Page 250 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 11. Integration and outlook
Especially the potential for increased CH4 production can have pronounced effects on the future climate. However, to fully represent the heterogeneity of these ecosystems, we need to expand the studies to include more lake and soil cores. As stressed in Chapter 6, we need extensive and representative datasets that do cover the diversity of these ecosystems in terms of microbial diversity, GHG production potential, and sensitivity to climate change.
We are only at the beginning of understanding the crucial role of microorganisms in the CH4 feedback loops of a warming Arctic. Currently, evidence is building up that warming will disproportionately induce microbial CH4 production over CH4 consumption, with all the potential consequences this entails (Anisimov 2007; Coolen and Orsi 2015; Helbig et al. 2017; Dean et al. 2018). This is indeed also what we observed in our studies on the warming- and substrate-induced methane cycling potential of Alaskan thermokarst lakes in Chapter 7. In the five-year anoxic warming experiment of permafrost soils described in Chapter 9, we saw that, even though GHG production rates are low, stable methanogenic conditions are being reached on the long term. Such trends have also been shown on decadal timescales. An 11-year-long study on artificial ponds highlighted a disproportionate increase of methanogenesis over methanotrophy, which exceeded temperature-based predictions (Zhu et al. 2020). Disproportionate increases of CH4 emissions would result in a feedback loop that would be even stronger than currently estimated.
It, however, remains an open question whether such an expected disproportionate increase in CH4 production will really occur in the field. The concept of the permafrost “methane bomb” has been discussed in literature (Zhiliba et al. 2011; Anisimov et al. 2012; Treat and Frolking 2013). According to database studies, the CH4 seep scenario for the next 1,000 years does indicate that a sudden increase in CH4 emissions from Siberian wetlands is unlikely, and that its impact to global temperatures is low (ca. +0.01°C within the 21st century) (Anisimov et al. 2012). Also, studies on Eastern Greenland permafrost-affected soils have shown that wet conditions after thaw can potentially reduce total carbon emissions, thereby minimizing the likelihood of sudden CH4 peaks (Elberling et al. 2013; Treat and Frolking 2013). However, studies on CH4 cycle microorganisms are often in contrast with these model-based scenarios that do not include microbial activities. Therefore, there is an urgent need to include microbial data in models and model estimations. The impact of microbial activity is enormous, but the net effect of microbial activities on the concentration of CH4, CO2, and N2O is currently not
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