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Bradyrhizobium japonicum

Also known as
B. japonicum

Bradyrhizobium japonicum is a soil bacterium that forms nitrogen-fixing nodules on soybean roots. It is the rhizobium most commonly used in soybean inoculants. Soybeans are not native to North America, so fields without a history of soybeans may lack enough of the bacterium.

What is Bradyrhizobium japonicum?

Bradyrhizobium japonicum is one of the rhizobia, bacteria that live in partnership with legumes. It infects soybean roots and triggers the formation of nodules, where it carries out biological nitrogen fixation, converting nitrogen gas from the air into forms the plant can use. The plant supplies the bacteria with carbon in return.

The species was originally named Rhizobium japonicum. It was moved to the new genus Bradyrhizobium in 1982; the name refers to its slow growth compared with other rhizobia.

Why soybean fields are inoculated

The partnership is specific: soybeans nodulate effectively with Bradyrhizobium strains adapted to them, not with the rhizobia of other legumes. Because soybeans originated in East Asia, North American soils did not naturally contain these bacteria, and populations built up only where soybeans were grown and inoculated. Inoculation is therefore strongly recommended in fields with no recent soybean history and is widely used elsewhere to ensure good nodulation.

Survival and performance

  • Populations can decline in fields out of soybeans for several years, in very acidic soils, or after flooding or drought.
  • Strains differ in how effectively they fix nitrogen and how well they compete with bacteria already in the soil.
  • Inoculant quality depends on delivering enough live cells, commonly stated as colony-forming units per seed.

Frequently asked questions

No. Other legumes, such as alfalfa, peas and clovers, require their own rhizobia.

Not always. In fields with a recent history of well-nodulated soybeans, yield responses to inoculation are often small and inconsistent. Many growers inoculate anyway as low-cost insurance. See double inoculation for more intensive approaches.

  • Rhizobia

    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.

  • Nodulation

    Nodulation is the formation of root nodules on a legume, such as soybean, by nitrogen-fixing rhizobia bacteria. The nodules house the bacteria and are where atmospheric nitrogen is converted into a form the plant can use.

  • Biological nitrogen fixation BNF

    Biological nitrogen fixation (BNF) is the conversion of nitrogen gas from the air into ammonia by certain bacteria and archaea, using the enzyme nitrogenase. In agriculture, the most important example is rhizobia fixing nitrogen inside the root nodules of legumes.

Sources

  1. Transfer of Rhizobium japonicum to Bradyrhizobium gen. nov., D. C. Jordan, International Journal of Systematic Bacteriology, 1982
  2. Soybean inoculation guidance, Land-grant university extension