Geographic patterns of symbiont abundance and adaptation in native Australian Acacia-rhizobia interactions

1. The importance of plant-soil interactions in land reclamation, and the management and restoration of functioning native ecosystems, is becoming widely recognized. However, relatively little is known about broad-scale patterns of genetic variation and adaptation in wild plant-soil symbiotic interactions. 2. The current study is part of a larger project examining patterns of genetic variation and adaptation in host and symbiont populations across their geographical ranges using two widespread native Australian Acacia spp. (A. salicina, A. stenophylla) and associated populations of rhizobial bacteria. 3. A total of 58 sites were characterized with regard to symbiont population sizes, soil chemistry and environmental parameters. Rhizobial abundance was negatively correlated with a small number of soil factors, including nitrogen, and positively correlated with organic carbon and cation exchange capacity. 4. There were clear differences between host species in seedling growth responses in glasshouse trials using pots inoculated with native soils. While A. salicina grew equally well with soils from A. stenophylla and A. salicina sites, A. stenophylla grew best when inoculated with its own soils, indicating broad-scale adaptation to its own rhizobia. 5. Rhizobial abundance in these soils was also strongly correlated to variation in nodulation and host growth, indicating that abundance may be a good indicator of the relative effectiveness of rhizobial populations. 6. The clear differences in specificity and effectiveness of host-symbiont associations, even among related species, suggests that better knowledge of these systems at multiple spatial scales is central to understanding the factors that influence the ecology and evolution of plant and soil communities, and has potential to increase the cost-effectiveness of restoration programs.