Potential carbon emissions dominated by carbon dioxide from thawed permafrost soils

Abstract

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...

Temperature and moisture effects on greenhouse gas emissions from deep active-layer boreal soils

“…Using two metanalyses of aerobic and anaerobic permafrost soil incubations, Schädel et al (2016) showed that C release was highly sensitive to temperature and that soils released far more (220-520 %) C under aerobic conditions. Our incubation was fully aerobic, but its results are consistent with the conclusion that respiration in the form of CO 2 is likely to dominate the high-latitude C feedback, and that aerobic soils, and the conditions under which currently waterlogged soils may drain, deserve particular attention.…”

“…The Q 10 values observed in this experiment were low (all less than 2.0, even when controlling for changes in soil moisture). Temperature sensitivities of ∼ 2 are more typical (Dutta et al, 2006;Schädel et al, 2016), although the temperature sensitivity of C release can change over time of incubation (Dutta et al, 2006) and vary between soil fractions cycling over different time horizons (Karhu et al, 2010;Schädel et al, 2014). Observed surface CO 2 fluxes at this CPCRW site exhibited a Q 10 of 5.1 ± 1.4 over a temperature range of 3.5-15 • C (C. Anderson, personal communication, 2016); however, these surface fluxes were measured over multiple months and include root respiration preventing any direct comparison.…”

“…(Kim et al, 2012). The common anaerobic conditions following permafrost thaw are expected to lower CO 2 emissions but increase those of CH 4 (Treat et al, , 2014, but emissions from aerobic soils will likely dominate the permafrost C feedback (Schädel et al, 2016). Decadal warming and drying trends in Alaska (Bieniek et al, 2014) thus seem likely to increase GHG emissions from soils, and laboratory incubation experiments are critical in understanding these dynamics .…”

Increased nitrous oxide emissions from Arctic peatlands after permafrost thaw

“…Furthermore, the positive climate change feedback of N 2 O will be stronger under aerobic conditions than under anaerobic conditions. Because N 2 O has an almost 300 times stronger global warming potential than CO 2 on a 100-y time horizon (12), a postthaw N 2 O release would further enhance the radiative forcing stemming from C gases (8,9).…”

“…With thawing, a vast pool of immobile C stored in permafrost (7) becomes available for decomposition and remobilization, triggering greenhouse gas emissions of carbon dioxide (CO 2 ) and methane (CH 4 ). Thus, gaseous C release from thawing permafrost is being studied extensively to evaluate the magnitude of the permafrost-carbon feedback to the climate (8,9). Often overlooked, however, is the fact that permafrost soils are also large N reservoirs, with a conservative estimate of 67 billion tons of total N in the upper 3 m (10).…”