Soil microbial legacies and drought mediate diversity-invasibility relationships in non-native communities

High native species diversity generally suppresses non-native invasions,
but many ecosystems are now characterized by non-native assemblages that
vary in species diversity. How this non-native species diversity affects
subsequent invaders and its environmental dependence remains unclear.
Here, we conducted a plant-soil feedback experiment to investigate how
non-native plant species diversity affects the growth of subsequent
non-native plants, the role of soil microbes in this process, and the
dependence of these patterns on drought. We found that under well-watered
conditions, the biomass of subsequent invaders was higher with soil
inocula generated by high non-native diversity, which was associated with
higher arbuscular mycorrhizal fungal richness. However, under drought
conditions, the biomass of subsequent invaders did not depend on soil
inocula generated by non-native diversity. Our study reveals soil
microbial legacies likely contribute to the commonly observed
co-occurrence of multiple non-native species in nature and the importance
of environmental conditions for these effects.