Legacy effects of intensified drought on the soil microbiome in a mesic grassland

The soil microbiome response to global change drivers remains largely unclassified, with current studies showing contrasting results to several global change drivers. One such driver, drought, is increasing in severity and frequency due to climate change. Further, legacy effects, or long-lasting impacts after drought has subsided, could have lasting impacts on the soil microbiome with important consequences for ecosystem functioning. Thus, our study aimed to understand how the soil microbiome responds after a severe, prolonged drought and whether legacy effects persist post-drought. We measured soil microbial community response in a mesic grassland for two years after a four-year experimental drought in a native tallgrass prairie. The experimental drought was imposed either (1) chronically by reducing each growing season rainfall event by 66% or (2) intensely by completely eliminating growing season rainfall until an similar to 45% reduction in mean annual rainfall (climatic average) was achieved. The bacterial community showed no legacies in the first season post-drought in response to either chronic or intense drought but showed an increased abundance of Verrucomicrobia and decreased richness in both treatments in the second growing season after the drought treatments ended. In the first and second post-drought growing seasons, we found small differences in beta diversity between the control and intense drought treatment for fungal communities but not for the chronic drought treatment. Further, we found that the two main phyla of fungi, Ascomycota and Basidiomycota, showed reduced relative abundance post-drought in the intense drought treatment. Overall, few legacies in soil microbial communities persisted after a four-year experimentally induced drought. However, our results show that the nature of the drought-chronic versus intense-can differentially impact fungal versus bacterial short-term legacies. These results suggest that the soil microbiome is for the most part drought resistant (responds little during drought) and/or resilient (quickly recovers post-drought, since a study at the same site found that drought impacted the bacterial community) in this mesic grassland.