Studies on the Inoculation and Competitiveness of a Rhizobium leguminosarum Strain in Soils Containing Indigenous Rhizobia

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Abstract
The competitiveness of a Rhizobium leguminosarum strain was investigated at two separate locations in field inoculation studies on commercially grown peas. The soil at each location (sites I and II) contained an indigenous R. leguminosarum population of ca. 3 × 104 rhizobia per g of soil. At site I it was necessary to use an inoculum concentration as large as 4 × 107 CFU ml−1 (2 × 106 bacteria seed−1) to establish the inoculum strain in the majority of nodules (73%). However, at site II the inoculum strain formed only 33% of nodules when applied at this (107 CFU ml−1) level. Establishment could not be further improved by increasing the inoculum concentration even as high as 109 CFU ml−1 (9.6 × 107 bacteria seed−1). The inoculum strain could be detected at both sites 19 months after inoculation. Analysis by intrinsic antibiotic resistance patterns and plasmid DNA profiles indicated that a dominant strain(s) and plasmid pool existed among the indigenous population at site II. Competition experiments were carried out under laboratory conditions between a dominant indigenous isolate and the inoculum strain. Both strains were shown to be equally competitive.