The impact of wildfire on biogeochemical fluxes and water quality in 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 are 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-fire (9 years) and post-fire (4 years) 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 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 3 years post-fire. Aquatic C and N losses the first 12 months post-fire were 7 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 (Ca²⁺, Mg²⁺, K⁺ ,SO${}_{\mathrm{4}}^{-\mathrm{2}}$ , Cl⁻ ,NH${}_{\mathrm{4}}^{+}$ , total organic N) 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 mobilization processes, respectively. Three years after the fire, it appears that dissolved fluxes of nutrients have largely returned to pre-fire conditions, but there is still net release of CO₂.