Over the past decades, Europe has been experiencing frequent and severe extreme weather events (EWEs) like droughts, heatwaves, forest fires, and heavy rains. In the context of global warming, an escalation in the frequency of extreme summers is anticipated. Extreme heatwaves and droughts have significant consequences not only for above-ground crops but may also for below-ground microorganisms.

Our study was carried out at the Global Change Experimental Facility (GCEF) UFZ field research station in Bad Lauchstädt, Saxony-Anhalt, Germany. Over the summers of 2014-2019, a total of 180 soil samples were collected between July and September from three distinct land-use types. Initially, our study was designed to explore the interactive effects of future climate scenarios, including a warming of 0.6 °C and altered precipitation patterns, along with land-use changes on soil microbiomes.

However, in 2018, central Europe experienced an exceptionally hot and dry summer, followed by intense drought conditions that persisted into the subsequent summer of 2019. These consecutive extreme summers provided a unique opportunity to investigate the impact of such conditions on soil microorganisms. As previous research has demonstrated the influence of climate manipulations on soil microbial biomass, diversity, and community structure, we now want to answer the question what happens to the soil microbiomes under naturally occurring extreme summers. 

Based on the 6-year soil samples, we use metagenomics to explore the impacts of extreme summers on microbial community and functional traits at the GCEF. The resulting data showed that extreme summer significantly impacted the soil microbiomes, including archaea, bacteria, fungi and viruses, in both cropland and grassland. Functional analysis highlighted the genes associated with microbial antioxidant mechanisms, cell wall biosynthesis, heat shock proteins, and sporulation as potential contributors to the enrichment of taxa during drought conditions. Furthermore, annotation of contigs and metagenome-assembled genomes (MAGs) indicated that Actinobacteria may possess a competitive advantage in extreme summers, potentially attributed to the biosynthesis of geosmin and 2-methylisoborneol. The expression levels of selected genes and MAGs were confirmed by metatranscriptomics conducted in 2022 summer.

Although the enriched functional traits may play a role in enhancing microbial drought tolerance, it is important to note that overall microbial biomass and activities were significantly decreased during extreme summers, as previously observed in a separate GCEF study. In contrast to extreme summers, the future climate scenarios induced similar changes in soil microbiomes, albeit to a lesser extent. These findings provide valuable insights into the response of soil microbial communities to extreme weather events and the potential mechanisms underlying their adaptation to drought conditions.

Our findings bring new insights into the microbial adaptations to the extreme summers and also open various questions yet to be answered, such as: what are the key factors inducing selectivity in soil microbial communities for three domains and DNA/RNA viruses? which microbial functional traits are more effective in extreme summers? Is there a drought legacy under long-term future climate scenarios? If so, how the adaptive changes play a role under extreme summers? To address these questions, long-term research stations such as the GCEF will prove to be invaluable resources.

A warmer planet means more EWEs. We look forward to adopting multi-omics approaches and fostering collaborations with scientists worldwide to enhance our understanding of how EWEs impact soil ecosystems.