Function: Nitrogen fixation

In cultivated areas leguminous plants (Fabaceae) are the most common nitrogen fixing organisms. They form a symbiosis with host-specific nitrogen fixing Rhizobium bacteria. The bacteria live in root nodules of the plants. The bacteria provide plants with nitrogen and they receive in return products of photosynthesis (energy).

In other habitats cyanobacteria in the soil, in the Baltic Sea or in symbiosis with fungi forming lichens are important in biological nitrogen fixation. Cyanobacteria obtain their energy by photosynthesis. In forests alders (Alnus) form a symbiosis with Frankia bacteria. Frankia live in root nodules of the alders.

There are some nitrogen fixing bacteria in the soil that live on the surface of roots without forming root nodules. Some soil bacteria also fix nitrogen by receiving their energy from decaying organic compounds.

Rajala J. (ed.) 2006. Luonnonmukainen maatalous. Helsingin yliopiston Maaseudun tutkimus- ja koulutuskeskus, julkaisu no 80. 494 p. [In Finnish]