High-latitude ecosystems store approximately 1700 Pg of soil carbon (C), which is twice as much C as is currently contained in the atmosphere. Permafrost thaw and subsequent microbial decomposition of permafrost organic matter could add large amounts of C to the atmosphere, thereby influencing the global C cycle. The rates at which C is being released from the permafrost zone at different soil depths and across different physiographic regions are poorly understood but crucial in understanding future changes in permafrost C storage with climate change. We assessed the inherent decomposability of C from the permafrost zone by assembling a database of long-term (>1 year) aerobic soil incubations from 121 individual samples from 23 high-latitude ecosystems located across the northern circumpolar permafrost zone. Using a three-pool (i.e., fast, slow and passive) decomposition model, we estimated pool sizes for C fractions with different turnover times and their inherent decomposition rates using a reference temperature of 5 °C. Fast cycling C accounted for less than 5% of all C in both organic and mineral soils whereas the pool size of slow cycling C increased with C : N. Turnover time at 5 °C of fast cycling C typically was below 1 year, between 5 and 15 years for slow turning over C, and more than 500 years for passive C. We project that between 20 and 90% of the organic C could potentially be mineralized to CO2 within 50 incubation years at a constant temperature of 5 °C, with vulnerability to loss increasing in soils with higher C : N. These results demonstrate the variation in the vulnerability of C stored in permafrost soils based on inherent differences in organic matter decomposability, and point toward C : N as an index of decomposability that has the potential to be used to scale permafrost C loss across landscapes.

Permafrost is a distinct feature of the terrestrial Arctic and is vulnerable to climate warming. Permafrost degrades in different ways, including deepening of a seasonally unfrozen surface and localized but rapid development of deep thaw features. Pleistocene ice-rich permafrost with syngenetic ice-wedges, termed Yedoma deposits, are widespread in Siberia, Alaska, and Yukon, Canada and may be especially prone to rapid-thaw processes. Freeze-locked organic matter in such deposits can be re-mobilized on short time-scales and contribute to a carbon-cycle climate feedback. Here we synthesize the characteristics and vulnerability of Yedoma deposits by synthesizing studies on the Yedoma origin and the associated organic carbon pool. We suggest that Yedoma deposits accumulated under periglacial weathering, transport, and deposition dynamics in non-glaciated regions during the late Pleistocene until the beginning of late glacial warming. The deposits formed due to a combination of aeolian, colluvial, nival, and alluvial deposition and simultaneous ground ice accumulation. We found up to 130gigatons organic carbon in Yedoma, parts of which are well-preserved and available for fast decomposition after thaw. Based on incubation experiments, up to 10 of the Yedoma carbon is considered especially decomposable and may be released upon thaw. The substantial amount of ground ice in Yedoma makes it highly vulnerable to disturbances such as thermokarst and thermo-erosion processes. Mobilization of permafrost carbon is expected to increase under future climate warming. Our synthesis results underline the need of accounting for Yedoma carbon stocks in next generation Earth-System-Models for a more complete representation of the permafrost-carbon feedback

Increasing temperatures in northern high latitudes are causing permafrost to thaw 1 , making large amounts of previously frozen organic matter vulnerable to microbial decomposition 2 . Permafrost thaw also creates a fragmented landscape of drier and wetter soil conditions 3,4 that determine the amount and form (carbon dioxide (CO 2 ), or methane (CH 4 )) of carbon (C) released to the atmosphere. The rate and form of C release control the magnitude of the permafrost C feedback, so their relative contribution with a warming climate remains unclear 5,6 . We quantified the e ect of increasing temperature and changes from aerobic to anaerobic soil conditions using 25 soil incubation studies from the permafrost zone. Here we show, using two separate meta-analyses, that a 10 • C increase in incubation temperature increased C release by a factor of 2.0 (95% confidence interval (CI), 1.8 to 2.2). Under aerobic incubation conditions, soils released 3.4 (95% CI, 2.2 to 5.2) times more C than under anaerobic conditions. Even when accounting for the higher heat trapping capacity of CH 4 , soils released 2.3 (95% CI, 1.5 to 3.4) times more C under aerobic conditions. These results imply that permafrost ecosystems thawing under aerobic conditions and releasing CO 2 will strengthen the permafrost C feedback more than waterlogged systems releasing CO 2 and CH 4 for a given amount of C.High-latitude ecosystems store almost twice as much C in soils than what is contained in the atmosphere 7,8 . As the global climate warms, northern high latitudes are experiencing rapid increases in temperature 9 that have the potential to not only increase C emissions from previously frozen C in permafrost and the active layer 10 but also to indirectly affect the C cycle through changes in regional and local hydrology. Warmer temperatures increase thawing of icerich permafrost and the melting of ground ice, which causes the land surface to collapse into the space that was previously filled by ice resulting in thermokarst terrain 11 . Permafrost thawing can also gradually increase active layer thickness (seasonally thawed ground), causing poorly drained soil conditions in lowlands or drier conditions in uplands where natural drainage can increase 3 . On the other hand, permafrost thaw and collapse can cause soils to become waterlogged where anaerobic conditions prevail and C is released in the form of CO 2 and CH 4 . One major uncertainty in determining the climate forcing impact of permafrost ecosystems is understanding the relative magnitudes of the effects of shifting subsurface hydrology versus increasing temperatures on greenhouse gas release in permafrost ecosystems.In addition to soil temperature and moisture, the chemical composition (for example, carbon to nitrogen ratio) 12 , physical protection by soil minerals, microbial community dynamics, and other environmental controls, such as pH and nutrient availability, also impact the amount of C released to the atmosphere 13 . While temperature and soil moisture (that is, oxygen availability) a...

