis
Recent evolutionary modeling suggests that the common ancestor of this clade acquired some kind of genetic predisposition, a set of changes that made nodulation possible but did not guarantee it. From that precursor state, individual lineages appear to have gained (and frequently lost) the ability to form root nodules through additional evolutionary steps.14Nature Communications. Shifts in evolutionary lability underlie independent gains and losses of root-nodule symbiosis in a single clade of plants Losses actually outnumber gains considerably, which is why many lineages within the clade no longer nodulate. This pattern makes sense when you consider the cost of nitrogen fixation: maintaining the partnership is metabolically expensive, so once soil nitrogen becomes abundant enough, plants that abandon nodulation and redirect those resources toward growth can outcompete their fixing neighbors.
The Metabolic Cost of Fixing Your Own Nitrogen
Nitrogen fixation is not free. The plant pays its bacterial partner in carbon compounds derived from photosynthesis, and the energy cost is substantial. A metabolic model of soybean estimated the carbon cost at roughly 4 grams of carbon per gram of nitrogen fixed, which translated to a predicted grain yield reduction of about 27% compared to a hypothetical non-nodulating soybean receiving the same nitrogen from the soil.15PubMed. A genome-scale metabolic reconstruction of soybean and Bradyrhizobium diazoefficiens reveals the cost-benefit of nitrogen fixation That is a steep penalty, which helps explain why fixation pays off mostly in nitrogen-poor environments.
Plants are pragmatic about this trade-off. When soil nitrogen is plentiful, many legumes reduce nodulation or shut it down entirely. Experimental work has confirmed that the carbon cost savings from bacterial nitrogen fixation are apparent only when soil nitrogen is low; at higher nitrogen levels, the benefit of the symbiosis essentially disappears.16AoB PLANTS. Symbiotic nitrogen fixation reduces belowground biomass carbon costs of nitrogen acquisition under low, but not high, nitrogen availability This is why heavily fertilized farm fields are poor environments for observing active nitrogen fixation, and why fixation matters most in degraded, pioneer, or low-input systems.
Nitrogen Fixation Without Nodules
Nodules are the most efficient way plants access biologically fixed nitrogen, but they are not the only way. Several other forms of nitrogen fixation contribute to ecosystems without any root nodules involved.
Free-Living Soil Bacteria
Many soil bacteria, including species of Azotobacter, Azospirillum, and Clostridium, can fix nitrogen independently while living near plant roots. This free-living fixation in the rhizosphere is widespread across terrestrial environments.17PubMed Central. To Fix or Not To Fix: Controls on Free-Living Nitrogen Fixation in the Rhizosphere However, the rates are typically low compared to what nodulated plants achieve or what farmers apply as fertilizer. Free-living fixers generally do not depend heavily on nitrogen fixation to meet their own needs, treating it more as a supplement than a primary nitrogen source.18The ISME Journal. Complex N acquisition by soil diazotrophs: how the ability to release exoenzymes affects N fixation by terrestrial free-living diazotrophs The contribution matters most in nutrient-poor systems where even small amounts of new nitrogen can shift the competitive balance among plants.
Endophytic Bacteria Inside Plant Tissues
Some nitrogen-fixing bacteria live inside the stems, leaves, and roots of grasses and other non-leguminous crops without forming visible nodules. Sugarcane is a well-studied example. Researchers have isolated nitrogen-fixing bacteria from the leaves, stems, and roots of sugarcane plants grown in nitrogen-free sand, confirming that these endophytes carry active nitrogenase enzymes.19PubMed Central. Isolation and Characterisation of Endophytic Nitrogen Fixing Bacteria in Sugarcane Brazilian sugarcane has long been noted for producing high yields on low-nitrogen soils, and endophytic fixation is thought to contribute part of the explanation, though the exact share of nitrogen provided this way remains debated.
