Invader-induced soil abiotic and biotic conditions have little impact on the competitive interactions between a native and an invasive legume

Abstract

• No competition occurred between native Psoralea pinnata and invasive A. saligna . • Each legume species associated with different rhizobia and used divergent nitrogen sources. • Australian rhizobia were naturally present in pristine Fynbos soils. • Acacia invasion and spread may not be limited by rhizobium availability in Fynbos soils. Invasive Australian acacias cause substantial biodiversity impacts in South Africa's Core Cape Subregion (CCR) via positive plant-soil feedbacks, driven by high growth rates and leaf litter input by these legumes and their associations with nitrogen-fixing rhizobia. However, it is unclear whether acacia-induced soil abiotic conditions, or the association between acacias and different types of rhizobia, or both, are driving the invasive performance of acacias. Here we aim to determine how familiar (i.e., Australian) vs. novel (i.e., South African) rhizobium associations and acacia leaf litter affect competition between the invasive Acacia saligna (Labill.) H.L.Wendl. and native Psoralea pinnata L.. We grew these two legumes alone and together in pots containing Psoralea -conditioned soils. Seedlings grown alone were subjected to Australian bradyrhizobia inoculum treatments, while seedlings grown in mixture received a combination of inoculum and topsoil conditioned by acacia leaf litter in a fully factorial design. We found that neither the addition of Australian rhizobia, nor acacia-conditioned topsoil, enhanced the performance of A. saligna . Surprisingly, these treatments also did not hinder P. pinnata ’s performance relative to that of A. saligna . Next generation sequencing (NGS) data revealed that each legume species associated with distinct (and familiar) rhizobia, regardless of inoculum treatment. Acacia saligna primarily met its nitrogen requirements by utilising soil nitrogen, while P. pinnata obtained much of its nitrogen via biological fixation. This apparent differentiation in nitrogen acquisition, coupled with the unique rhizobial associations of the two species, may explain the absence of strong asymmetric competition between them. This may also explain why P. pinnata is able to persist under Acacia invasion in the field and suggests that this native legume is a good candidate for restoring acacia-invaded habitats.