Forest fires in Canadian permafrost region: the combined effects of fire and permafrost dynamics on soil organic matter quality

Heidi Aaltonen,Kristiina Karhu,Frank Berninger,Jukka Pumpanen,Marjo Palviainen,Viktor Bruckman,Kajar Köster,Christina Biasi,Egle Köster,Xuan Zhou

doi:10.1007/s10533-019-00560-x

Heidi Aaltonen, Kristiina Karhu + Show 8 more

Open Access

https://doi.org/10.1007/s10533-019-00560-x

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Abstract

Wildfires burn approximately 1% of boreal forest yearly, being one of the most significant factors affecting soil organic matter (SOM) pools. Boreal forests are largely situated in the permafrost zone, which contains half of global soil carbon (C). Wildfires advance thawing of permafrost by burning the insulating organic layer and decreasing surface albedo, thus increasing soil temperatures. Fires also affect SOM quality through chemical and physical changes, such as the formation of resistant C compounds. The long-term post-fire effects on SOM quality, degradability and isotopic composition are not well known in permafrost forests. We studied the effect of forest fires on the proportional sizes of SOM pools with chemical fractionation (extracting with water, ethanol and acid) of soil samples (5, 30 and 50 cm depths) collected from a fire chronosequence in the upland mineral soils of the Canadian permafrost zone. We also determined the 13C and 15N isotopic composition of soil after fire. In the topsoil horizon (5 cm) recent fire areas contained a smaller fraction of labile SOM and were slightly more enriched with 15N and 13C than older fire areas. The SOM fraction ratios reverted towards pre-fire status with succession. Changes in SOM were less apparent deeper in the soil. Best predictors for the size of recalcitrant SOM fraction were active layer depth, vegetation biomass and soil C/N ratio, whereas microbial biomass was best predicted by the size of the recalcitrant SOM fraction. Results indicated that SOM in upland mineral soils at the permafrost surface could be mainly recalcitrant and its decomposition not particularly sensitive to changes resulting from fire.

Highlights

Forest fires burn 5–20 M ha of boreal forest each year (Stocks et al 2004; Sukhinin et al 2004; Tan et al 2007; Flannigan et al 2009) and as much as 80% of the boreal forest region is located in the circumpolar permafrost zone (Helbig et al 2016)
We studied the effect of forest fires on the proportional sizes of soil organic matter (SOM) pools with chemical fractionation of soil samples (5, 30 and 50 cm depths) collected from a fire chronosequence in the upland mineral soils of the Canadian permafrost zone
Around half of the global soil carbon (C) pool resides in the permafrost soils of the Northern hemisphere and yet one-fourth of this permafrost is predicted to thaw by 2100 (Davidson and Janssens 2006)

Summary

Introduction

Forest fires burn 5–20 M ha of boreal forest each year (Stocks et al 2004; Sukhinin et al 2004; Tan et al 2007; Flannigan et al 2009) and as much as 80% of the boreal forest region is located in the circumpolar permafrost zone (Helbig et al 2016). The albedo of the soil surface may change due to the destruction of vegetation and darkening of the soil surface (Bret-Harte et al 2013; French et al 2016). All of these factors can lead to further warming of the subsoil and thawing of permafrost (Yoshikawa et al 2002; Randerson et al 2006; Gibson et al 2018). Thawing of permafrost means that soil organic matter (SOM) that was previously protected by subzero temperatures, becomes susceptible to decomposition (Dutta et al 2006; Harden et al 2012; Walz et al 2017). The thawing permafrost has been expected to cause significant positive feedback to climate change through greenhouse gases released from decomposing, previously frozen organic matter (Schadel et al 2016)

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