Abstract
Permafrost stores globally significant amounts of carbon (C) which may start to decompose and be released to the atmosphere in form of carbon dioxide (CO2) and methane (CH4) as global warming promotes extensive thaw. This permafrost carbon feedback to climate is currently considered to be the most important carbon-cycle feedback missing from climate models. Predicting the magnitude of the feedback requires a better understanding of how differences in environmental conditions post-thaw, particularly hydrological conditions, control the rate at which C is released to the atmosphere. In the sporadic and discontinuous permafrost regions of north-west Canada, we measured the rates and sources of C released from relatively undisturbed ecosystems, and compared these with forests experiencing thaw following wildfire (well-drained, oxic conditions) and collapsing peat plateau sites (water-logged, anoxic conditions). Using radiocarbon analyses, we detected substantial contributions of deep soil layers and/or previously-frozen sources in our well-drained sites. In contrast, no loss of previously-frozen C as CO2 was detected on average from collapsed peat plateaus regardless of time since thaw and despite the much larger stores of available C that were exposed. Furthermore, greater rates of new peat formation resulted in these soils becoming stronger C sinks and this greater rate of uptake appeared to compensate for a large proportion of the increase in CH4 emissions from the collapse wetlands. We conclude that in the ecosystems we studied, changes in soil moisture and oxygen availability may be even more important than previously predicted in determining the effect of permafrost thaw on ecosystem C balance and, thus, it is essential to monitor, and simulate accurately, regional changes in surface wetness.
Highlights
We investigated the contribution of deep soil organic carbon (SOC)-derived CO2 using radiocarbon measurements by sampling in the peatland plateau and at the wetland margin (Fig. 1a), where the greatest rates of previously-frozen C release were expected (Jones et al, 2016)
In the ecosystems we studied, our results provide direct field evidence that oxic conditions are required for high rates of previouslyfrozen SOC release, and that post-thaw soil moisture is a key control of the permafrost C feedback to climate in boreal forests and peatlands (Schädel et al, 2016; Schuur et al, 2015)
We studied fire-induced permafrost thaw in well-drained forests sites and thaw resulting in the formation of collapse wetlands in peatland sites
Summary
Soils in the northern circumpolar permafrost region (17.8 × 106 km 0 m depth) represent the largest terrestrial carbon store, containing > 1000 Pg C (Hugelius et al, 2014; Tarnocai et al, 2009), which has accumulated over thousands of years (Gorham et al, 2007; Harden et al, 1992; Mackay, 1958; Zoltai, 1995). Estop-Aragonés et al.2008; Camill, 2005; Harden et al, 2012), decompose and enter the atmosphere as CO2 or CH4, potentially exacerbating climate change (Schuur et al, 2015). This permafrost carbon feedback is missing in Earth system models (Ciais et al, 2013) and its inclusion may result in high-latitude ecosystems being predicted to become sources rather than sinks of C during the 21st century (Koven et al, 2011). The magnitudes and timings of soil organic carbon (SOC) loss from permafrost are highly uncertain, with estimates of 37–347 Pg C by 2100 (Schaefer et al, 2014). Accurately projecting future rates of CO2 release from permafrost is essential for predicting the magnitude of this feedback
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