Biological Nitrogen Fixation
Biological Nitrogen Fixation (BNF) 
Soil microorganisms have developed specific systems that make them capable of adapting and surviving to diverse conditions of nutrient availability or starvation, as well as getting through different stress factors to which they are exposed in the environments in which they live. Among these changes there are physiological and morphological adaptations, and also specialised systems that allow the utilization of available nutrients and keep an optimal metabolic state. In particular, as an adaptation to the nitrogen starvation, a group of bacterial species named diazotrophs, has developed the ability to fix atmospheric N2 by reduction to assimilable forms (like ammonia) for plants and other bacteria that constitute the N biogeochemical cycle. Due to this remarkable feature, these microorganisms play an essential role in recycling and manteinance of the N cycle. Furthermore, BNF is an important factor in soil conservation and agronomical and livestock productivity .
Three groups of bacteria can be distinguished among diazotrophs depending on the way they carry out the biological fixation of N2. The first group can fix N2 in free-living state whereas the second one includes bacteria that need to establish an association with a host plant, but without any vegetal structure modification. The third group is formed by bacteria that carry out the fixation under symbiotic state. The bacterial genus Rhizobia (from the Greek “riza”= root , “bios”= life) is found in this last group and its members have the special characteristic of associating closely to their host plants roots (legumes as soybean, pea and others). There, special structures, known as nodules, are formed, and the bacteria inside these nodules suffer a differentiation process that leads them to fix atmospheric N2 .
- Peoples M. B. and D. F. Herridge. 1990. Nitrogen fixation by legumes in tropical and sub-tropical agriculture. Adv. Agron. 44:155-224.
- Peoples M. B. and E. T. Craswell. 1992. Biological nitrogen fixation: Investments, expectations and actual contributions to agriculture. Plant Soil 141:13-39.
- Peoples M. B., D. F. Herridge and J. K. Ladha. 1995. Biological nitrogen fixation: an efficient source of nitrogen for sustainable agricultural production. Plant and Soil. 174: 3–28.
- Spaink H. P., A. Kondorosi, and P. J. J. Hooykaas. (Eds.). 1998, The Rhizobiaceae, Kluwer Academic Publishers, Dordrecht, The Netherlands.
- Hablieb C. M. and P. W. Luden. 2000. Regulation of biological nitrogen fixation. J. Nutr. 130: 1081-1084.
- Leigh, G.J. 2002. Nitrogen fixation at the millennium. London: Elsevier Science
- Sprent, J. I. 2002, Nodulation in legumes, Royal Botanic Gardens, Kew, RU.
- Gage, D. J. 2004. Infection and invasion of roots by symbiotic, nitrogen-fixing rhizobia during nodulation of temperate legumes. Microbiol Mol Biol Rev. 68: 280-300.