Abstract
AbstractThe decomposition of thawing permafrost organic matter (OM) to the greenhouse gases (GHG) carbon dioxide (CO2) and methane forms a positive feedback to global climate change. Data on in situ GHG fluxes from thawing permafrost OM are scarce and OM degradability is largely unknown, causing high uncertainties in the permafrost‐carbon climate feedback. We combined in situ CO2 and methane flux measurements at an abrupt permafrost thaw feature with laboratory incubations and dynamic modeling to quantify annual CO2 release from thawing permafrost OM, estimate its in situ degradability and evaluate the explanatory power of incubation experiments. In July 2016 and 2019, CO2 fluxes ranged between 0.24 and 2.6 g CO2‐C m−2 d−1. Methane fluxes were low, which coincided with the absence of active methanogens in the Pleistocene permafrost. CO2 fluxes were lower three years after initial thaw after normalizing these fluxes to thawed carbon, indicating the depletion of labile carbon. Higher CO2 fluxes from thawing Pleistocene permafrost than from Holocene permafrost indicate OM preservation for millennia and give evidence that microbial activity in the permafrost was not substantial. Short‐term incubations overestimated in situ CO2 fluxes but underestimated methane fluxes. Two independent models simulated median annual CO2 fluxes of 160 and 184 g CO2‐C m−2 from the thaw slump, which include 25%–31% CO2 emissions during winter. Annual CO2 fluxes represent 0.8% of the carbon pool thawed in the surface soil. Our results demonstrate the potential of abrupt thaw processes to transform the tundra from carbon neutral into a substantial GHG source.
Highlights
Frozen ground of the northern hemisphere contains about 800 Pg organic carbon (Hugelius et al, 2014), which has been protected from decomposition for hundreds to thousands of years KNOBLAUCH ET AL.Journal of Geophysical Research: BiogeosciencesSusanne Liebner, Axel Steinhof, Mikhail N
We found that greenhouse gas fluxes were dominated by CO2 and that the minor importance of methane was due to the absence of methane producing microorganisms in the Pleistocene permafrost
The aims of the current study were to (a) quantify the in situ decomposability of Pleistocene and Holocene permafrost organic matter (OM), (b) evaluate how far data from short- and long-term laboratory incubations may be used to predict in situ CO2 and methane fluxes, and (c) quantify the amount of permafrost OM released as CO2 during one year
Summary
Frozen ground (permafrost) of the northern hemisphere contains about 800 Pg organic carbon (Hugelius et al, 2014), which has been protected from decomposition for hundreds to thousands of years KNOBLAUCH ET AL.Journal of Geophysical Research: BiogeosciencesSusanne Liebner, Axel Steinhof, Mikhail N. Grigoriev Methodology: Christian Beer, Alexander Schuett, Lewis Sauerland, Susanne Liebner, Axel Steinhof, Janet Rethemeyer Project Administration: Mikhail N. Another 500 Pg organic carbon are stored in the seasonally unfrozen surface active layer of permafrost soils and perennially unfrozen sediments (Hugelius et al, 2014). The GHG release from thawing permafrost is seen as one of the major tipping elements in the climate due to the related positive feedback mechanism (Beer, 2008; Heimann & Reichstein, 2008). Still it is one of the key uncertainties when simulating the future global carbon cycle and climate. Model simulations on the fate of soil carbon in the permafrost region differ vastly between a net carbon gain of about 50 Pg and a loss of 140 Pg carbon until 2100 (McGuire et al, 2018)
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