Mutualisms can be disrupted when non-native plants are introduced into novel environments, potentially impacting their establishment success. Introduced species can reassemble mutualisms by forming novel associations with resident biota or by maintaining familiar associations when they are co-introduced with their mutualists. Invasive AustralianAcaciaspecies in South Africa have formed nitrogen-fixing rhizobium mutualisms using both pathways.Here we examined the contributions of novel vs familiar rhizobial associations to the performance ofAcacia salignaacross different soils within South Africa’s Core Cape Subregion (CCR), and the concomitant impacts of exotic rhizobia on the endemic legume,Psoralea pinnata. We grew each legume with and without AustralianBradyrhizobiumstrains across various CCR soil types in a glasshouse. We identified root nodule rhizobium communities associating with seedlings grown in each treatment combination using next-generation sequencing (NGS) techniques.Our results show that different CCR soils affected growth performances of seedlings for both species while the addition of Australian bradyrhizobia affected growth performances ofA. saligna, but notP. pinnata. NGS data revealed that each legume associated mostly with their familiar rhizobial partners, regardless of soil conditions or inoculum treatment.Acacia salignapredominantly associated with Australian bradyrhizobia, even when grown in soils without inoculum, whileP. pinnatalargely associated with native South AfricanMesorhizobiumstrains.Our study suggests that exotic Australian bradyrhizobia are already present and widespread in pristine CCR soils, and that mutualist limitation is not an impediment to further acacia invasion in the region. The ability ofP. pinnatato sanction AustralianBradyrhizobiumstrains suggests that this species may be a good candidate for restoration efforts following the removal of acacias in CCR habitats.
