Enzyme-mediated decomposition of soil organic matter (SOM) is controlled, amongst other factors, by organic matter properties and by the microbial decomposer community present. Since microbial community composition and SOM properties are often interrelated and both change with soil depth, the drivers of enzymatic decomposition are hard to dissect. We investigated soils from three regions in the Siberian Arctic, where carbon rich topsoil material has been incorporated into the subsoil (cryoturbation). We took advantage of this subduction to test if SOM properties shape microbial community composition, and to identify controls of both on enzyme activities. We found that microbial community composition (estimated by phospholipid fatty acid analysis), was similar in cryoturbated material and in surrounding subsoil, although carbon and nitrogen contents were similar in cryoturbated material and topsoils. This suggests that the microbial community in cryoturbated material was not well adapted to SOM properties. We also measured three potential enzyme activities (cellobiohydrolase, leucine-amino-peptidase and phenoloxidase) and used structural equation models (SEMs) to identify direct and indirect drivers of the three enzyme activities. The models included microbial community composition, carbon and nitrogen contents, clay content, water content, and pH. Models for regular horizons, excluding cryoturbated material, showed that all enzyme activities were mainly controlled by carbon or nitrogen. Microbial community composition had no effect. In contrast, models for cryoturbated material showed that enzyme activities were also related to microbial community composition. The additional control of microbial community composition could have restrained enzyme activities and furthermore decomposition in general. The functional decoupling of SOM properties and microbial community composition might thus be one of the reasons for low decomposition rates and the persistence of 400 Gt carbon stored in cryoturbated material.

Austrian Polar Research Institute Vienna Austria; University of Bergen Centre for Geobiology Department of Biology Bergen Norway

Austrian Polar Research Institute Vienna Austria; University of Vienna Department of Ecogenomics and Systems Biology Division of Archaea Biology and Ecogenomics Vienna Austria

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

International Atomic Energy Agency Joint FAO IAEA Division for Nuclear Techniques in Food and Agriculture Soil and Water Management and Crop Nutrition Laboratory Vienna Austria

Leibniz Universität Hannover Institut für Bodenkunde Hannover Germany

Leibniz Universität Hannover Institut für Bodenkunde Hannover Germany; VN Sukachev Institute of Forest Siberian Branch of Russian Academy of Sciences Krasnoyarsk Russia

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

University of Vienna Department of Microbiology and Ecosystem Science Division of Terrestrial Ecosystem Research Vienna Austria

University of Vienna Department of Microbiology and Ecosystem Science Division of Terrestrial Ecosystem Research Vienna Austria; Austrian Polar Research Institute Vienna Austria

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