Genetic diversity and symbiotic efficiency of black wattle-nodulating rhizobia in soils of Rio Grande do Sul State, Brazil

Black wattle is the third most cultivated forest species in Brazil. Besides its economic importance, black wattle is also used to restore degraded areas, where soils are generally acid and Al levels high. This study intended to investigate the genetic diversity of indigenous rhizobia in soils in Rio Grande do Sul and to select isolates that fix N-2 efficiently at low pH. Fifty Bradyrhizobium sp. isolates were obtained, which, along with the recommended strains BR 3067 and BR 3068, were molecularly characterized by PCR with primer BOX A 1-R. DNA band patterns of the isolates were used to construct a dendrogram by which the Shannon diversity index was calculated. Ten isolates were further tested for low pH tolerance and Al presence. Eight isolates were selected for a nitrogen-fixing greenhouse trial. High genetic diversity was observed among the isolates. Ten clusters were formed based on a 70 % similarity cut-off level and a diversity index of 4.3. The presence of Al did not affect the tested isolates, while growth decreased at pH 4.5. With respect to the symbiotic efficiency, T6-16 and V-7 were the most effective isolates, similar to the recommended strain BR 3068.