Effects of forest fire on soil organic matter quality in Canadian permafrost region

Theme 1. Environmental protection
Session Name 1.10 Biogeochemical cycles in Arctic forests
Presentation Type Poster
Author(s) Heidi Aaltonen (University of Helsinki, Finland), Kajar Köster (University of Helsinki, Finland), Frank Berninger (University of Helsinki, Finland), Egle Köster (University of Helsinki, Finland), Xuan Zhou (University of Helsinki, Finland)
Abstract text

Increasing temperatures on the northern hemisphere may increase frequency of forest fires. A large proportion of boreal forests in this region grow on permafrost soils, storing 50 % of global soil carbon.  Recent predictions have indicated that 25 % of the permafrost thaws during this century, which might further be facilitated by fires removing the insulating organic layer. Thawing permafrost results in the decomposition of previously frozen soil organic matter (SOM), but the scale of emissions are dependent on degradability of SOM. Chemical properties of SOM enable separation of different SOM pools. We conducted chemical fractionation of SOM, and analyzed 15N and 13C isotopic composition of samples collected from a fire choronosequence in Canadian permafrost region.  At 5 cm depth, recent fire areas had a smaller fraction of labile SOM than older areas, while at 30 and 50 cm there was no clear pattern with years since fire. Size of the most resistant SOM fraction was best predicted by: active layer depth (ALD), vegetation biomass (VB) and C/N, depending on soil depth. δ15N values at 5 cm soil depth were slightly enriched on recent fire area, while older fire areas were depleted. Fires affect SOM quality mostly in the top (5 cm) soil and as succession proceeded, the ratios of the SOM fractions reverted towards pre-fire status. Results indicated that the soil at permafrost surface contains mostly recalcitrant SOM, not very sensitive to changes in ALD and VB, meaning possible CO2 emissions from decomposing permafrost could be less than previously thought.