Biogeochemical and hydrological impacts of a low-severity wildfire in the wetland-dominated zone of discontinuous permafrost

Northern regions are experiencing rapid climate warming. As these regions warm, the occurrences of naturally ignited wildfires are increasing in frequency, severity and area burned, calling for a more thorough understanding of post-fire eco-hydrological impacts. Changes in runoff chemistry, and soil moisture and thermal regimes, have been attributed to the significant loss of organic matter (OM) and exposure of deeper soils, leading to enhanced permafrost degradation, ground surface subsidence and the conversion of peat landscapes from long-term C sinks to sources. However, low-severity wildfires often result in minor OM loss. Due to the significant and immediate threats posed to the health of ecosystems and local communities, the impacts of large, high-severity burns have been a primary research focus while the implications of low-severity wildfires remain understudied. Boreal peatlands in the zone of discontinuous permafrost are ecologically-sensitive areas, where even minor land surface disturbances, such as wildfire, cause changes in surface vegetation and soil properties that alter the water and surface energy balances. In 2014, a low-severity wildfire burned approximately half of a 5 hectare treed permafrost plateau in the wetland-dominated landscape of the Scotty Creek drainage basin, Northwest Territories, Canada, located in the zone of discontinuous permafrost. This provided a unique opportunity to examine post-fire changes in runoff chemistry, plateau energy dynamics, water inputs, and ground thaw regimes within a single, partially burned landform unit. Beginning in March 2016, approximately 1.5 years following the wildfire event, runoff water samples were collected from the saturated layer as thaw progressed. Intensive repeated measurements of ground thaw dynamics, coupled with laboratory analyses of changes in near-surface (0-20 cm depth) peat physical and hydraulic properties, were used to explain changes in runoff water chemistry. Seasonal peat porewater showed elevated nutrient and ion concentrations in the burned site, likely due to the translocation of dense ash and char particulates from the surface, and leachate from dead OM. Physical properties showed substantial changes of pore size and structure between burned and unburned samples over the upper 20 cm of peat. Single-porosity ash and char particulates that clogged burned peat pore spaces created smaller interparticle pores characterized by lower saturated hydraulic conductivity and greater soil moisture retention. Longer porewater residence time promoted diffusive solute exchange between relatively dilute mobile porewater and more concentrated porewater was retained within smaller diameter and immobile pores. The higher thermal conductivity of wetter soils, enhanced by more consistent and uniform incident shortwave radiation resulting due to canopy removal, promoted deeper and more homogeneous ground thaw, releasing previously frozen permafrost porewater back into the soil solution. This study links biogeochemical and hydrological impacts of a wildfire to generate a more comprehensive understanding of how a permafrost plateau responds to a low-severity burn. Results suggests that post-fire changes in pore structure, runoff flowpath, and porewater chemistry were initiated by the incorporation of particulates from above-ground sources and augmented by abiotic forces