Abstract
Permafrost peatlands store globally significant amounts of soil organic carbon (SOC) that may be vulnerable to climate change. Permafrost thaw exposes deeper, older SOC to microbial activity, but SOC vulnerability to mineralization and release as carbon dioxide is likely influenced by the soil environmental conditions that follow thaw. Permafrost thaw in peat plateaus, the dominant type of permafrost peatlands in North America, occurs both through deepening of the active layer and through thermokarst. Active layer deepening exposes aged SOC to predominately oxic conditions, while thermokarst is associated with complete permafrost thaw which leads to ground subsidence, inundation and soil anoxic conditions. Thermokarst often follows active layer deepening, and wildfire is an important trigger of this sequence. We compared the mineralization rate of aged SOC at an intact peat plateau (∼70 cm oxic active layer), a burned peat plateau (∼120 cm oxic active layer), and a thermokarst bog (∼550 cm anoxic peat profile) by measuring respired 14C–CO2. Measurements were done in fall when surface temperatures were near-freezing while deeper soil temperatures were still close to their seasonal maxima. Aged SOC (1600 yrs BP) contributed 22.1 ± 11.3% and 3.5 ± 3.1% to soil respiration in the burned and intact peat plateau, respectively, indicating a fivefold higher rate of aged SOC mineralization in the burned than intact peat plateau (0.15 ± 0.07 versus 0.03 ± 0.03 g CO2–C m−2 d−1). None or minimal contribution of aged SOC to soil respiration was detected within the thermokarst bog, regardless of whether thaw had occurred decades or centuries ago. While more data from other sites and seasons are required, our study provides strong evidence of substantially increased respiration of aged SOC from burned peat plateaus with deepened active layer, while also suggesting inhibition of aged SOC respiration under anoxic conditions in thermokarst bogs.
Highlights
Northern permafrost peatlands are global hot spots for soil organic carbon (C) storage, with up to 250 kg C m−2 and an estimated total storage of 150 Pg within the northern circumpolar permafrost region (Hugelius et al 2014)
Site description and experimental design Study sites were located in the discontinuous permafrost zone (Brown et al 1997) of western Canada (59.5°N, 117.2°W), and included a burned peat plateau affected by wildfire in 2007, an intact peat plateau not burned at least in the last 70 years, and an adjacent thermokarst bog where we differentiate between a developmentally young thermokarst bog site near its edge and a mature thermokarst bog site in its center
We identified the transition from Sphagnum to sylvic peat at the young and mature bog, which indicates the shift from peat plateau to thermokarst bog vegetation and the timing of collapse (O’Donnell et al 2012)
Summary
Northern permafrost peatlands are global hot spots for soil organic carbon (C) storage, with up to 250 kg C m−2 and an estimated total storage of 150 Pg within the northern circumpolar permafrost region (Hugelius et al 2014). The discontinuous permafrost zone of western Canada is a major peatland region with >150 000 km of permafrost peatlands (Hugelius et al 2014). Peatland development in this region started about 9000 years ago, but permafrost only started aggrading after the Holocene thermal maximum and became widespread following a climate cooling around 1200 years ago (Zoltai 1995, Pelletier et al 2017). Northern regions are rapidly warming (Johannessen et al 2004) and permafrost thaw (Payette et al 2004, Romanovsky et al 2010, Baltzer et al 2014) will expose vast stores of permafrost peatland C to microbial activity, and to potential mineralization and emission into the atmosphere as greenhouse gases carbon dioxide (CO2) and methane (CH4). The magnitude and timing of greenhouse gas emissions derived from recently thawed soil C represents a critical uncertainty for our understanding of the permafrost C feedback to climate change (Schuur et al 2015)
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