Effects of environmental variables on the competition between inoculum strains and naturalized populations of Rhizobium trifolii for nodulation of Trifolium subterraneum L. and on rhizobia persistence in the soil

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Abstract
Experiments were conducted at six field sites in south-eastern Australia with three cultivars of Trifolium subterraneum L, inoculated with diverse strains of Rhizobium trifolii. The various locations provided different edaphic and climatic conditions for plant growth, and naturalized populations of R. trifolii provided competition in forming nodules for the inoculum strains. Success of the inocula in forming nodules in the first year was used as an index of competitiveness, and success in the second year as an index of persistence. Inoculum strains formed a majority of nodules in the first year but fewer in the second. Inocula generally persisted better at those sites which had smaller populations of naturalized rhizobia. Competitiveness and persistence did not appear to be related characteristics, although some strains were superior to others especially in terms of persistence. Values for strain persistence tended to be similar at locations where the Mt Barker cultivar was grown, indicating an effect of host on the expression of this character. Significant effects of soil texture, soil pH, or growth temperature (individually) on the relative competitiveness and persistence of the inoculum strains could not be measured, although the cumulative effects of these (and other) environmental variables may have been substantial. The inoculum strains could be assembled into groups according to strong relationships of nodulating performance at each harvest. Within these distinct groups, the nodulation behaviour of the strains relative to each other remained approximately constant, irrespective of the absolute levels of competitiveness or persistence, the location of the experiment, or the year of the harvest.

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