Moss-Cyanobacteria Partnerships
In boreal forests across Scandinavia, Canada, and Russia, feather mosses harbor cyanobacteria on their leaf surfaces. These cyanobacteria fix atmospheric nitrogen and can contribute up to half of the total nitrogen input in these nitrogen-limited forests.20PubMed Central. Moss-cyanobacteria associations as biogenic sources of nitrogen in boreal forest ecosystems The nitrogen eventually makes its way into the forest soil as the moss decomposes.21Applied Soil Ecology. Long-term fate of nitrogen fixation in Pleurozium schreberi Brid (Mit.) moss carpets in boreal forests This is not plant nitrogen fixation in the traditional sense — the moss is not a vascular plant and the cyanobacteria are doing the work — but the partnership is ecologically critical in some of the planet’s largest biomes. Interestingly, when atmospheric nitrogen deposition increases from pollution, fixation rates in these moss associations can decline, suggesting a feedback mechanism that partially offsets added nitrogen.22PubMed Central. The impact of simulated chronic nitrogen deposition on the biomass and N₂-fixation activity of two boreal feather moss-cyanobacteria associations
Nitrogen Fixers as Ecosystem Builders
Nitrogen-fixing plants play an outsized role in ecological succession, the process by which bare ground gradually accumulates soil, nutrients, and biological complexity. On recently exposed surfaces like mine spoils, volcanic deposits, and glacial moraines, nitrogen is often the scarcest nutrient, and fixing plants are among the first colonizers. Research on mine reclamation sites in Germany found that pioneer legumes drove higher soil nitrogen levels through their root nodules, and areas with denser legume colonization had significantly more soil nitrogen than unvegetated ground.23Restoration Ecology. Linkage between root systems of three pioneer plant species and soil nitrogen during early reclamation of a mine site in Lusatia, Germany
Both legumes and actinorhizal plants serve as drivers of primary succession, facilitating soil development and the establishment of neighboring non-fixing species.24Functional Ecology. Evidence for phosphorus cycling parity in nodulating and non‐nodulating N2‐fixing pioneer plant species in glacial primary succession On Mount St. Helens, for instance, alder thickets developed soils with measurably higher nutrient levels and greater plant cover compared to areas dominated by lupine patches alone, though the increased soil fertility did not automatically lead to invasion by later-successional species, possibly because of dry conditions or other limiting factors.25Plant Ecology. Nitrogen-fixers Alnus and Lupinus influence soil characteristics but not colonization by later successional species in primary succession on Mount St. Helens The takeaway is that nitrogen fixers build the nutrient foundation for future ecosystems, but other factors like water availability and seed dispersal determine how quickly that foundation gets used.
Agricultural Use of Nitrogen-Fixing Plants
Farmers have used nitrogen-fixing plants to maintain soil fertility for thousands of years, and the practice remains central to sustainable agriculture. Cover cropping with legumes like crimson clover, hairy vetch, and winter peas adds biologically fixed nitrogen to the soil between cash crop seasons. Green manure legumes like lupins, when incorporated into crop rotations, can contribute substantial nitrogen. In a long-term study of no-tillage rotations, lupins used as green manure added roughly 300 kilograms of nitrogen per hectare to the system, which was essential for maintaining a positive nitrogen balance.26Field Crops Research. Influence of no-tillage and frequency of a green manure legume in crop rotations for balancing N outputs and preserving soil organic C stocks
In agroforestry, nitrogen-fixing trees serve double duty: they shade crops while enriching the soil. Inga trees over coffee plantations are a classic example, though as noted earlier, the fixation rate in young trees can be modest. Actinorhizal species like Casuarina are planted in coastal areas of the tropics both for wind protection and for soil improvement, and alders play similar roles in temperate agroforestry systems. Choosing the right fixing species for a given site involves matching the plant to local soil pH, climate, and the presence of compatible bacteria, since fixation rates vary enormously with conditions.
Environmental Conditions That Limit Fixation
Nitrogen fixation does not happen at a constant rate. Soil pH has a major influence on which nitrogen-fixing bacteria thrive and how active they are. Some important rhizobial genera, like Bradyrhizobium, are most abundant in mildly acidic soils (pH 4.5 to 5.5), while others like Rhizobium prefer higher pH. Neutral soil (pH 6.5 to 7.5) tends to support the greatest overall microbial diversity. Alkaline soils above roughly pH 7.8 appear to be particularly inhospitable to nitrogen-fixing microorganisms, with significantly reduced abundance compared to other pH ranges.27iScience. Environmental tipping points for global soil nitrogen-fixing microorganisms
Beyond pH, phosphorus availability frequently limits fixation. Nitrogenase requires a lot of energy, and the cellular machinery that produces that energy depends on phosphorus. In many tropical soils where nitrogen fixation should theoretically be advantageous, phosphorus scarcity caps how much fixing actually occurs. Water stress, extreme temperatures, and soil salinity also reduce fixation rates, either by harming the bacteria directly or by impairing the plant’s ability to support its microbial partners. This is why a nitrogen-fixing species that performs well in one region can be disappointing when planted elsewhere without attention to soil chemistry.
