Fluvial Export of Carbon and Nutrients from Boreal Catchments Disturbed by Fire and Permafrost Thaw

Abstract Increased frequency and intensity of fire in the boreal forest can degrade permafrost and redistribute large pools of carbon and nitrogen that influence global climate. However, these changes are difficult to observe and predict at regional spatial scales and decadal or longer timescales. As receiving features of landscapes, streams integrate biogeochemical activity within the contributing area and might provide signals of disturbance and recovery at the catchment scale. We modeled solute concentrations as a function of fire history (0–100% catchment area burned less than one to > 79 years ago), permafrost, topography, and vegetation in 75 catchments of Interior Alaska. Concentrations of dissolved organic carbon (DOC) declined significantly with increasing proportion of the catchment burned, indicating that fire reduced stores of organic matter in the catchment, likely within the forest floor. We applied parameters estimated by regression models to predict that complete burning of catchments would reduce yield of DOC by an average of 46% (– 1.6 kg km –2 d –1 ) during the summer low-flow period. Solute concentrations were uninfluenced by time since fire, suggesting decadal-scale legacies of fire on fluvial exports or heterogeneous trajectories of ecosystem recovery following fire. Elevated concentrations of dissolved inorganic nitrogen in streams draining less extensive permafrost indicated potential for nitrogen export from catchments upon permafrost thaw. Concentrations of weathering products (e.g., sodium, chloride, sulfate) were weakly influenced by fire or permafrost extent, suggesting that disturbance primarily affected export of carbon, nitrogen, and phosphorus by reducing organic stocks and biological nutrient retention. Stream chemistry reflects the fire history and permafrost extent of boreal forest ecosystems, providing a spatially integrative indicator of ecosystem response and recovery from disturbance.