3. 2. Methane flux from rewetting

General patterns of responses

Net CH4 flux is the result of the balance between the two off setting processes of methanogenesis (microbial production under anaerobic conditions) and methanotrophy (microbial consumption) (Dutaur and Verchot, 2007). Methanogenesis occurs via the anaerobic degradation of organic matter by methanogenic archaea within the archaeal phylum Euryarchaeota (Thauer, 1988). Methanotrophy occurs by methanotrophs metabolizing CH4 as their source of carbon and energy (Hanson and Hanson, 1996). In anoxic soils, emergent vegetation also influences CH4 flux to the atmosphere, as plants enable oxygen transport to the rhizosphere, transport through aerenchymateous tissue, and the production of labile substrates via root exudation (Joabsson et al. 1999).

The reported effects of rewetting on CH4 fluxes are variable. Rewetting reduced CH4 consumption or increased CH4 production in arable land (Syamsul Arif et al., 1996; Kessavalou et al., 1998; Hergoualc’h et al., 2008), peatland (Kettunen et al., 1996; Blodau and Moore, 2003; Dinsmore et al., 2009) and tropical forest (Silver et al., 1999). In a wheat-fallow cropping system, CH4 consumption declined by about 60% for 3 to 14 d after rewetting (Kessavalou et al., 1998). In peatland, a pulse of CH4 was observed after water table drawdown (Moore and Knowles, 1990; Shurpali et al., 1993) and significant pulses of CH4 fluxes were produced with both drainage (700 μg m−2 h−1 above the pre-change mean) and rewetting (over 160 μg m−2 h−1 above the value of prior to rewetting) within 1 or 2 days in a mesocosm study (Dinsmore et al., 2009). In contrast, other studies have reported that rewetting increased CH4 consumption, or reduced CH4 production, both in the field (Davidson et al., 2004; Borken et al., 2006; Davidson et al., 2008; Fiedler et al., 2008) and laboratory (Czepiel et al., 1995; West and Schmidt, 1998). In incubation experiments with alpine soil, CH4 oxidation increased significantly from 11 pmol CH4 (g dry weight)−1 h−1 to -29.5 − -67.0 pmol CH4 (g dry weight)−1 h−1 9 days after rewetting (West and Schmidt, 1998). Enhanced CH4 oxidation was promoted after rewetting for days to weeks in peatland (Öquist and Sundh, 1998; Kettunen et al., 1999; Goldhammer and Blodau, 2008) and rice field (Ratering and Conrad, 1998). However, in an in situ water table drawdown experiment, CH4 production declined in hummocks but stayed constant in hollows relative to control plots, suggesting a strong role of plant-mediated release of CH4 in some peatland microforms (Strack and Waddington, 2007). In summary, studies report a large uncertainty in CH4 responses after rewetting and there are much smaller responses in magnitude but fewer observations compared to other gases (Table 1, Fig. 2).



Mechanisms and drivers

In general, CH4 production rates are controlled by the availability of suitable substrates, alternative electron acceptors for competing redox reactions (i.e., sulfate reduction), the nutritional status of the ecosystem (i.e., bog versus fen), water table position or soil moisture content, temperature, and soil salinity (Thauer, 1988; Hanson and Hanson, 1996; Dutaur and Verchot, 2007).

The mechanisms and drivers underlying changes in CH4 flux following rewetting are complex because they involve the response of both methanogenesis and methanotrophy to changes in soil environment, particularly soil moisture, and availability of electron donors and acceptor that determine the redox status of soil. Rewetting can increase the availability of water-soluble C substrates (Zsolnay and Görlitz, 1994; Stark and Firestone, 1995), which soil methanotrophs utilize as an electron source (Whittenbury et al., 1970). In unfrozen soils, there was no correlation between soil temperature and CH4 consumption, suggesting strong substrate limitation on methanotrophs (Borken et al., 2006). Borken et al. (2006) also found that methanotrophs were stressed when water contents were below 0.15 g cm−3 (in the A horizon), thus rewetting can alleviate osmotic stress and promote the growth and activity of soil methanotrophs (Schnell and King, 1996; West and Schmidt, 1998). While several studies have shown that experimental drought increased CH4 consumption rates (cf. Borken et al., 2006, Davidson et al., 2008), Fiedler et al. (2008) found no evidence of increased methanotrophy in response to natural drought in forest soils. Methanotrophs responded quickly to water table manipulations in peat soil (Blodau and Moore, 2003). Rewetting also can inhibit methanotrophic activity in more poorly drained soils, for example if oxygen diffusion becomes limiting (Striegl, 1993). Because methanogenesis requires anaerobic soil conditions, drought typically suppresses CH4 production, while rewetting increases it. Methanogenic populations require some time to re-establish after rewetting (Fetzer et al., 1993).

In addition to environmental controls, both methanotrophy and methanogenesis are sensitive to interactions and competition with other microbial redox processes. Drying and rewetting of soils can increase SO4 pools through remineralization of organic sulfate and/or reoxidation of iron sulfides. This can stimulate sulfate reduction and effectively suppress methanogenesis (c.f., Blodau and Moore, 2003). In thick organic soils, this is more likely to occur in surface layers that experience fluctuating water tables rather than more saturated deeper peat layers (Goldhammer and Blodau, 2008). Overall, the mechanisms and drivers responsible for the various response of CH4 to thawing have not been clearly explored to our knowledge and further research is needed to identify the mechanisms controlling the response after rewetting at multiple ecosystems.