COLONIZATION OF RHIZOSPHERES BY BRADYRHIZOBIUM SP IN RELATION TO STRAIN PERSISTENCE AND NODULATION OF SOME PASTURE LEGUMES

High soil mineral N and broadcast sowing four perennial pasture legumes inoculated with a mixture of three strains of Bradyrhizobium led to the failure of the inoculant to establish in host plant rhizospheres. This contrasted with much better survival of the same three strains within the rhizospheres of four annual and two perennial legumes when sown in drill rows into an adjacent area of the same soil that had low nitrate concentrations. Use of adhesive with the inoculum enhanced its establishment within the rhizosphere and increased the nodule representation for most legume-strain combinations compared to inoculation without adhesive. When adhesive was used and when seed was drill-sown into the soil low in nitrate, changes in rhizosphere populations of inoculum strains generally exhibited three distinct phases. The first phase occurred between sowing and seedling emergence, and typically involved a decline in the size of the inoculum population from that on the seed to that in rhizosphere at emergence. This was followed by a period of rapid colonization of the rhizosphere to a common population density. Finally, the third phase was usually, but not always, characterized by a further increase in numbers with the onset of nodule decay. For these freely nodulating pasture legumes, neither the seed inoculum level nor establishment within the rhizosphere was reflected in competitive ability for nodule formation, and saprophytic competence was not a function of nodule representation. Strain CB756str generally performed poorly compared to two other strains. Counts of inoculum strains in the rhizosphere of freely nodulating legumes had no predictive capacity for the nodulation success of a strain.