Rapid carbon loss and slow recovery following permafrost thaw in boreal peatlands

Abstract

Permafrost peatlands store one-third of the total carbon (C) in the atmosphere and are increasingly vulnerable to thaw as high-latitude temperatures warm. Large uncertainties remain about C dynamics following permafrost thaw in boreal peatlands. We used a chronosequence approach to measure C stocks in forested permafrost plateaus (forest) and thawed permafrost bogs, ranging in thaw age from young (<10 years) to old (>100 years) from two interior Alaska chronosequences. Permafrost originally aggraded simultaneously with peat accumulation (syngenetic permafrost) at both sites. We found that upon thaw, C loss of the forest peat C is equivalent to ~30% of the initial forest C stock and is directly proportional to the prethaw C stocks. Our model results indicate that permafrost thaw turned these peatlands into net C sources to the atmosphere for a decade following thaw, after which post-thaw bog peat accumulation returned sites to net C sinks. It can take multiple centuries to millennia for a site to recover its prethaw C stocks; the amount of time needed for them to regain their prethaw C stocks is governed by the amount of C that accumulated prior to thaw. Consequently, these findings show that older peatlands will take longer to recover prethaw C stocks, whereas younger peatlands will exceed prethaw stocks in a matter of centuries. We conclude that the loss of sporadic and discontinuous permafrost by 2100 could result in a loss of up to 24 Pg of deep C from permafrost peatlands.

Inconsistent Response of Arctic Permafrost Peatland Carbon Accumulation to Warm Climate Phases

“…Moreover, a long‐term hiatus in the accumulation record may occur, as has been detected in the age‐depth model of Sei2 (see also Routh et al, ; Sannel et al, ). In addition, the very low CARs, when followed by thaw, could also be the results of peat decomposition of the formerly frozen peat layers upon thaw (see Jones et al, ; O'Donnell et al, ). In contrast, permafrost thaw may have resulted in also very high ACARs (>50 g C m −2 year −1 ) as recorded in Ind4 and Ind5, where Sphagnum is encouraged to grow in situ by the hydrological changes brought about by warming (Zhang et al, ).…”

“…Intense permafrost thawing eventually causes peatland collapse and saturation with thaw water potentially leading to increase in carbon sequestration through increased NPP and decreased CO 2 emissions, although wetting promotes CH 4 emissions (Swindles et al, ). Also some studies suggest that critical loss of sporadic and discontinuous permafrost in the coming century may lead to a loss of the large deep C storage (Jones et al, ; Schuur et al, ). Thereby in the future the net effect can either be an increase or a reduction in ACARs, or even turning the site to a net C source (Chaudhary et al, ; Wang et al, ; Zhang et al, ).…”

“…Interlinked changes in temperature and moisture conditions may trigger shifts in vegetation composition (e.g., Zhang et al, ) and consequently cause significant changes in C accumulation patterns (Charman et al, ; Treat et al, ). Moreover, permafrost thaw may expose substantial quantities of old stored organic C to decomposition (Jones et al, ; O'Donnell et al, ). This could potentially be released to the atmosphere as carbon dioxide (CO 2 ) and/or methane (CH 4 ), leading to a positive climate feedback (Hodgkins et al, ).…”

Long‐term Impacts of Permafrost Thaw on Carbon Storage in Peatlands: Deep Losses Offset by Surficial Accumulation

“…Inundation results in anaerobic conditions associated with elevated emissions of CH 4 (Wickland et al, ) but can also restrict respiration of old C that previously was respired aerobically in the active layer (Schädel et al, ) further enhancing C accumulation. Despite high accumulation rates of surface peat following thaw, studies along Alaskan thaw chronosequences indicated large net losses of C in the initial decades to centuries following thaw, in the order of <500 to 3,500 g C m −2 yr −1 during the first decade following thaw (Jones et al, ; O'Donnell et al, ). However, various field studies from areas with different permafrost and developmental histories have found no direct evidence of losses of old soil C as CO 2 or CH 4 following thaw at the magnitude that would be necessary for such large net C losses (Cooper et al, ; Estop‐Aragonés, Cooper, et al, ; Estop‐Aragonés, Czimczik, et al, ; Klapstein et al, 2014).…”

Greenhouse gas balance over thaw‐freeze cycles in discontinuous zone permafrost

“…In contrast, poorly drained peatland permafrost sites typically thaw and then flood, forming collapse scar wetlands that are dominated by anaerobic conditions [ Bellisario et al , ]. Poorly drained syngenetic permafrost sites may also experience high C loss rates [e.g., O'Donnell et al , ] or decreased C accumulation following thaw [ Jones et al , , ] as large stores of previously (and rapidly) frozen (i.e., microbially unavailable) labile C become accessible [ Schädel et al , ]. However, at our site, the bulk of the peat was apparently established under wetland conditions prior to the initiation of permafrost [ Zoltai , ].…”