Tundra wildfire triggers long-term lateral nitrogen loss

Climate change is triggering widespread ecosystem disturbance across the permafrost zone, including substantial increases in the extent and severity of wildfire in Arctic tundra. This shift in disturbance regime could influence multiple aspects of terrestrial and aquatic biogeochemistry. Tundra wildfire may cause terrestrial vegetation shifts, increasing productivity and nutrient demand; alternatively, wildfire regimes may intensify lateral nutrient loss from the landscapes into adjacent river networks. To address this unknown, we collected water samples from 60 burned and unburned watersheds around the Anaktuvuk River fire scar in northern Alaska. We used a novel aerial sampling technique to collect samples three times during the flow seasons of 2017 and 2018, ten years after the wildfire. Despite a decade of ecosystem recovery, we observed nearly a doubling of total dissolved nitrogen concentration, primarily due to elevated organic nitrogen and secondarily from inorganic nitrogen increases. Isotopic analysis suggests that burn-mobilized lateral nitrogen flux comes from old soil nitrogen, not newly-fixed inputs from vegetation shifts. These findings indicate that tundra wildfire could destabilize nitrogen previously stored in permafrost, potentially exacerbating terrestrial nitrogen limitation and altering aquatic and estuarine ecosystems in the permafrost zone.