Natural abundance of 15N of N derived from the atmosphere by different strains of Bradyrhizobium in symbiosis with soybean plants

To quantify the BNF contribution to legumes using the N-15 natural abundance technique, it is important to know the abundance of N-15 of the plants grown entirely dependent on BNF (value 'B'). The aim of the study was to determine the N-15 natural abundance of N-2 fixed by different Bradyrhizobium strains in symbiosis with one soybean cultivar. Treatments consisted of soybean plants cultivated with and without inoculation with ten Bradyrhizobium strains, in five replicates planted in Leonard jars in a sand/vermiculite mixture. Plants were harvested after 46 days. The 'B' values of the aerial tissue ('B-s') ranged from -2.6 to -3.9 %. There was a tendency for the 'B-s' values of plants inoculated with strains of B. elkanii to be more negative than plants inoculated with other strains. All 'B' values of the whole plant were less than 1 unit of delta N-15 (%) different from zero, suggesting that the symbioses have little tendency to show significant isotopic fractionation during N-2 fixation, but there is considerable depletion in N-15 of the N translocated to the shoot tissue.