Contrasting Carbon and Nitrogen Stocks and Ice Contents along Two Arctic Hillslope Toposequences

Hillslope processes affect rates of transport, deposition, and decomposition, impacting the distribution of soil organic carbon (SOC) stocks in many regions. In the Arctic, hillslope processes are further impacted by the added complexity of permafrost-affected solifluction and other lateral mass movements, cryoturbation, and patterned-ground formation. Despite increasing numbers of studies on permafrost-region SOC stocks, quantitative information across soil toposequences in the continuous permafrost zone remain limited and hill-toe deposits comprise only about 2.5% of existing soil profiles for Alaska. In this study, two toposequences in the Arctic Foothills, north of the Brooks Range of Alaska were investigated. The soils of both toposequences were formed on loess over glacial till and support moist acidic tundra vegetation. Several locations along the toposequence (encompassing summit/shoulder, backslope/footslope, and toeslope/basin positions) were sampled by opening soil pits, taking soil cores, or a combination of both to a depth of 2-3 m. Ice-wedge polygons were present at summit/shoulder and toeslope/basin positions. Ice-wedge polygons in the basins were clearly defined, with deep inundated troughs, while the ice-wedge polygons at the summit/shoulder were marked by surficial soil cracks with drier troughs. The thickness of the surficial organic horizons was greater in the toeslope/basin, and the basin contained deep peat deposits infused with ice. In the backslope/footslope, surficial organic horizons varied depending on frost boil features. Both hillslopes contained large C and N stocks across all positions to 1 m depth. Burial of surface organic horizons into the mineral subsoil through cryoturbation was prevalent in all positions. Ice content was high in all positions but it was greater in the basin. While C and N stocks to 1 m in the basin were less compared to other positions due to the massive amount of ice, peat deposits continued below 1 m depth while mineral horizons were already found at this depth in the other positions. A better understanding of the distribution of soil genetic horizons, and ice content along toposequences in hilly permafrost terrains is critical for determining C and N stocks in the Arctic and for informing and improving modeling efforts.