Plant communities mediate the interactive effects of invasion and drought on soil microbial communities

Soil microbiomes could play a major role in ecosystem responses to escalating anthropogenic global change. However, we currently have a poor understanding of how soil microbes will respond to interacting global change factors and if responses will be mediated by changes in plant community structure. We used a field experiment to assess changes in soil fungal and bacterial communities in response to plant invasion, experimental drought, and their combination. In addition, we evaluated the relative importance of direct versus indirect pathways of invasion and drought through changes in associated plant communities with structural equation models. We found that fungal communities were interactively structured by invasion and drought, where fungal richness was lowest with invasion under ambient conditions but highest with invasion under drought conditions. Bacterial richness was lower under drought but unaffected by invasion. Changes in the plant community, including lower plant richness and higher root biomass, moderated the direct effects of invasion on microbial richness. Fungal and bacterial functional groups, including pathogens, mutualists, and nitrogen metabolizers, were also influenced by plant community changes. In sum, plant communities mediated the effects of interacting global change drivers on soil microbial community structure, with significant potential consequences for community dynamics and ecosystem functions.