Native Plant Diversity Generates Microbial Legacies That Either Promote or Suppress Non-Natives, Depending on Drought History

Diverse native plant communities resist non-native plants more than species-poor communities, in part through resource competition. The role of soil biota in diversity–invasibility relationships is poorly understood, although non-native plants interact with soil biota during invasions. We tested the responses of non-native plants to soil biota generated by different native plant diversities. We applied well-watered and drought treatments in both conditioning and response phases to explore the effects of ‘historical’ and ‘contemporary’ environmental stresses. When generated in well-watered soils, the microbial legacies from higher native diversity inhibited non-native growth in well-watered conditions. In contrast, when generated in drought-treated soils, the microbial legacies from higher native diversity facilitated non-native growth in well-watered conditions. Contemporary drought eliminated microbial legacy effects on non-native growth. We provide a new understanding of mechanisms behind diversity–invasibility relationships and demonstrate that temporal variation in environmental stress shapes relationships among native plant diversity, soil biota and non-native plants.