Evaluating the quantity and biodegradability of soil organic matter in some Canadian Turbic Cryosols

Abstract Turbic Cryosols contain almost 80% of the soil organic carbon (C) in the Canadian Arctic. Predicted warming of up to 8 °C in the circumpolar permafrost zone by the end of the 21st century will increase the thickness of the active layer and release C that was previously protected from microbial degradation by freezing. Little is known about the disposition, biochemical composition, and potential biodegradability of this soil organic matter (SOM). In this study we characterized SOM in the active layer and near-surface permafrost in three Turbic Cryosols in the Northwest Territories, Canada. We used physical (size fractionation), chemical (sequential cold- and hot-water extraction), and biological (100-d bioassay for mineralizable C) methods to estimate the quantities of labile SOM. Soil C and N storage in the profiles (measured to 70–110 cm) ranged from 15 to 46 kg C m− 2 and 1.3 to 3.1 kg N m− 2, with 16 to 78% contained in horizons with cryoturbation features and 4 to 66% in perennially frozen horizons. The concentrations of C and N in the whole soil and all fractions typically declined with depth. The proportion of total C and N in particulate organic matter in horizons with cryoturbation features was large (usually > 30%) compared to other ecosystems, whereas that in the water extracts was < 5%, and the fraction of potentially mineralizable C was < 2%. The contribution of C and N to the total storage in each SOM fraction in the perennially frozen horizons with cryoturbated features was generally larger than that observed in mineral horizons of the active layer with cryoturbated features. Our results suggest that the amount of labile SOM in horizons with cryoturbation features is small and that the relative bioavailability of SOM in near surface permafrost horizons with cryoturbated features is similar or greater than the SOM in the active layer horizons with cryoturbated features.