Competitiveness of inoculant strains of Rhizobium leguminosarum bv. trifolii in red clover using repeated inoculation and increased inoculum levels

Abstract

The effects of inoculation of red clover with Rhizobium leguminosarum bv. trifolii strains 285 and 7612 were studied. In greenhouse experiments repeated inoculations were made. Strain 285 occupied all nodules when included, and strain 7612 increased its nodule occupancy with repeated inoculation. In field studies, where native, ineffective red clover bacteria were present, increased levels of inoculum were added, which increased dry matter production and nitrogen content of the plants. Yields from plants inoculated with strain 7612 were higher but dependent on inoculum concentration. The number of early nodules occupied by inoculant bacteria increased with increasing inoculum concentrations of strain 7612 but not of strain 285. The inoculant strain occupancy of late nodules was unaffected by inoculum levels in both cases. Acetylene reduction and accumulation of 14C-labelled metabolites in nodules situated at different distances from the shoots and in nodules of different ages, including overwintering nodules, were studied. Nodule placement did not influence the biological activity of the nodules. Old nodules were less active on a dry matter basis, but since they are larger, they may have contributed to the nitrogen balance of the plants as much as young nodules. Overwintering the plants did not change this pattern. Key words: competition, inoculation, nodule life cycle, Rhizobium.