Symbiotic Nitrogen Fixation in Soybean

Abstract

Nitrogen is the key component of vegetable protein for human and animal consumption. Although 78% of the atmosphere by volume is consisted of molecular nitrogen, this huge amount is not available for plants, animals or human. Only the bacteria that have nitrogenase enzyme can reduce atmospheric nitrogen and thus they called as a “nitrogen fixers”. Nitrogen fixers reduce molecular nitrogen to amino acids and protein through ammonia (Fritsche, 1990; Lindemann and Glower, 2003). Nitrogen fixation process realizes either by free-living, associative or symbiotic nitrogen fixers. In symbiotic relation microorganism infects the plant root through infection thread and lives in the nodule forming structure. Afterwards plant supply component of nitrogenase and organic compounds to microorganism whereas microorganism supply reduced nitrogen to plant. Associative microorganisms are not infecting to plant however they colonize in rhizosphere and use of root exudates to successful nitrogen fixation. Free living fixers are independent and they need neither infect to plant nor rhizosphere exudates for fixation. Although the fixation rate vary depends on the nitrogen fixer type, the most effective fixation occurs in symbiotic relation with legumes. Soybean itself represents 77% of the N fixed by the crop legumes by fixing 16.4 Tg N annually, fixation by soybean in the U.S., Brazil and Argentina is calculated at 5.7, 4.6 and 3.4 Tg, respectively (Herridge et al. 2008). Plant and microorganism are particular for each other, thus only certain microorganism can infect particular plant whereas the appropriate rhizobium of soybean is called as Bradyrhizobium japonicum. The shape and size of the nodules are also particular for legumes, the soybean nodules are round and in same cases big as pea. Effective nodules are large in size and reddish in the inside colour. Legumes have an important role for both human nutrition and animal feeding, however, soybeans are unique in legumes with contents of 40% protein and 21% oil as well as isoflavones. Thus, soybean is the most widely grown protein/oilseed crop in the world, with both North and South America producing large portions of the world’s supply of this remarkable crop. In case of legume introduce to soil for the first time, appropriate rhizobium strain has to be inoculated for successful nitrogen fixation. In many cases some rhizobium bacterium might be existed in the soils, nevertheless, due to the insufficient number and activity (Gok and Onac, 1995), inoculation should be repeated. No successful nodulation as well as nitrogen fixation should be expected without inoculation with appropriate and healthy rhizobium strain by convenient inoculation method. A number of methods available to used in