Evaluation of Indigenous Bradyrhizobium of the Soybean Rhizosphere Proliferation in Delmarva Soils of Maryland, USA.

An important requirement for agronomically useful Rhizobium-soybean interaction is the ability of inoculant bacterial strains to compete with diverse native rhizobial strains.  Therefore, the characteristics of the rhizobial field populations should routinely be assessed for the development of strategies for effective nodulation of soybeans by select inoculants.  Rhizosphere soils and soybean nodules from eight randomly select Delmarva soybean fields were screened for indigenous bradyrhizobia over three growing seasons. Ten isolates were identified using 16S rRNA and characterized by FAME analysis. Drought and salt tolerance of the isolates were also tested due to the elevated temperatures of the Delmarva region during the years of evaluation. The 16S rRNA gene sequence separated the Bradyrhizobium isolates from the nodules into two species, Bradyrhizobium japonicum and B. elkanii. The soil isolates were distinct from the other isolates and matched with Bradyrhizobium sp. and related closely to reference strain, B. japonicum USDA TA11Nod+. This study showed that the appearance of the soybean and the genotypic characteristics of microsymbionts were good indicators of the activity in the soybean rhizosphere of the Delmarva soils.