Page 221 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

several studies on permafrost (Steven et al. 2007; Lipson et al. 2013). Overall, an increase in dissimilatory sulfate reduction gene abundance was observed for AL and TL, whereas a decrease was found for PF. For assimilatory sulfate reduction, a general decrease could be explained by an overall decreased availability of carbon, which suppresses growth. An overall decrease in sulfate reduction genes is obvious due to the expected depletion of sulfate under anoxic conditions. However, cryptic sulfur cycling that can be linked to iron could supply oxidized sulfur species (Holmkvist, Ferdelman and Jørgensen 2011). Overall, studies on sulfur cycling in permafrost indicate that changes in redox conditions after thaw stimulate sulfate reduction (Mackelprang et al. 2016). This is in line with a study on the effects of fire thaw found that sulfate reduction genes were more abundant in anoxic deep soil layers (Tas et al. 2014). However, on the long-term it seems that dissimilatory sulfate reduction is mainly limited by sulfate availability, but experimental evidence is needed to support our observations.
The assumption that initial GHG fluxes and the short-term thaw response are associated with
the labile organic matter pool while long-term GHG fluxes are controlled by the slower degradable organic matter pool agrees with the CAZy-based analysis. This analysis revealed a substantial increase of functional traits which are potentially involved in degrading cellulose
and hemicellulose. This links to scenarios in which the microbial community is able to adapt to anaerobic utilization of less labile organic carbon from dead plant material after long-term thaw
exposure of permafrost (Mayumi et al. 2016; Yu et al. 2018). Similarly, an increased lignin 9 decomposition contributed by Proteobacteria was revealed in active layer soils of Arctic tundra
soils after depleting soil labile C through a 975-day laboratory incubation (Tao et al. 2020). Through these microbial functional modulations, permafrost thaw and long-term exposure to warming does result in steady GHG releases, even at low temperatures. Further temperature increases may increase GHG production rates given that less degradable carbon is particularly sensitive to warming (Davidson and Janssens 2006).
Conclusions
This study used an anaerobic incubation system to investigate the potential of microbial response to environmental changes in which labile carbon is steadily depleted, and older carbon
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