Abstract
Climate warming in arctic/subarctic ecosystems will result in increased frequency of forest fires, elevated soil temperatures and thawing of permafrost, which have implications for soil organic matter (SOM) decomposition rates, the CO2 emissions and globally significant soil C stocks in this region. It is still unclear how decomposability and temperature sensitivity of SOM varies in different depths and different stages of succession following forest fire in permafrost regions and studies on long term effects of forest fires in these areas are lacking. To study this question, we took soil samples from 5, 10 and 30 cm depths from forest stands in Northwest Canada, underlain by permafrost, that were burnt by wildfire 3, 25 and over 100 years ago. We measured heterotrophic soil respiration at 1, 7, 13 and 19 °C. Fire had a significant effect on the active layer depth, and it increased the temperature sensitivity (Q10) of respiration in the surface (5 cm) and in the deepest soil layer (30 cm) in the 3-year-old area compared to the 25- and more than 100-year-old areas. Also the metabolic quotient (qCO2) of soil microbes was increased after fire. Though fires may facilitate the SOM decomposition by increasing active layer depth, they also decreased SOM quality, which may limit the rate of decomposition. After fire all of these changes reverted back to original levels with forest succession.
Highlights
Permafrost soils cover 24% of the land area in the northern hemisphere, and these soils store around 50% of the total global soil carbon (C) pool (Tarnocai et al, 2009)
The changes to the active layer depth resulting from fire were reflected in the temperature response of soil respiration which varied from 1.7 to 3.0 in our study areas (Fig. 1)
Our results indicated that fire had a substantial effect on the temperature response of soil respiration in boreal forests with underlying permafrost, while there were no significant differences in the heterotrophic respiration levels between the age classes
Summary
Permafrost soils cover 24% of the land area in the northern hemisphere, and these soils store around 50% of the total global soil carbon (C) pool (Tarnocai et al, 2009). Climate warming has been greatest in high latitudes (IPCC, 2013), and it is estimated that 25% of the permafrost area will thaw by 2100 (Davidson and Janssens, 2006). Rising soil temperatures and thawing permafrost may increase soil organic matter (SOM) decomposition and release of C from the soil, which may further amplify climate warming (Allison and Treseder, 2011). C stored in permafrost may be relatively labile since it is not necessarily protected by physical barriers, such as microaggregates, or stabilized by humification (Michaelson et al, 2004). The lability of SOM is dependent on stabilization or physical protection, and on the molecular structure, which is affected, for example, by the origin of the SOM (Schmidt et al, 2011)
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