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Research article - Peer-reviewed, 2021

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

Granath, Gustaf; Evans, Christopher D.; Strengbom, Joachim; Folster, Jens; Grelle, Achim; Stromqvist, Johan; Kohler, Stephan J.


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 gm(-2), respectively. Net CO2 loss associated with soil and biomass respiration was similar to 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 gm(-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+,SO4-2, Cl-, NH4+, 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.

Published in

2021, Volume: 18, number: 10, pages: 3243-3261