Rhizobia
- Also known as
- rhizobium, rhizobial bacteria
Rhizobia are soil bacteria that form nodules on the roots of legumes, such as soybeans, peas and alfalfa, and inside those nodules convert nitrogen from the air into a form the plant can use. Each legume partners with specific rhizobial species.
What are rhizobia?
Rhizobia is the collective name for soil bacteria that can live in partnership with legume plants and fix nitrogen. They are not a single genus: the group includes Rhizobium, Bradyrhizobium, Sinorhizobium (also called Ensifer), Mesorhizobium and others. Rhizobia can live freely in soil, but they fix nitrogen in agriculturally meaningful amounts only inside the root nodules they form with a compatible host.
How the partnership works
Legume roots release signaling compounds, mainly flavonoids, into the soil. Compatible rhizobia respond by producing signals of their own, called Nod factors, which the root recognizes. The bacteria then enter the root, usually through a root hair, and the plant builds a nodule around them. Inside the nodule, the bacteria change into a nitrogen-fixing form and convert nitrogen gas from the air into ammonia through biological nitrogen fixation. The plant supplies the bacteria with sugars in exchange.
The relationship is a symbiosis: both partners benefit. For the plant, it can replace much of the nitrogen that would otherwise have to come from soil or fertilizer.
Host specificity
Rhizobia are selective. The signaling exchange works only between matching partners, so each legume forms effective nodules only with particular rhizobial species. Examples:
| Legume | Rhizobia |
|---|---|
| Soybean | Bradyrhizobium japonicum and related Bradyrhizobium species |
| Alfalfa | Sinorhizobium meliloti |
| Peas, lentils, faba beans | Rhizobium leguminosarum |
| Common bean | Rhizobium species, including R. tropici and R. etli |
This is why inoculant products are sold for a specific crop, and why a soybean inoculant does nothing for alfalfa.
Rhizobia inoculants
Where the right rhizobia are absent or scarce in a field, growers apply them as a microbial inoculant, usually on the seed. Inoculation matters most in fields that have not grown the legume recently, and in conditions that harm existing rhizobial populations, such as flooding, drought, very acidic soil or high temperatures. Even where rhizobia are present, growers often inoculate to introduce more effective strains. The number of live bacteria is given on the label as colony-forming units.
Why rhizobia matter
Rhizobia allow legumes to meet much of their nitrogen requirement without nitrogen fertilizer. In soybeans, reviews commonly put the share of the crop’s nitrogen supplied by fixation at about half or more, varying with soil nitrogen, yield level and conditions. Legumes grown in rotation can also leave residual nitrogen for the following crop.
Rhizobia at Locus
Locus sells soybean rhizobia inoculants in liquid, planter-box and multi-strain system formats.
Frequently asked questions
Not always. Fields with a recent history of well-nodulated soybeans usually carry a rhizobial population. Inoculation is most important in fields new to soybeans or without soybeans for several years, and after flooding, drought or other stresses. Many growers inoculate every year because the cost is low relative to the risk of poor nodulation.
Rhizobia are one group of nitrogen-fixing bacteria: the ones that form nodules with legumes. Other bacteria fix nitrogen while living freely in soil or in loose association with plant roots.
In agriculturally meaningful amounts, no. Rhizobia fix nitrogen efficiently only inside a nodule, where the plant supplies energy and controls oxygen levels.
Sources
- Chapter 23: Nitrogen fixation, in iGrow Soybeans, South Dakota State University Extension
- Revisiting biological nitrogen fixation dynamics in soybeans, Peer-reviewed review, PMC8529188
- Nitrogen management for soybeans, University of Delaware Cooperative Extension