Plant-soil feedbacks do not explain invasion success of Acacia species in introduced range populations in Australia

Legumes, especially acacias, are considered amongst the most successful invaders globally. However there is still very little known about the role of soil microbial communities in their invasion success in novel ranges. We examined the role of the soil microbial community in the invasion success of four Acacia species (A. cyclops, A. longifolia, A. melanoxylon and A. saligna) and a close relative Paraserianthes lophantha, introduced into novel regions within Australia using a "black-box" approach. Seed and soil material were collected from multiple populations within each species' native and introduced range within Australia and used in a plant-soil feedback experiment to assess the effect of the soil microbial community on plant growth and nodulation. We found no effect, either positive or negative, of soil origin on species' performance, however there was a significant interaction between species and seed origin. Seed origin had a significant effect on the biomass of two species, A. cyclops and A. saligna. A. cyclops plants from the native range performed better across all soils than plants from the introduced range. The opposite trend was observed for A. saligna, with plants from the introduced range performing better overall than plants from the native range. Seed or soil origin did not have a significant effect on the presence and number of nodules suggesting that rhizobia do not constrain the invasion success of these legumes. Our results suggest that plant-soil feedbacks are unlikely to have played a significant role in the invasion success of these five species introduced into novel regions within Australia. This may be due to the widespread occurrence of acacias and their associated soil microbial communities throughout the Australian continent.