Arctic ecosystems are warming rapidly, which is expected to promote soil organic matter (SOM) decomposition. In addition to the direct warming effect, decomposition can also be indirectly stimulated via increased plant productivity and plant-soil C allocation, and this so called "priming effect" might significantly alter the ecosystem C balance. In this study, we provide first mechanistic insights into the susceptibility of SOM decomposition in arctic permafrost soils to priming. By comparing 119 soils from four locations across the Siberian Arctic that cover all horizons of active layer and upper permafrost, we found that an increased availability of plant-derived organic C particularly stimulated decomposition in subsoil horizons where most of the arctic soil carbon is located. Considering the 1,035 Pg of arctic soil carbon, such an additional stimulation of decomposition beyond the direct temperature effect can accelerate net ecosystem C losses, and amplify the positive feedback to global warming.

Austrian Polar Research Institute Vienna Austria

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 Center for Geomicrobiology Aarhus Denmark

Department of Earth Sciences University of Gothenburg Gothenburg Sweden

Department of Ecogenomics and Systems Biology University of Vienna Vienna Austria

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

Department of Microbiology and Ecosystem Science University of Vienna Vienna Austria

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

Department of Physical Geography Stockholm University Stockholm Sweden

Institute of Microbiology Ernst Moritz Arndt University Greifswald Germany

Institute of Soil Science Leibniz Universität Hannover Hannover Germany

Lancaster Environment Centre Lancaster University Lancaster UK

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

VN Sukachev Institute of Forest Siberian Branch of Russian Academy of Sciences Krasnoyarsk Russia

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