Climate change is creating widespread ecosystem disturbance across the permafrost zone, including a rapid increase in the extent and severity of tundra wildfire. Previously rare, increased tundra wildfire has unknown consequences for lateral nutrient flux from terrestrial to aquatic environments. Lateral loss of nutrients could reduce carbon uptake and slow recovery of already nutrient-limited tundra ecosystems. To investigate the long-term effects of tundra wildfire on lateral nutrient export, we analyzed water chemistry in and around the 10-year-old Anaktuvuk River fire scar in northern Alaska. Using an aerial sampling method, we collected water samples from the outlets of 24 burned and 20 unburned watersheds during snowmelt, at peak growing season, and after plant senescence in 2017 and 2018.
Results/Conclusions
After a decade of ecosystem recovery, active-layer depth and aboveground biomass had recovered in burned watersheds, but litter and belowground biomass remained ~25% lower. Despite lower organic matter stocks, dissolved organic nutrients were substantially elevated in burned watersheds, with higher flow-weighted concentrations of organic carbon (15% higher), organic nitrogen (50% higher), organic phosphorus (72% higher), and organic sulfur (120% higher). Chemical proxies of water flowpath indicated greater interaction with mineral soils in burned watersheds, but optical analysis suggested that recent plant growth, not mineral soils, were the main source of elevated nutrients in burned watersheds. Burned and unburned watersheds had similar δ15N-NO3-, indicating that exported nitrogen was of pre-burn origin (i.e. not recently fixed). Lateral nitrogen flux from burned watersheds was 2- to 3-fold higher than observed rates of nitrogen fixation in this area. These findings indicate that tundra wildfire initially increases nutrient availability, but on longer timescales destabilizes nitrogen and other nutrients previously stored in permafrost via plant uptake and leaching. This lateral nutrient loss could exacerbate terrestrial nutrient limitation after disturbance or serve as an important nutrient release mechanism during succession.