Page 36 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 1. General introduction
Recent modeling work suggests that the CH4 climate feedback through to the year 2100 from permafrost regions may be relatively small compared to other feedbacks (Gao et al. 2013; Schaefer et al. 2014; Lawrence et al. 2015). However, these studies use models that likely do not adequately represent microbial responses in the Arctic. Modeling based on microbial dynamics predicts a much larger feedback from permafrost environments (30–90 Tg CH4 yr−1) (McCalley et al. 2014) than models lacking microbial activity (6–15 Tg CH4 yr−1) (Gao et al. 2013); incorporating deep C deposits and thermokarst activity into non-microbial models produces a slightly higher range (8–26 Tg CH4 yr−1) by 2100 (Schneider von Deimling et al. 2015b). These large-scale physically-based models also do not account for small-scale processes such as anoxic and oxic zones within soil pores that are temporally variable, yet may play an important role in microbial CH4 production and oxidation (Ebrahimi and Or 2017), or local-scale processes such as thaw-induced land subsidence. These processes may be important at regional scales in balancing soil drying (promoting CH4 oxidation) and soil wetting (promoting CH4 production) (Koven et al. 2015; Helbig et al. 2017). The potential strength of the permafrost CH4 feedback thus remains uncertain up to 2100 and beyond. A thorough understanding of the microbial mechanisms is needed to better quantify their contributions to the global GHG budget. A substantial lack of data on microbial life within these ecosystems highlights an important knowledge gap that hampers accurate model predictions on how warm the Earth will be in the future. A major part of this thesis focuses on the effects of warming on permafrost ecosystems.
Conclusions and thesis outline
There is considerable uncertainty on how natural CH4 production and consumption processes will be affected by a combination of altering environmental factors in response to climate change. This thesis focuses on potential changes of natural CH4 cycling under human interference and future warming scenarios. This work explores the role of these changes across a range of natural and manmade environments.
Chapter 2 introduces the diversity of CH4 cycling microorganisms. These organisms co-occur in a wide variety of both natural and manmade ecosystems. Globally, methanogens are
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