Competitiveness of indigenous strains of Bradyrhizobium on Desmodium intortum CV Greenleaf in three soils of South East Queensland

Abstract

Six spontaneous antibiotic (streptomycin + rifampicin) resistant strains of B radyrhizobium were tested for rhizosphere and nodulating competitiveness on Desmodium intortum. The resistant strains were obtained from parental strains that were selected from indigenous soil populations nodulating D. intortum in south east Queensland. The parental strains represented five dominant groups of isolates obtained using intrinsic antiboditic resistance and serological typing techniques. The sixth parental strain CB627 was that recommended for D. intortum. Test strains were reintroduced into their soil of origin and into two other soils in numbers approximately equal to the populations in the original soils. The number of each test strain in the soil and rhizosphere and the proportion of nodules formed by these strains were assessed for two sowing occasions. All strains colonized the soil and rhizosphere. There was an overriding effect of soil type on numbers of bradyrhizobia in the soil and the rhizosphere and only a small interaction effect between soil and strain. There were markedly different responses in the proportion of nodules formed by the test strains. A strain from Beerwah soil (CB3098) failed to form nodules in Tumbulgum soil in the first sowing, but all strains formed nodules in Beerwah and Samford soils. Strain CB3097, from Tumbulgum soil, was the most competitive of all strains throughout the experiment and in all soil types tested. There was no relationship between number of bradyrhizobia in either the soil or in the rhizophere and the proportion of nodules formed by a strain. Strain CB3097 has potential as a new inoculum strain for D. intortum.