Drought eliminates soil microbially mediated indirect competitive advantage among exotic plants

Ecosystems world-wide are experiencing an accelerating accumulation of exotic plant species, posing serious threats to biodiversity and ecosystem functioning. While soil microbially mediated indirect interactions play a crucial role in shaping plant community composition, their contribution to accumulation of exotics remains poorly understood, especially under changing environmental conditions. Here, we examined how native and exotic plants impact the growth and competitiveness of later-arriving native and exotic plants through soil microbial legacies generated in different environmental conditions. We found that soil microbial legacies generated by exotics under well-watered conditions conferred a competitive advantage to later-arriving exotics over natives, but legacies generated by natives had no such effect. This indirect advantage for exotics was linked to a higher relative abundance of pathogens in soils conditioned by exotics, which was negatively correlated with native growth and positively associated with the competitive effects of exotics on natives. However, drought eliminated this pathogen-mediated indirect advantage among exotics by preventing pathogen accumulation. This study highlights the critical importance of considering soil microbially mediated indirect interactions between native and exotic species for accurately predicting the accumulation of exotic plant species under climate change.