Page 216 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 9. Long-term warming effects on permafrost soil microbial communities
long-term. This was mainly assigned to the presence of more stable complex organic compounds that are more inert and are therefore degraded much slower (Dutta et al. 2006; Lee et al. 2012).
The formation of anoxic conditions is largely dependent on soil moisture levels (Lee et al. 2012; Treat et al. 2015), since oxygen diffusion is limited in water-saturated soils which has been shown in a previous study on polygonal tundra soils (Liebner et al. 2011). Increased water saturation is expected to coincide with (especially ice-rich) permafrost thaw, which stimulates wetland and thermokarst lake formation (Schuur et al. 2015). The gradual depletion of alternative terminal electron acceptors (e.g. NO2-/NO3-, Fe(III), SO42-) results in the formation of conditions suitable for methanogenesis (Achtnich, Bak and Conrad 1995; Lee et al. 2012; Wagner 2017). In our study, the ratio of CO2/CH4 was approaching 1, and CH4 production rate remained fairly constant after 300 days. This could indicate a depletion of alternative terminal electron acceptors. Similar observations were made by Knoblauch et al. (2018) where, on the long term, anoxic conditions also lead to stable though low rates of CH4 production. Although methanogenesis became more important relative to CO2 production, the abundance (both relative and absolute) of methanogenic archaea decreased, even though the relative abundance of hydrogenotrophic methanogens increased. This stresses the potential limitation of methanogenesis due to reduced substrate provision by fermentative microbial groups as well as a depletion of labile carbon. At the same time, it suggests that hydrogenotrophic methanogenesis is gaining in importance relative to acetoclastic methanogenesis in the course of long-term permafrost soil incubations. In more detail, methanogenesis is facilitated by the formation of a fermentative system that degrades organic matter into substrates such as acetate, H2 and CO2, but also formate and ethanol (Treat et al. 2015). Acetate formation has been observed in northern peatland, bog and fen environments (Duddleston et al. 2002; Hädrich et al. 2012; Hodgkins et al. 2014). The presence of acetate is linked to the presence of labile organic matter (Prater, Chanton and Whiting 2007; Ewing et al. 2015). Several studies have shown that acetate formation is stimulated by labile organic matter fractions of vascular plants (Chanton et al. 1995; Popp et al. 1999; Prater, Chanton and Whiting 2007). In our study, the first peak in CH4 production could therefore be linked with higher abundances of labile organic carbon which was present in situ. When labile organic matter pools get depleted, more stable
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