Gifted Species - Biological Nitrogen-Fixation
Biological nitrogen fixation remains mainly confined to a few distinct nutritional types of prokaryotes, some of which are free-living while others live in symbiotic association with eukaryotic partners. This process contributes annually about 6O% to the earth's newly fixed nitrogen budget. Table shows the ranges and types of biological nitrogen fixers. They have been variously classified as free living or symbiotic (Rhizobium-legume association). Free-living can further be classified into phototrophic (Nostoc), chemotrophic (Klebsiella), aerobic (Azotobacter) or anaerobic (Chromatium) depending upon the state of existence and mode of nutrition. Agronomically the most important nitrogen fixing systems are Rhizobium-legume association, Frankia (actinorhiza1)-woody plant association, Anabaena-Azolle and Cyanobacteria-rice system. Rhizobium-legume association as you will know invariably occurs in the form of root nodules.
Table: Examples of Gifted Species