Effects of fire on decomposition: assessing the relative importance of soil environment versus charring on decomposition in boreal conifer forests

Abstract:

Boreal forests are experiencing significant changes in climate and disturbance regimes, including increases in the frequency and severity of fires. Fires impact the carbon (C) cycle of this region in many ways, including through changes to C inputs to the ecosystem (i.e., loss of all living vegetation, followed post-fire regrowth), changes in mycorrhizal relationships, the altering soil temperature and moisture regimes, and the charring of surface organic soil. All of these factors have the potential to impact decomposition rates. We were interested in comparing the relative importance of changes in soil temperature and moisture (soil environmental conditions) versus surface organic soil quality (charring) on decomposition rates. To disentangle the effects of environmental factors versus charring on mass loss, we performed a reciprocal transplant experiment. Our design included burned and unburned feather moss litter, collected from the field and placed within litterbags, which were then placed into triplicate burned and unburned black spruce dominated stands in interior Alaska. Litterbags were collected after one, three, and seven years, after which mass loss and changes in C and N pools were quantified. Exponential decomposition (k) values varied with litter type (burned/unburned) by environment (burned/unburned site) interactions. Averaged across both types of environments, decomposition rates were almost double for unburned versus burned litter. Decomposition rates were approximately 30 percent faster for unburned versus burned sites. Our results to date show that changes to soil quality due to charring have a larger effect in controlling post-burn decomposition rates than changes in soil environmental conditions.