Engineering Nitrogen Fixation Into Crops That Cannot Do It
The potential prize is enormous: if staple cereal crops like rice, wheat, and maize could fix their own nitrogen, global dependence on synthetic nitrogen fertilizer would drop dramatically, reducing both costs and the environmental damage from fertilizer runoff. Researchers have pursued this goal through several strategies.28PubMed Central. Biological nitrogen fixation in cereal crops: Progress, strategies, and perspectives
One approach involves transferring the nitrogen-fixation gene cluster directly into plant cells. In one notable experiment, a minimal set of nine nitrogen-fixation genes from a soil bacterium was inserted into the model plant Arabidopsis. The transgenic plants showed moderate nitrogenase activity and grew with higher biomass than controls under low-nitrogen conditions.29PubMed Central. Biological nitrogen fixation in cereal crops: Progress, strategies, and perspectives – Section: Third-generation N-self-fertilizing cereal crops: Engineering autonomous N fixation in cereal crops The result, if independently validated, would represent a major milestone, though moving from a tiny lab plant to a high-yielding field crop involves scaling problems that have not yet been solved.
A different strategy focuses on improving the microbial side: engineering the bacteria that already associate with cereal roots to fix more nitrogen. Researchers identified plant compounds that stimulate biofilm production in nitrogen-fixing bacteria, then used genetic engineering to increase nitrogen fixation in rice plants carrying those bacteria.30PubMed. Genetic engineering for enhanced biological nitrogen fixation in cereal crops Both paths face the same fundamental obstacle that nature solved with nodules: protecting nitrogenase from oxygen while maintaining plant growth. Without a structure equivalent to a legume nodule, the oxygen problem limits how much nitrogen an engineered cereal could realistically fix.
There is also the question of whether engineering nodulation itself into cereals is feasible. Since all nodulating plants share a common ancestor with a predisposition for the trait, the genetic foundation for nodulation may involve dozens or hundreds of coordinated changes rather than a simple switch. Researchers studying Parasponia, the lone non-legume nodulator, have found that some of the same genetic pathways used in legume nodulation were independently co-opted, which is encouraging but also underscores how deeply embedded the trait is in plant biology.11PubMed Central. Comparative genomics of the nonlegume Parasponia reveals insights into evolution of nitrogen-fixing rhizobium symbioses Engineering an entirely new organ on a cereal root would be a project measured in decades, not years.
Commonly Confused Plants and Misconceptions
A surprisingly common misconception is that all plants with a legume-like growth habit fix nitrogen. Honey locust trees (Gleditsia triacanthos), while technically in the legume family, are generally considered non-nodulating or extremely poor fixers under most field conditions. Kentucky coffeetree is another legume-family member whose nodulation status is uncertain or negligible. Being in the legume family raises the probability of nitrogen fixation, but it does not guarantee it.
Another frequent confusion involves grasses. People sometimes hear that sugarcane or certain tropical grasses “fix nitrogen” and assume they do so in the same way as legumes. They do not. Grasses lack nodules entirely. What they have are endophytic bacteria living loosely inside their tissues, contributing modest and variable amounts of nitrogen. The contribution is real but far smaller per plant than what a well-nodulated legume or alder achieves.
Finally, there is the persistent idea that planting any legume will automatically improve your soil. If the right rhizobium strain is not present in the soil, the legume will not nodulate effectively and will simply draw nitrogen from the soil like any other plant. This is why inoculants — commercial preparations of the correct rhizobium strains — are sold alongside legume seeds for agricultural use. Soil pH, phosphorus levels, and moisture also need to be in the right range for fixation to work well. Simply scattering clover seed and hoping for the best often disappoints.