Increased deep soil respiration detected despite reduced overall respiration in permafrost peat plateaus following wildfire

Abstract

Wildfire in boreal permafrost peatlands causes a thickening and warming of the seasonally thawed active layer, exposing large amounts of soil carbon to microbial processes and potential release as greenhouse gases. In this study, conducted in the discontinuous permafrost zone of western Canada, we monitored soil thermal regime and soil respiration throughout the 2016 growing season at an unburned peat plateau and two nearby peat plateaus that burned 16 and 9 years prior to the study. Maximum seasonal soil temperature at 40 cm depth was 4 °C warmer in the burned sites, and active layers were ∼90 cm thicker compared to the unburned site. Despite the deeper and warmer seasonally thawed active layer, we found higher soil respiration in the unburned site during the first half of the growing season. We partitioned soil respiration into contribution from shallow and deep peat using a model driven by soil temperatures at 10 and 40 cm depths. Cumulative estimated deep soil respiration throughout the growing season was four times greater in the burned sites than in the unburned site, 32 and 8 g C m-2 respectively. Concurrently, cumulative shallow soil respiration was estimated to be lower in the burned than unburned site, 49 and 80 g C m-2 respectively, likely due to the removal of the microbially labile soil carbon in the shallow peat. Differences in deep contribution to soil respiration were supported by radiocarbon analysis in fall. With effects of wildfire on soil thermal regime lasting for up to 25 years in these ecosystems, we conclude that increased loss of deep, old, soil carbon during this period is of similar magnitude as the direct carbon losses from combustion during wildfire and thus needs to be considered when assessing overall impact of wildfire on carbon cycling in permafrost peatlands.