Categories of Gifted Species
This group of nitrogen fixers has been divided into three categories:
For example the Rhizobium infecting clovers and peas are called trifolii and leguminosarum respectively. The genus Bradyrhizobium is not so rigid with regard to the host partners. All the strains of Rhizobium and Bradyrhizobium produce nodules on the host, but Azorhizobium produces nodules on the stem of Sesbania rostrata and Aeschynomene afraspera.
Parasponia is the only non-legume genus where the bacterial partner is Rhizobium. This observation has raised the hope of creating nitrogen fixing associations between Rhizobium and important crops like cereals. Nitrogen fixing nodules on the roots of woody plants like Alnus, Casuarino, Myria are formed by symbiotic association with Frankia a member of actinomycetes. Anabaena-Azolla association is experimentally shown to be a strong source of nitrogen fertiliser in wetland rice agriculture. Free-living heterocysts cyanobacteria are also known to contribute significant amount of nitrogen to the wetland rice ecosystem. Lichens are associations between fungi and N2-fixing cyanobacteria. They are very important source of N2 in barren harsh also known to occur in tropical plants like Psychotria partner.