Global warming in mid-latitude alpine regions results in permafrost thawing, together with greater availability of carbon and nutrients in soils and frequent freeze-thaw cycles. Yet it is unclear how these multifactorial changes will shape the 1 m-deep permafrost microbiome in the future, and how this will in turn modulate microbiallymediated feedbacks between mountain soils and climate (e.g. soil CO2 emissions). To unravel the responses of the alpine permafrost microbiome to in situ warming, we established a three-year experiment in a permafrost monitoring summit in the Alps. Specifically, we simulated conditions of warming by transplanting permafrost soils from a depth of 160 cm either to the active-layer topsoils in the north-facing slope or in the warmer south-facing slope, near the summit. qPCR-based and amplicon sequencing analyses indicated an augmented microbial abundance in the transplanted permafrost, driven by the increase in copiotrophic prokaryotic taxa (e.g. Noviherbaspirillum and Massilia) and metabolically versatile psychrotrophs (e.g. Tundrisphaera and Granulicella); which acclimatized to the changing environment and potentially benefited from substrates released upon thawing. Metabolically restricted Patescibacteria lineages vastly decreased with warming, as reflected in the loss of alpha-diversity in the transplanted soils. Ascomycetous sapro-pathotrophs (e.g. Tetracladium) and a few lichenized fungi (e.g. Aspicilia) expanded in the transplanted permafrost, particularly in soils transplanted to the warmer south-facing slope, replacing basidiomycetous yeasts (e.g. Glaciozyma). The transplantation-induced loosening of microbial association networks in the permafrost could potentially indicate lesser cooperative interactions between neighboring microorganisms. Broader substrate-use microbial activities measured in the transplanted permafrost could relate to altered soil C dynamics. The three-year simulated warming did not, however, enhance heterotrophic respiration, which was limited by the carbon-depleted permafrost conditions. Collectively, our quantitative findings suggest the vulnerability of the alpine permafrost microbiome to warming, which might improve predictions on microbially-modulated transformations of moun-tain soil ecosystems under the future climate. (c) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Rising temperatures in the Arctic and the expansion of plants to higher latitudes will significantly alter belowground microbial communities and their activity. Given that microbial communities are major producers of greenhouse gases, understanding the magnitude of microbial responses to warming and increased carbon input to Arctic soils is necessary to improve global climate change models. In this study, active layer and permafrost soils from northern Greenland (81 degrees N) were subjected to increased carbon input, in the form of plant litter, and temperature increase, using a combined field and laboratory approach. In the field experiment, unamended or litter-amended soils were transplanted from the permafrost layer to the top soil layer and incubated for one year, whereas in the laboratory experiment active layer and permafrost soils with or without litter amendment were incubated at 4 degrees C or 15 degrees C for six weeks. Soil microbial communities were evaluated using bacterial 16S and fungal ITS amplicon sequencing and respiration was used as a measure of microbial activity. Litter amendment resulted in similar changes in microbial abundances, diversities and structure of microbial communities, in the field and lab experiments. These changes in microbial communities were likely due to a strong increase in fast-growing bacterial copiotrophic taxa and basidiomycete yeasts. Furthermore, respiration was significantly higher with litter input for both active layer and permafrost soil and with both approaches. Temperature alone had only a small effect on microbial communities, with the exception of the field-incubated permafrost soils, where we observed a shift towards oligotrophic taxa, specifically for bacteria. These results demonstrate that alterations in High Arctic mineral soils may result in predictable shifts in the soil microbiome.