| Summary: | Soils in the permafrost region have acted as carbon sinks for thousands of years. However, as a result of global warming, permafrost soils are thawing and will potentially release more greenhouse gases (GHGs) such as methane (CH4) and carbon dioxide (CO2). To address the large heterogeneities of GHG releases, this study focused on the relationship between CO2 and CH4 emissions and soil parameters, as well as the evolution of microbial abundance during a permafrost thaw experiment representing the extent of an Arctic summer season. Two depths from three Lena Delta cores taken along a transect from upland to floodplain were incubated anoxically for 68 days at two different temperatures (4 °C and 20 °C) and an assessment of microbiological abundance (CH4 producers and aerobic CH4 oxidizers) was performed in parallel. Samples from located in upland or slope position remained in a lag phase during the whole incubation, while those from located in the floodplain showed high production of CH4 (6.5x103 µgCH4-C.gC-1) and CO2 (6.9x103 µgCO2-C.gC-1). Periodic flooding likely allowed the establishment of favorable methanogenic conditions. The presence of higher copy numbers of methanogenic archaea in the active layer of the floodplain than in the upland and slope from the beginning (1.5 to 9.6 times higher) until the end of the incubation time (11 to 700 times higher) supported this hypothesis. In addition, our study pointed out different anaerobic CO2 production (methanogenesis and other respiration) pathways according to landscape position.
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