RESPONSES OF FIELD BEAN (Phaseolus vulgaris L.) TO LEVELS OF Rhizobium leguminosarum bv. phaseoli INOCULATION IN SOILS CONTAINING EFFECTIVE R. leguminosarum bv. phaseoli POPULATIONS

Abstract

Greenhouse studies were conducted to determine the effect of adding Rhizobium leguminosarum bv. phaseoli inocula to field beans (Phaseolus vulgaris L.) growing in soils already containing R. leguminosarum bv. phaseoli. Indigenous R. leguminosarum bv. phaseoli populations in the 12 soils used ranged from 1.1 × 101 to 4 × 105 rhizobia g−1 of soil. Antibiotic-resistant isolates of R. leguminosarum bv. phaseoli strain 3644 were used as inocula and inoculum levels ranged from 104 to 108 bacteria per seed. N-15 isotope dilution methods with barley as a nonfixing control plant were used to determine N2 fixation levels. Bean plants grown in soils containing greater than 8 × 10 R. leguminosarum bv. phaseoli did not show positive responses to added rhizobia, except in one soil where the inoculum formed a significant proportion of nodules on inoculated plants. Plants growing in soils with less than 8 × 103R. leguminosarum bv. phaseoli did show increased levels of plant N accumulation and dry matter production in response to rhizobium addition if the level of soil mineral N was less than 25 μg N g−1 soil. Nodule occupancy by the marked R. leguminosarum bv. phaseoli isolate increased only in soils containing 8 × 103R. leguminosarum bv. phaseoli or less. The resident population of rhizobia in many of the soils was determined to be effective in N2 fixation since the proportion of N derived from N2 fixation did not increase in response to inoculation. Increasing the number of R. leguminosarum bv. phaseoli added per seed from 104 to 108 did not generally increase the effectiveness of the added inocula. Responses of beans to R. leguminosarum bv. phaseoli inoculation can only be expected in soils with low levels of resident R. leguminosarum bv. phaseoli and mineral N.Key words: Field bean, nodule occupancy, N-15 dilution, competition, N2 fixation