Summary For successful restoration of native vegetation on nitrogen-depauperate soils, an understanding of nitrogen-fixing relationships between plant host species and their bacterial symbionts is critical. Each of three geographically restricted Acacia species (A. fulva, A. nano-dealbata, and A. trachyphloia) and three widely distributed species (A. dealbata, A. implexa, and A. melanoxylon) were inoculated with 20 different rhizobial (Bradyrhizobium spp.) strains. The strains comprised two obtained from each of 10 different host species, including the six Acacia species listed above plus a further four species, A. cangaiensis, A. cincinnata, A. deanei, A. mearnsii. Neither restricted nor widely distributed species grew more effectively with their own strains than with strains isolated from other species. Thus, host species with restricted geographical ranges did not demonstrate greater specialization in their symbiotic associations with rhizobia than widespread species. Highly significant variation was observed between the strains obtained from each host species with respect to their ability to promote effective plant growth across all host species. In many cases, strains that were highly effective at promoting growth for one host species, were comparatively ineffective in combination with other host species. Strains thus exhibited host specificity in their ability to fix nitrogen. These findings indicate that choosing appropriate rhizobial strains for inoculation prior to revegetation is critical and should be made carefully for both restricted and widespread species.
Key words distribution, nitrogen fixation, revegetation, symbiotic interactions.