Permafrost Disturbance Impacts on Dissolved Ion Loads and Nitrogen Dynamics

Physical disturbance (slope failure) and thermal perturbation (deep thaw) of the active layer alter the physical hydrology and the dissolved loads of solutes and nutrients of arctic watersheds and downstream water bodies, however the seasonality and duration of these changes, and the processes responsible for them, are poorly understood. To examine these dimensions of permafrost change, we investigated the seasonal fluxes of major ions and dissolved N species from small catchments that were affected by recent slope failures. The catchments represent a range of disturbance extent and hydrological connectivity, including an undisturbed control catchment.

Key differences in the impacts of slope disturbance on the fluvial export of major ions, and nitrogen from these headwater catchments were observed. The export of ions increased for all streams (disturbed and undisturbed) in response to deep thaw in warm years. However, the impact of physical disturbance was largely limited to increased ion concentrations late in the season when baseflow was driven by rainfall and soil water drainage. In contrast, nitrate (NO₃^-) concentrations were substantially affected by physical disturbance. A comparison of a disturbed and undisturbed catchment shows the two streams had similar concentrations of all dissolved N prior to slope failure, yet five years following failure, NO₃^- concentrations in the disturbed watershed were two orders of magnitude higher than in the undisturbed catchment. NO₃^- was especially high following late season rainfall and isotopic evidence shows that late season NO₃^-is not from rainfall but microbially produced (mineralized).

Results indicate that the impact of disturbance on ion and N loads is limited by discharge and hydrological connectivity of the disturbed areas, that the drainage of upper permafrost (soils below normal active layer depth) is the likely source of enhanced ion and mineralized NO₃^-, and that impacts are evident several years after disturbance.