Cryoturbation, the burial of topsoil material into deeper soil horizons by repeated freeze-thaw events, is an important storage mechanism for soil organic matter (SOM) in permafrost-affected soils. Besides abiotic conditions, microbial community structure and the accessibility of SOM to the decomposer community are hypothesized to control SOM decomposition and thus have a crucial role in SOM accumulation in buried soils. We surveyed the microbial community structure in cryoturbated soils from nine soil profiles in the northeastern Siberian tundra using high-throughput sequencing and quantification of bacterial, archaeal and fungal marker genes. We found that bacterial abundances in buried topsoils were as high as in unburied topsoils. In contrast, fungal abundances decreased with depth and were significantly lower in buried than in unburied topsoils resulting in remarkably low fungal to bacterial ratios in buried topsoils. Fungal community profiling revealed an associated decrease in presumably ectomycorrhizal (ECM) fungi. The abiotic conditions (low to subzero temperatures, anoxia) and the reduced abundance of fungi likely provide a niche for bacterial, facultative anaerobic decomposers of SOM such as members of the Actinobacteria, which were found in significantly higher relative abundances in buried than in unburied topsoils. Our study expands the knowledge on the microbial community structure in soils of Northern latitude permafrost regions, and attributes the delayed decomposition of SOM in buried soils to specific microbial taxa, and particularly to a decrease in abundance and activity of ECM fungi, and to the extent to which bacterial decomposers are able to act as their functional substitutes.

] Austrian Polar Research Institute Vienna Austria [2] Division of Archaea Biology and Ecogenomics Department of Ecogenomics and Systems Biology University of Vienna Vienna Austria

] Austrian Polar Research Institute Vienna Austria [2] Division of Terrestrial Ecosystem Research Department of Microbiology and Ecosystem Science University of Vienna Vienna Austria

] Department of Biology Centre for Geobiology University of Bergen Bergen Norway [2] Austrian Polar Research Institute Vienna Austria

] Department of Biology Centre for Geobiology University of Bergen Bergen Norway [2] Austrian Polar Research Institute Vienna Austria [3] Division of Archaea Biology and Ecogenomics Department of Ecogenomics and Systems Biology University of Vienna Vienna Austria

] Institut für Bodenkunde Leibniz Universität Hannover Hannover Germany [2] VN Sukachev Institute of Forest Siberian Branch of Russian Academy of Sciences Akademgorodok Russia

] Institute of Genomics and Systems Biology Argonne National Laboratory Argonne IL USA [2] Computation Institute University of Chicago Chicago IL USA

] Institute of Genomics and Systems Biology Argonne National Laboratory Argonne IL USA [2] Department of Ecology and Evolution University of Chicago Chicago IL USA

Central Siberian Botanical Garden Siberian Branch of Russian Academy of Sciences Novosibirsk Russia

Department of Biology Centre for Geobiology University of Bergen Bergen Norway

Department of Earth Science Centre for Geobiology University of Bergen Bergen Norway

Department of Ecosystems Biology University of South Bohemia České Budějovice Czech Republic

Division of Ecosystem Modelling Institute of Coastal Research Helmholtz Zentrum Geesthacht Geesthacht Germany

Institut für Bodenkunde Leibniz Universität Hannover Hannover Germany

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