Page 202 - 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
thaw and long-term warming of permafrost soils, we hypothesize that the microbial communities adapt to the depletion of available carbon and alternative terminal electron acceptors, through taxonomic and functional shifts, in spite of the heterogeneity of initial geochemistry and the microbial community between active layer and permafrost. To test this hypothesis, we conducted a lab scale incubation experiment at 4°C for over five years under anoxic conditions. Full metagenomic sequencing at the start and end of the incubation was coupled to measurements of geochemistry as well as CO2 and CH4 production rates. The results can provide insights into the effects of boundary conditions on GHG production for future modeling studies.
Materials and methods
Study site and sampling
The study site was located on Samoylov Island (72° 22′ N, 126° 30’ E), in the Lena River Delta in Northeast Siberia, Russia. Samoylov Island developed during the Holocene and is underlain by continuous permafrost. The mean annual air temperature (MAAT) is -12.5°C (1998-2011) and mean annual precipitation is 125 mm, and the mean soil temperatures (MST) is 4.1°C at depth of 20 cm in the warmest month of August (mean air temperature 8.5°C in August) (Boike et al. 2013). On the island, ice-wedge polygons were extensively developed with low-lying polygon centers and elevated polygon rims on the surface. The dominating vascular plant species in the polygon centers is the sedge Carex aquatilis (Schwamborn, Rachold and Grigoriev 2002; Kutzbach, Wagner and Pfeiffer 2004). Samples were taken from a polygon center in spring 2011 when the soil was entirely frozen. The location and sampling procedures of the polygon used for this study were described before (Walz et al. 2017). According to our previous study, oxygen became depleted in the upper centimeters due to water logging (Liebner et al. 2011). Limited by remoteness, logistics and harsh cold sampling season, samples retrieved from the field in the frozen season are scarce, therefore limiting the use in multiple experiments and replication studies. In this study, samples from the active layer (AL, 15-22 cm), transition layer (TL, 30-34 cm) and permafrost (PF, 42-51 cm) were used for incubations. For this study, we performed GHG flux analyses (three biological replicates for GHG production), chemical analysis (two technical replicates), and a time-series of molecular analyses (three samples for
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