Unraveling the Complex Drivers of CO2 and CH4 Flux in Permafrost Soils

Abstract:

Permafrost contains large stocks of organic carbon (C) that are vulnerable to decomposition following thaw, which could increase greenhouse gas (GHG) emissions leading to a potential C-climate feedback. Despite their global importance, GHG emissions from thawing permafrost are difficult to predict due to their complex mechanisms. The objective of this study was to determine the mechanisms controlling GHG flux from permafrost soil, comparing CH₄ and CO₂ production. We simulated permafrost thaw under drained and anoxic conditions at 1 and 15 °C, and measured CH₄ and CO₂ production. We also measured soil chemical and biological parameters (e.g. mid-infrared spectroscopy, iron speciation, soil redox, and next generation sequencing of the 16S gene).

All treatments produced considerable amounts of CO₂ (oxic, 15 °C: 0.3-2.0 mg CO₂-C gdws^-1). CH₄ production was highly variable (anoxic, 15 °C: 0.4-67 μg CH₄-C gdws^-1), which was not explained by soil C content (2-603 μg CH₄ g soil C^-1). We explored the reasons behind this seemingly random variability in CH₄ production, and found that it can be explained by the activity of non-methanogenic anaerobes and substrate supply. For example, we found that the activity of iron reducers improved the fit of CH₄ production model, reducing second order bias correction (AICc) from 80 to 38, as did a gross measure of anaerobic activity (AICc reduced from 80 to 60), however neither was statistically significant (p>0.05). In methanogenesis, the lability, rather than the total chemistry of the dissolved organic matter, was important for determining gas production, but the opposite was found to be important for predicting CO₂ production. Differences in methanogen populations likely also contributed to the variability in the CH₄ production, and further analysis of the 16S gene abundances will elucidate this. In summary, production of CH₄ depends not only on the methanogens themselves, but also on the activity of the non-methanogenic anaerobes and the lability of the DOM. Thus, methanogenesis should be viewed as a process dependent on a consortium of anaerobes. Because the prediction of GHG fluxes is critical to our understanding of the impact of permafrost C on the global C cycle, a more mechanistic understanding of the processes governing anaerobic GHG flux will be crucial.