Effect of Nitrate on Nodulation and Nitrogen Fixation of Soybean

1.1 Biological nitrogen fixation and nitrogen nutrition in soybean plants Biological nitrogen fixation is one of the most important processes for ecosystem to access available N for all living organisms. Although N2 consists 78% of atmosphere, but the triple bond between two N atoms is very stable, and only a few group of prokaryotes can fix N2 to ammonia by the enzyme nitrogenase. Annual rate of natural nitrogen fixation is estimated about 232 x 106 t, and the 97% depends on biological nitrogen fixation (Bloom, 2011). This exceeds the rate of chemical nitrogen fertilizer uses about 100 x 106 t฀in 2009. Soybean can use N2, though symbiosis with nitrogen fixing soil bacteria, rhizobia, to make root nodules for harboring them. Soybean (Glycine max [L.] Merr.) is a major grain legume crop for feeding humans and livestock. It serves as an important oil and protein source for large population residing in Asia and the American continents. The current global soybean production was 231 x 106 t in 2008 (FAOSTAT). It is a crop predominantly cultivated in U.S.A., Brazil, Argentina and China, which together contribute nearly 87 percent of the total world produce in 2008. Soybean has become the raw materials for diversity of agricultural and industrial uses. Soybean seeds contain a high proportion of protein, about 40% based on dry weight, therefore, they require a large amount of nitrogen to get a high yield. About 8 kg N is required for 100 kg of soybean seed production. Soybean can use atmospheric dinitrogen (N2) by nitrogen fixation of root nodules associated with soil bacteria, rhizobia. Soybean plants can absorb combined nitrogen such as nitrate for their nutrition either from soil mineralized N or fertilizer N. It is well known that heavy supply of nitrogen fertilizer often causes the inhibition of nodulation and nitrogen fixation. Therefore, only a little or no nitrogen fertilizer is