Climate change, snow cover and microbial diversity in mountain soils: critical processes during snowmelt

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In alpine zones, soil microbial diversity and activity are strongly dependent on the annual patterns of snowpack formation. It is under the insulating winter snowpack where the highest microbial biomass is found. At snowmelt, there is a drastic and still not well understood rapid turnover: the winter bacterial and fungal communities entirely disappear and are replaced by spring communities. Global warming is affecting the mountain ranges by increasing the annual mean temperatures, reducing the winter snow cover, advancing the time of snowmelt and increasing the frequency of freeze-and-thaw cycles. Understanding how the soil microbiome is affected by warming and in turn affects it, e.g. by releasing more greenhouse gas, is essential for establishing predictive scenarios. We plan to investigate the soil microbial biodiversity (including, for the first time, protists) and its functions before, during and after snowmelt, the modalities of the microbial turnover during snowmelt and how it is affected by warming. Our study will be carried out on three mountains in the Alps (Davos, Switzerland), at three different altitudes and in three seasons, with a collection of more than 200 soil samples. Vegetation, relevant soil and climate parameters will be recorded. In addition, we will mimic the effects of climate warming by experimentally modifying the snow cover - compacting and removing to simulate earlier melt at six sites. We will use high-throughput sequencing of RNAs to simultaneously obtain information on both composition and function of the soil communities. Because we are looking for short-term temporal shifts, we will extract RNA to target only living cells. Briefly, the ribosomal RNAs will be used to identify the bacterial, fungal and protistan communities and their interactions; messenger RNAs will characterize bacterial and fungal functional profiles and metabolic pathways active before, during and after thaw and how the experimental treatments affect these dynamics. Our study will contribute to the fundamental understanding of spring soil microbial dynamics in the mountains responsible of soil organic matter mineralization at thaw and the biogeochemical cycles contributing to greenhouse gas emissions as a result of climate warming. The multiple qualifications assembled in our German-Swiss collaborative project (applicant: protistology, environmental PCR, bioinformatics; T. Urich: metatranscriptomics, bacterial functions in soils; C. Rixen: effects of snow cover on soil and vegetation, knowledge of the sampling sites; M. Bonkowski: ecology, plant-microbe interactions, soil food webs) will ensure a successful completion of the project. Our project will thus make a significant contribution to understanding the functioning of alpine and subalpine ecosystems.