Legacy effects of an invasive legume more strongly impact bacterial than plant communities in a Mediterranean-type ecosystem

Invasive plants can leave long-lasting impacts that influence ecosystem processes and management, even after their removal from the environment. Understanding these impacts and their legacy effects on native plant-soil interactions is important to assist ecological restoration. This study examines the legacy effects of the invasive Acacia saligna (Labill.) H.L. Wendl. on soil characteristics, as well as plant and bacterial communities, within Cape Fynbos conservation areas, aiming to assess the potential for restoration following the invader's removal. We compared attributes of ecosystem functioning and biodiversity in sites cleared and invaded by A. saligna, as well as uninvaded reference sites, in three conservation areas. We assessed soil chemical properties, nitrogen (N) isotope signatures (as a proxy for the sources of N uptake by a native plant and A. saligna), and plant and soil bacterial community diversity and composition. The effect of A. saligna removal was mostly contingent on the conservation area studied, though we observed consistent patterns for plant and bacterial diversity according to site type. In some conservation areas, we found signs of recovery towards reference site values for soil abiotic characteristics, e.g. potassium and soil organic carbon. Yet, in other cases removal of the invader was associated with higher soil pH and higher phosphorus availability. Soil conditions in cleared sites influenced the nitrogen sources used by A. saligna, shifting towards soil-derived nitrogen, but did not influence the native species tested. While we observed signs of native plant community recovery after clearing, soil bacterial communities remained comparable to those in invaded sites. The lag in bacterial community recovery was driven by soil pH changes caused by A. saligna invasion. Our findings show that invasive legume removal can directly facilitate native vegetation recovery but that some legacy effects may hinder or delay soil bacterial community recovery. Impacts of soil legacy effects can be contingent upon the taxa examined and the context of management or invasion history of sites. Thus, passive restoration, such as invasive species removal, may be slowed down.