Extreme drought triggers transition to an alternative soil microbial state

Soil microbial communities play a pivotal role in regulating ecosystem functioning1 but they are increasingly threatened by human-driven perturbations, including climate extremes, which are predicted to increase in frequency and intensity with climate change2. It has been demonstrated that soil microbial communities are sensitive to climate extremes, such as drought3,4, and that effects can be long-lasting5,6. However, considerable uncertainties remain concerning the response of soil microbial communities to increases in the intensity and frequency of climate extremes, and their potential to trigger transitions to alternative, and potentially deleterious, taxonomic and functional states7. Here we demonstrate that extreme, frequent drought induces a shift to an alternative soil microbial state characterised by strongly altered bacterial and fungal community structure of reduced complexity and functionality. Moreover, we found that this drought-induced alternative microbial state persisted after returning soil to its previous moisture status. However, bacterial communities were able to adapt by increasing their growth capacity, despite being of reduced diversity. Abrupt transitions to alternative states are well documented in aquatic and terrestrial plant communities in response to human-induced perturbations, including climate extremes8,9. Our results provide experimental evidence that such transitions also occur in soil microbial communities in response to extreme drought with potentially deleterious consequences for soil health.