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

Abundância natural de ¹⁵ N do N derivado da atmosfera por diferentes estirpes de Bradyrhizobium em simbiose com plantas de soja

ABSTRACT:

To quantify the BNF contribution to legumes using the ¹⁵N natural abundance technique, it is important to know the abundance of ¹⁵N of the plants grown entirely dependent on BNF (value ‘B’). The aim of the study was to determine the ¹⁵N natural abundance of N₂ 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 δ¹⁵N (‰) different from zero, suggesting that the symbioses have little tendency to show significant isotopic fractionation during N₂ fixation, but there is considerable depletion in ¹⁵N of the N translocated to the shoot tissue.

Key words: ¹⁵N Isotopic fractionation; ‘B’ value; Biological nitrogen fixation