Spontaneous antibiotic-resistant mutants were selected from five indigenous isolates of Rhizobium which had been shown to dominate nodule populations of Desmodium intortum cv. Greenleaf at three field sites in eastern, subtropical Australia, and from CB627, the commercially recommended strain for Desmodium species in Australia. These mutants were compared with their respective parent strains for similarity of antigenic composition and symbiotic nitrogen fixation on D. intortum. No mutant had altered antigenic properties but some mutants from two of the six strains had reduced nitrogen fixing capacity. Mutants similar to the parent strain in the tested characteristics were chosen for ecological studies.
The antibiotic—resistant strains were used as inoculum in a glasshouse trial to investigate ecological and nodulating characteristics of the indigenous strains when introduced into their soil of origin and into other soil types. The strains were distributed throughout the soil by mixing prior to sowing, to avoid any positional advantage regarding root colonization that may have occurred with seed inoculation.
The factors strain, soil and time interacted to affect the numbers of inoculum strain in both soil and rhizosphere. Soil exerted the stronger influence on the growth rates of the rhizobia, which were lowest in Tumbulgum soil and highest in Beerwah soil.
There was very little difference in growth over the 52 day sampling period between the indigenous strains when reintroduced into their soil of origin and CB627 in the same soil type, in either soil or rhizosphere. The total number of rhizobia decreased slightly in all three soil types regardless of strain treatment. In the rhizosphere the total number of rhizobia increased in Beerwah and Samford soils but not Tumbulgum soil, indicating that soil factors may override the rhizosphere effect. The rhizosphere stimulated slightly growth of the inoculum strains in the rhizosphere of plants in Beerwah soil but appeared to reduce growth of certain inoculum strains in the rhizospheres in Tumbulgum soil and all inoculum strains in the rhizospheres in Samford soil. Competition and/or antagonism from biological factors may account for this observation.
The strain of Beerwah origin (a sandy soil) grew relatively poorly in Tumbulgum soil (a clay) and vice versa. However the indigenous isolates did not show superior colonizing characteristics in their soils of origin, suggesting that only a low degree of adaptation exists.
Strains varied in their nodulating ability over the different soil types. The pH differences between soils may have influenced the nodulation patterns. The proportions of nodules occupied by the introduced strains could not be consistently correlated with any of their growth characteristics. The indigenous isolates occupied higher proportions of nodules than CB627 in all soils, with minor exceptions. This evidence suggests that CB627 has poor 'nodulating competitiveness' and that the indigenous isolates may be better suited for use as inoculum by virtue of their apparently superior nodulating competitiveness. This experiment indicates that screening of indigenous isolates for dominant strains may provide useful inoculum strains. Continuation of the experiment will determine the ability of these indigenous isolates and CB627 to persist into a second season.