Methane production as key to the greenhouse gas budget of thawing permafrost

Abstract

9 6 Russian Academy of Sciences, Siberian Branch, Mel'nikov Permafrost Institute, Yakutsk, Russia 10 11Permafrost thaw liberates frozen organic carbon, which is decomposed to carbon 12 dioxide (CO 2 ) and methane (CH 4 ). The release of these greenhouse gases (GHGs) 13 forms a positive feedback to atmospheric CO 2 and CH 4 concentrations and accelerates 14 climate change 1, 2 . Current studies report a minor importance of CH 4 production in 15 water-saturated (anoxic) permafrost soils 3,4,5,6 and a stronger permafrost carbon-16 climate feedback from drained (oxic) soils 1, 7 . Here we show through 7-year laboratory 17 incubations that equal amounts of CO 2 and CH 4 are formed in thawing permafrost 18 under anoxic conditions after stabile CH 4 -producing microbial communities have 19 established. Less permafrost carbon was mineralized under anoxic conditions but 20 more CO 2 -C equivalents were formed than under oxic conditions when taking the 21 higher global warming potential (GWP) of CH 4 into account 8 . An organic carbon 22 decomposition model, calibrated with the observed decomposition data, predicts until 23 2100 a higher loss of permafrost carbon under oxic conditions (113±58 g CO 2 -C kgC -1 ) 24 but a twice as high production of CO 2 -C equivalents (241±138 g CO 2 -C-eq. kgC -1 ) under 25 anoxic conditions. These findings challenge the view of a stronger permafrost carbon-26 climate feedback from drained soils 1, 7 and emphasize the importance of CH 4 27 production in thawing permafrost on climate relevant time scales. 28To test these hypotheses, we combined long-term incubation studies (> 7 years) of 56 permafrost samples with numerical modelling and simulated both oxic and anoxic GHG 57 production from thawing permafrost until 2100. Permafrost samples (n=29) from two sites in 58 northeast Siberia (Holozene river deposits, Pleistocene Yedoma sediments) were incubated 59 under oxic and anoxic conditions at 4 °C (ref 15). Samples were pre-incubated for four years 60 until constant CH 4 production rates were recorded in most of the anoxic samples before the 61 start of the main experiment spanning another three years (Suppl. Fig. S1). 62Maximum CO 2 production rates were observed both under oxic and anoxic conditions at the 63 beginning of the pre-incubation phase 15 but anoxic CH 4 production only started after a lag-64 phase lasting from few weeks up to several years. The multi-annual lag-phases indicate a 65 low abundance of methanogenic communities in the original samples, which may be caused 66 by permafrost formation under conditions not suitable for methanogenesis, e.g. in dry soils 16, 67 17 . However, these communities were activated after suitable conditions prevailed for long 68 enough time. Only four samples showed no CH 4 production even after seven incubation 69 years (Suppl. Table S1). The gene-copy numbers of the key enzyme of methanogenic 70 archaea (mcrA) were below the detection limit in these four samples (Suppl . Table S2). 71When inoculating these inactive samples with permafrost mate...

Increasing Organic Carbon Biolability With Depth in Yedoma Permafrost: Ramifications for Future Climate Change

“…C‐CO 2 and net C production potentials were higher in the modern AL than in the underlying TL sample. While yedoma OC has been previously shown to be highly biolabile upon thaw (Dutta et al, ; Knoblauch et al, , ; Vonk et al, ), our results demonstrate that GHG production from thawing yedoma permafrost OC increases with depth. Our results suggest that OC from thawed yedoma sediments will produce more GHG per unit OC as permafrost thaw depths increase (4.1 times more GHG production at 12‐ versus 9‐m depth at 3 °C and 9.9 times more GHG production at 13 °C).…”

Nutrient Release From Permafrost Thaw Enhances CH4 Emissions From Arctic Tundra Wetlands

“…We also provide circumstantial evidence that decadal‐time scale expansion of this common emergent macrophyte community is coincident with increased available nutrients in aquatic systems (Lougheed et al, ), which may have occurred in response to recent permafrost thaw. Verified by historical estimates of increasing CH 4 emissions (Andresen et al, ; Lara et al, ), recent permafrost thaw has likely increased regional CH 4 emission, but the magnitude of future change will likely be related to the extent of regional permafrost thaw and the rate of stabilization of new methanogenic microbial communities (Knoblauch et al, ). The long‐term monitoring legacy in aquatic tundra sites near Utqiaġvik (Villarreal et al, ) and across the Arctic (Callaghan et al, ) may not only represent a detailed archive of decadal‐time scale vegetation change but a well‐documented historical record that may be capable of estimating the timing of permafrost degradation and subsequent ecosystem impacts.…”

Ecosystem carbon response of an Arctic peatland to simulated permafrost thaw

“…However, hydrological and landscape changes associated with permafrost thaw and related changes in C release are difficult to predict, thereby posing a large uncertainty in predicting the future C balance in these regions. Currently, we lack sufficient understanding of the ratio of CO 2 vs. CH 4 of total gaseous C release, which will have important consequences for radiative forcing due to a ~30 times higher warming potential of CH 4 per mass unit (Knoblauch, Beer, Liebner, Grigoriev, & Pfeiffer, ; Myhre et al, ; Schädel et al, ; Schuur et al, ). An increasing number of studies show that permafrost thaw promotes CO 2 and CH 4 production potential in soils (Hodgkins et al, ; Schädel et al, ).…”

“…Hence, the atmospheric C release from permafrost soils is anticipated to increase substantially as permafrost thaws (Koven et al, ; Schneider von Deimling et al, ; Schuur et al, ; Zhuang et al, ). While the largest C losses are expected to occur when thawing takes place under oxic conditions due to the proportionally larger production of CO 2 vs. CH 4 (Lee, Schuur, Inglett, Lavoie, & Chanton, ; Schädel et al, ; Schuur et al, ; Treat et al, ), recent long‐term incubations predict an equally large permafrost–C feedback from anoxic soils, promoted by CH 4 production (Knoblauch et al, ).…”