The relationship between ecological variation and microbial genetic composition is critical to understanding microbial influence on community and ecosystem function. In glasshouse trials using nine native legume species and forty rhizobial strains, we find that bacterial rRNA phylotype accounts for 68% of inter-isolate variability in symbiotic effectiveness and 79% of host-specificity in growth response. We also find that rhizobial phylotype diversity and composition of soils collected from a geographic breadth of sites explains the growth responses of two acacia species. Positive soil microbial feedback between the two acacia hosts was largely driven by changes in diversity of rhizobia. Greater rhizobial diversity accumulated in association with the less responsive host species, Acacia salicina, and negatively affected the growth of the more responsive A. stenophylla. Together this work demonstrates correspondence of phylotype with microbial function, and demonstrates that the dynamics of rhizobia on host-species can feed back on plant population performance.
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