Soil microbial communities resist but ectomycorrhizal taxa shift in response to eight years of drought recurrence in a subalpine forest

Increasingly, forests ecosystems and their associated ecosystem services, are experiencing negative impacts of climate change induced droughts. Soil microbial communities, particularly those associated with trees, play a key role in mitigating these negative effects. In this study, we investigated the impact of eight years of recurring drought on the soil microbial communities of a subalpine spruce and larch forest, which we simulated through rainout shelters. A special focus was given to ectomycorrhizal fungi (ECM) through root-tip based investigation, as these mutualistic tree symbionts play an essential role in nutrient and water uptake. We assessed bacterial and fungal communities using amplicon sequencing. Additionally, root growth and soil parameters, including nitrate and ammonium concentrations, were quantified. Eight years of recurring drought did not lead to reductions in alpha diversity for neither ECM, nor soil fungi and bacteria. Furthermore, we found only small (< 7%), yet significant effects, on respective community composition. However, several ECM taxa displayed distinct responses to rainfall exclusion, suggesting that ECM display species-specific rather than genus level responses to drought. For soil parameters, drought led to a significant reduction in root density and nitrogen availability. The comparison of root tip- and amplicon-based ECM communities revealed substantial differences, indicating that soil amplicon sequencing and root-tip analyses provide complementary, not interchangeable, information. Our findings indicate that recurring drought had only limited effects on the soil microbiome of this high-elevation forest, while underscoring the importance of investigating individual taxa responses under climate change.