Page 199 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Introduction
Permafrost, which is classified as ground that stays frozen for at least two consecutive years, is widespread in the Arctic and subarctic regions. Permafrost-affected soils in these regions store approximately 1300 Pg carbon, which equals 50% of the global belowground organic carbon, and the major fraction (~ 1000 Pg) is stored in the upper three meters of soil (Hugelius et al. 2014). Over the last 30 years, high-latitude areas have warmed at a rate of 0.6°C per decade, which is twice as fast as the global average (IPCC 2013). Modeled extremes predict a temperature increase of up to 7-8°C by the end of this century (IPCC 2007).
Consequently, the thawing of permafrost exposes large organic carbon stocks to decomposition by soil microorganisms (Schuur et al. 2015). This in turn could release the sequestered frozen long-term carbon stocks into the atmosphere as the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4) (Dean et al. 2018; Knoblauch et al. 2018). Although recent data on carbon isotopes implies that CH4 derived from older carbon substrates is released relatively slowly, the increasing decomposition of permafrost carbon leads to a net positive contribution to the global atmospheric GHG budget (Douglas et al. 2020). This release initiates a positive climate feedback loop (Schuur et al. 2008; Pries, Schuur and Crummer 2013; Hayes et al. 2014; Anthony et al. 2018).
The active layer of permafrost-affected soils is exposed to seasonal freeze-thaw cycles, while the underlying permafrost is characterized by year-round below-zero temperatures and low water availability. This means that local conditions differ between layers characterized by seasonal thaw, episodic thaw and permafrost. The uppermost part of the permafrost, which is called the transition layer, is more prone to thaw than deeper permafrost. This layer differs in cryo-features, carbon, and moisture content from the underlying permafrost and is irregularly exposed to thaw (Ping et al. 2015). In a warming climate, permafrost carbon emissions interplay with local temperature changes, ground conditions, and hydrology over decadal to centennial time scales (Schuur et al. 2015). During thaw, water accumulation can lead to rapid gas diffusion limitation and a subsequent depletion of oxygen in the deep active layer, whereas drainage of melted water allows oxygen penetration into deeper soils. Thus, hydrology plays an important role in regulating the soil redox potential and hence the conditions for aerobic and anaerobic microbial metabolism (Mackelprang et al. 2016). In addition, changes in soil
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