Climate change in Arctic ecosystems fosters permafrost thaw and makes massive amounts of ancient soil organic carbon (OC) available to microbial breakdown. However, fractions of the organic matter (OM) may be protected from rapid decomposition by their association with minerals. Little is known about the effects of mineral-organic associations (MOA) on the microbial accessibility of OM in permafrost soils and it is not clear which factors control its temperature sensitivity. In order to investigate if and how permafrost soil OC turnover is affected by mineral controls, the heavy fraction (HF) representing mostly MOA was obtained by density fractionation from 27 permafrost soil profiles of the Siberian Arctic. In parallel laboratory incubations, the unfractionated soils (bulk) and their HF were comparatively incubated for 175 days at 5 and 15°C. The HF was equivalent to 70 ± 9% of the bulk CO2 respiration as compared to a share of 63 ± 1% of bulk OC that was stored in the HF. Significant reduction of OC mineralization was found in all treatments with increasing OC content of the HF (HF-OC), clay-size minerals and Fe or Al oxyhydroxides. Temperature sensitivity (Q10) decreased with increasing soil depth from 2.4 to 1.4 in the bulk soil and from 2.9 to 1.5 in the HF. A concurrent increase in the metal-to-HF-OC ratios with soil depth suggests a stronger bonding of OM to minerals in the subsoil. There, the younger 14 C signature in CO2 than that of the OC indicates a preferential decomposition of the more recent OM and the existence of a MOA fraction with limited access of OM to decomposers. These results indicate strong mineral controls on the decomposability of OM after permafrost thaw and on its temperature sensitivity. Thus, we here provide evidence that OM temperature sensitivity can be attenuated by MOA in permafrost soils.

5 N Sukachev Institute of Forest Siberian Branch of Russian Academy of Sciences Krasnoyarsk Russia

Austrian Polar Research Institute Vienna Austria

Bolin Centre for Climate Research Stockholm University Stockholm Sweden

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 Bioscience Centre for Geomicrobiology Aarhus Denmark

Department of Ecogenomics and Systems Biology University of Vienna Vienna Austria

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

Department of Environmental Science and Analytical Chemistry Stockholm University Stockholm Sweden

Department of Microbiology and Ecosystem Science University of Vienna Vienna Austria

Department of Natural Resources and the Environment University of New Hampshire Durham New Hampshire

Federal Institute for Geosciences and Natural Resources Hannover Germany

Institute of Biostatistics Leibniz Universität Hannover Hannover Germany

Institute of Microbiology Ernst Moritz Arndt University Greifswald Germany

Institute of Soil Science Leibniz Universität Hannover Hannover Germany

Max Planck Institute for Biogeochemistry Jena Germany

Soil Ecology University of Bayreuth Bayreuth Germany

Soil Science and Soil Protection Martin Luther Universität Halle Wittenberg Halle Germany

Thünen Institute of Climate Smart Agriculture Braunschweig Germany

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