The currently observed Arctic warming will increase permafrost degradation followed by mineralization of formerly frozen organic matter to carbon dioxide (CO2 ) and methane (CH4 ). Despite increasing awareness of permafrost carbon vulnerability, the potential long-term formation of trace gases from thawing permafrost remains unclear. The objective of the current study is to quantify the potential long-term release of trace gases from permafrost organic matter. Therefore, Holocene and Pleistocene permafrost deposits were sampled in the Lena River Delta, Northeast Siberia. The sampled permafrost contained between 0.6% and 12.4% organic carbon. CO2 and CH4 production was measured for 1200 days in aerobic and anaerobic incubations at 4 °C. The derived fluxes were used to estimate parameters of a two pool carbon degradation model. Total CO2 production was similar in Holocene permafrost (1.3 ± 0.8 mg CO2 -C gdw(-1) aerobically, 0.25 ± 0.13 mg CO2 -C gdw(-1) anaerobically) as in 34 000-42 000-year-old Pleistocene permafrost (1.6 ± 1.2 mg CO2 -C gdw(-1) aerobically, 0.26 ± 0.10 mg CO2 -C gdw(-1) anaerobically). The main predictor for carbon mineralization was the content of organic matter. Anaerobic conditions strongly reduced carbon mineralization since only 25% of aerobically mineralized carbon was released as CO2 and CH4 in the absence of oxygen. CH4 production was low or absent in most of the Pleistocene permafrost and always started after a significant delay. After 1200 days on average 3.1% of initial carbon was mineralized to CO2 under aerobic conditions while without oxygen 0.55% were released as CO2 and 0.28% as CH4 . The calibrated carbon degradation model predicted cumulative CO2 production over a period of 100 years accounting for 15.1% (aerobic) and 1.8% (anaerobic) of initial organic carbon, which is significantly less than recent estimates. The multiyear time series from the incubation experiments helps to more reliably constrain projections of future trace gas fluxes from thawing permafrost landscapes.

The community structure of sulfate-reducing bacteria (SRB) of a marine Arctic sediment (Smeerenburgfjorden, Svalbard) was characterized by both fluorescence in situ hybridization (FISH) and rRNA slot blot hybridization by using group-and genus-specific 16S rRNA-targeted oligonucleotide probes. The SRB community was dominated by members of the Desulfosarcina-Desulfococcus group. This group accounted for up to 73% of the SRB detected and up to 70% of the SRB rRNA detected. The predominance was shown to be a common feature for different stations along the coast of Svalbard. In a top-to-bottom approach we aimed to further resolve the composition of this large group of SRB by using probes for cultivated genera. While this approach failed, directed cloning of probe-targeted genes encoding 16S rRNA was successful and resulted in sequences which were all affiliated with the Desulfosarcina-Desulfococcus group. A group of clone sequences (group SVAL1) most closely related to Desulfosarcina variabilis (91.2% sequence similarity) was dominant and was shown to be most abundant in situ, accounting for up to 54.8% of the total SRB detected. A comparison of the two methods used for quantification showed that FISH and rRNA slot blot hybridization gave comparable results. Furthermore, a combination of the two methods allowed us to calculate specific cellular rRNA contents with respect to localization in the sediment profile. The rRNA contents of Desulfosarcina-Desulfococcus cells were highest in the first 5 mm of the sediment (0.9 and 1.4 fg, respectively) and decreased steeply with depth, indicating that maximal metabolic activity occurred close to the surface. Based on SRB cell numbers, cellular sulfate reduction rates were calculated. The rates were highest in the surface layer (0.14 fmol cell ؊1 day ؊1 ), decreased by a factor of 3 within the first 2 cm, and were relatively constant in deeper layers.

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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...

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