Impact of downslope soil transport on carbon storage and fate in permafrost dominated landscapes

Abstract:

A large fraction of high latitude permafrost-dominated landscapes are covered by soil mantled hillslopes. In these landscapes, soil organic carbon (SOC) accumulates and is lost through lateral transport processes. At present, these processes are not included in regional or global landsurface climate models. We present preliminary results of a soil transport and storage model over a permafrost dominated hillslope. In this model soil carbon is transported downslope within a mobile layer that thaws every summer. The model tracks soil transport and its subsequent storage at the hillslope's base. In a scenario where a carbon poor subsurface is blanketed by a carbon-rich surface layer, the progressive downslope soil transport can result in net carbon sequestration. This sequestration occurs because SOC is carried from the hilllsope’s near-surface layer, where it is produced by plants and is capable of decomposing, into depositional sites at the hillslope's base where it is stored in frozen deposits such that it's decomposition rate is effectively zero. We use the model to evaluate the quantities of carbon stored in depositional settings during the Holocene, and to predict changes in sequestration rate in response to thaw depth thickening expected to occur within the next century due to climate-change. At the Holocene time scale, we show that a large amount of SOC is likely stored in depositional sites that comprise only a small fraction of arctic landscapes. The convergent topography of these sites makes them susceptible to fluvial erosion and suggests that increased fluvial incision in response to climate-change-induced thawing has the potential to release significant amounts of carbon to the river system, and potentially to the atmosphere. At the time scale of the next century, increased thaw depth may increase soil-transport rates on hillslopes and therefore increase SOC sequestration rates at a magnitude that may partly compensate for the carbon release expected from permafrost thawing. Model guided field data collection is essential to reduce the uncertainty of these estimates.