The impact of wildfire on biogeochemical fluxes and water quality on boreal catchments

Abstract. Wildfires are the major disturbance in boreal ecosystems, and are of great importance for the biogeochemical cycles of carbon (C) and nutrients. However, these fire-induced impacts are hard to quantify and rarely assessed together at an ecosystem level incorporating both aquatic and terrestrial environments. Following a wildfire in Sweden in an area with ongoing monitoring, we conducted a pre- and post-fire multi-catchment investigation of element losses (combustion and leaching), and impacts on water quality. Direct C and nitrogen (N) losses through combustion were ca. 4500 g m⁻² and 100 g m⁻², respectively. Net CO₂ loss associated with soil and biomass respiration was ~ 150 g C m⁻² during the first year, but the ecosystem started to show net CO₂ uptake in June three years post-fire. Aquatic C and N losses during the first 12 months were 7 g m⁻² and 0.6 g m⁻², respectively. Hence, soil respiration comprised a non-negligible part of the post-fire C loss, whereas aquatic C losses were minor, and did not increase post-fire. However, other elements (e.g., Ca, S) exhibited ecologically relevant increases in fluvial export and concentration, with large peaks in the immediate post-fire period. The temporal dynamics of stream concentrations suggest the presence of faster- and slower-release nutrient pools with half-lives of around 2 weeks and 4 months, which we attribute to physicochemically and biologically mediated mobilisation processes, respectively. Three years after the fire, it appears that biogeochemical cycles have largely returned to pre-fire conditions, but there is still net release of CO₂.