Methane production in thawing permafrost can be substantial, yet often evolves after long lag phases or is even lacking. A central question is to which extent the production of methane after permafrost thaw is determined by the initial methanogenic community. We quantified the production of methane relative to carbon dioxide (CO2) and enumerated methanogenic (mcrA) gene copies in long-term (2-7 years) anoxic incubations at 4 degrees C using interglacial and glacial permafrost samples of Holocene and Pleistocene, including Eemian, origin. Changes in archaeal community composition were determined by sequencing of the archaeal 16S rRNA gene. Long-term thaw stimulated methanogenesis where methanogens initially dominated the archaeal community. Deposits of interstadial and interglacial (Eemian) origin, formed under higher temperatures and precipitation, displayed the greatest response to thaw. At the end of the incubations, a substantial shift in methanogenic community composition and a relative increase in hydrogenotrophic methanogens had occurred except for Eemian deposits in which a high abundance of potential acetoclastic methanogens were present. This study shows that only anaerobic CO2 production but not methane production correlates significantly with carbon and nitrogen content and that the methanogenic response to permafrost thaw is mainly constrained by the paleoenvironmental conditions during soil formation.

The organic carbon of permafrost affected soils is receiving particular attention with respect to its fate and potential feedback to global warming. The structural and activity changes of methanogenic communities in the degrading permafrost-affected wetlands on the Tibetan Plateau can serve as fundamental elements for modelling feedback interaction of ecosystems to climate change. Hence, we aimed at anticipating if and how the rapid environmental changes occurring especially on the high altitude Tibetan platform will affect methanogenic communities. We identified methanogenic community composition, activity and abundance in wetland soils with different hydrological settings, permafrost extent and soil properties and pinpoint the environmental controls. We show that despite a pronounced natural gradient, the Tibetan high elevation wetland soils host a large methanogenic core microbiome. Hydrogenotrophic methanogens, in particular Methanoregula, and H2-dependent methanogenesis were overall dominant although acetoclastic methanogens in addition to hydrogenotrophs were among the dominating taxa in a minerotrophic fen. Tracing the Methanoregula community of the Tibetan Plateau using public databases revealed its global relevance in natural terrestrial habitats. Unlike the composition, the activity and abundance of methanogens varied strongly in the studied soils with higher values in alpine swamps than in alpine meadows. This study indicates that in the course of current wetland and permafrost degradation and the loss in soil moisture, a decrease in the methane production potential is expected on the high Tibetan Plateau but it will not lead to pronounced changes within the methanogenic community structure. (C) 2017 The Authors. Published by Elsevier Ltd.