Forest floor chemical transformations in a boreal forest fire and their correlations with temperature and heating duration

Abstract

Boreal soils account for ~30% of the global soil organic carbon (C) stock. Wildfires are an important perturbation of this C pool, particularly affecting the top organic soil layer, which constitutes the forest floor. Alterations to the forest floor by fire are relevant to the soil C balance and have profound implications for soil properties. However, relationships between forest floor transformations and actual wildfire characteristics have not been established to date due to the logistical challenges of obtaining the necessary fire behaviour data, together with associated pre- and post-fire sample material. We used a high-intensity experimental wildfire to address this research gap, which enabled us to determine forest floor chemical transformations in a Canadian boreal forest in relation to temperature-time profiles for 18 sampling points during the fire. Forest floor samples taken pre- and post-fire were characterized using elemental and δ13C analysis, differential scanning calorimetry and 13C nuclear magnetic resonance. During this typical boreal crown fire average maximum temperature (Tmax) at the forest floor was 745°C (550<Tmax<976°C) with the average heating duration (t) >300°C being 176s (65<t<364s). Significant correlations were detected between the chemical characteristics of the pyrogenic (charred) forest floor layer and the temperature-time profiles at the corresponding sampling points. Higher Tmax and associated prolonged heating durations correlated with greater C enrichments, increased thermal recalcitrance and degree of aromaticity of the pyrogenic organic matter. These changes were particularly pronounced for Tmax>600-700°C, which is higher than the range of 300-500°C for aromaticity development previously reported from laboratory experiments. One reason for this discrepancy could be the generally much longer heating durations used in laboratory studies, and we therefore advise caution when extrapolating findings from laboratory studies to wildfire conditions. Almost half of the initial total C stock in the forest floor (20MgCha-1) was affected by fire, with ~24% of this fire-affected C transformed to pyrogenic organic matter. This pyrogenic material possessed variable, yet distinct, chemical characteristics when compared to unburnt forest floor, including higher recalcitrance and associated resistance to biological degradation. As some boreal regions already show a rise in fire severity and area burned linked to climate change, our findings suggest a potential accompanying increase in the more stable organic carbon stock, with important implications for the functioning and turnover of organic matter in boreal soils.