Competition in Soil between Effective and Ineffective Strains of Rhizobium leguminosarum biovar trifolii in the Nodulation of Red Clover, Trifolium pratense L., Studied with ELISA

Abstract

SUMMARY: The ability of two strains of Rhizobium leguminosarum biovar trifolii (formerly R. trifolii) to compete for nodule sites in soil with indigenous R. l. biovar trifolii was studied using ELISA. In pot experiments with soil containing effective indigenous strains, one of the inoculated strains, 285, was able to form nodules on red clover (Trifolium pratense L.), not only on the tap root but also on later-formed roots. Under field conditions, also in a soil containing effective indigenous R. l. biovar trifolii strains, both inoculated strains were detectable in minor proportions in the nodules of the established red clover. The inoculated strains survived in the soil during the field season. As the indigenous strains were effective, no increase in dry matter production was obtained in this experiment. However, in pot experiments in a soil with ineffective indigenous red clover bacteria, the other inoculated strain, 7612, known to be an effective nitrogen-fixing strain, was less successful and was only detectable incidentally. Although the nodule occupancy of the successful competitor strain, 285, only ranged between 7% and 27% on the upper tap root and was merely occasional on the other parts of the root in the ‘ineffective’ soil, increases in dry matter production were obtained, indicating that the nodules on a specific plant have varying significance for the apparent nitrogen fixation.