The Depth Distribution of Organic Carbon in Mineral Cryosols at Two Sites in the Canadian Arctic

Abstract

The northern circumpolar permafrost region covers over 15% of the earth’s land surface, and contains about 50% of global belowground organic carbon (OC). There has been a great deal of recent interest in cataloguing the distribution of organic carbon in these soils, as the Arctic is expected to experience particularly large warming in the coming century, which has potential to cause a release of carbon to the atmosphere and lead to a substantial positive feedback to warming. Despite this concern, little work has been done to elucidate the depth distribution of organic carbon in mineral Cryosols (permafrost-affected soils).

In this study, selected mineral Cryosols at Truelove Lowland, Devon Island, Nunavut and near Wright Pass, Yukon were examined. The depth distribution of OC in soils affected by cryoturbation (Turbic Cryosols) and soils not affected by cryoturbation (Static Cryosols) was compared using high depth-resolution GIS-based methods. Density of OC at Truelove Lowland ranged from 185 g OC m-2 cm-1 (1 g OC m-2 cm-1 = 0.1 kg OC m-3) near the surface to 28 g OC m-2 cm-1 at 90 cm depth in Turbic Cryosols and 178 g OC m-2 cm-1 near the surface to 11 g OC m-2 cm-1 at 25 cm depth in Static Cryosols. At Wright Pass, OC density ranged from 334 g OC m-2 cm-1 near the surface to 110 g OC m-2 cm-1 at 46 cm depth in Turbic Cryosols and 330 g OC m-2 cm-1 near the surface to 29 g OC m-2 cm-1 at 70 cm depth in Static Cryosols. When depth distribution was compared, it was found that Turbic Cryosols contain up to 125 g OC m-2 cm-1 more OC at depths of 20 to 45 cm at Truelove Lowland and up to 148 g OC m-2 cm-1 more OC at depths of 12 to 90 cm at Wright Pass. The high depth-resolution method employed in this study can determine OC content for any selected depth range, and could be employed with existing databases of OC to provide vertically-resolved measures of OC.

Several existing models of cryoturbation processes were evaluated using data from this study. Soil movement by diapirism or cryohydrostatic movement is an unlikely explanation for the development of cryoturbation features in examined soil. Cryostatic movement and differential frost heave processes, however, are possible explanations for the development of some cryoturbation features. Some evidence suggests that transport of OC in a dissolved state may be important in the genesis of cryoturbation forms.

Additionally, several Turbic Cryosol pedons were selected as representative with respect to soil morphology and presumed cryogenesis and were examined using carbon:nitrogen ratio and nuclear magnetic resonance spectroscopy to explore OC quality. In all near-surface samples from all pedons examined there was a high proportion of O-alkyl carbon (range from 58.5 to 60.2 %) and a high O-alkyl carbon to alkyl carbon ratio (range from 2.3 to 2.6). Near-surface OC in examined soils was labile compared to that of temperate ecosystems; however a high arctic nonsorted circle had similar OC lability and degree of humification at depth and near the soil surface (O-alkyl to alkyl carbon ratios of 3.0 and 2.6 and carbon to nitrogen ratios of 16.5 and 16.6, respectively), while a low arctic earth hummock had chemically transformed OC at depth (O-alkyl carbon contents of 39.0 and 60.2% and O-alkyl to alkyl carbon ratios of 0.8 and 2.4, respectively). These results are unreplicated, but highlight the need for further study of OC quality amongst different modes of cryoturbation and different cryoturbation-related landforms.