Methane production occurs in the anoxic zone of a peatland. Peat and plant litter decay contribute to CH4production (Baird et al., 2009; Williams and Crawford, 1984). Methane is produced by archaea and a number of bacteria within a complex food web (Segers, 1998). There are two different methanogen groups within the archaea; one which ferments acetate or similar organic compounds to produce CH4and CO2(as shown below), and one that oxidises hydrogen (H2) and reduces CO2to produce CH4(Gauci et al., 2004; Le Mer and Roger, 2001; Schimel, 2004).
The fermentation process responsible for producing CO2and CH4, given by:
C6H12O6 → 3 CO2+ 3 CH4
is dependent upon the consecutive actions of four microbial populations (Le Mer and Roger, 2001). These actions are: hydrolysis, acidogenesis, acetogenesis and, finally, methanogenesis (Le Mer and Roger, 2001). Acetate fermentation may be responsible for more than 67 % of CH4production, with the oxidation of H2and the reduction of CO2responsible for the remaining 33 % (Kotsyurbenko et al., 2004). However, the relative contributions of these two different methanogen groups may vary with increasing depth through the peat profile. The contribution of H2-
oxidising and CO2-reducting methanogens increases with depth to 50 – 100 % of CH4production (Kotsyurbenko et al., 2004). Bellisario et al. (1999) indicated that acetate fermentation was the more dominated method of CH4production in vegetated areas due to the input of fresh organic matter, with areas dominated by recalcitrant material were more likely to rely on CO2reduction for methanogenesis.
In many environments CH4production also requires C1 compounds, which are organic compounds that do not have carbon-carbon bonds. However, it may be that in northern peatlands, predominantly those with Sphagnum mosses, neither C1 compounds nor acetate are utilised by methanogens for CH4production, which suggests that the group of methanogens responsible for H2oxidation and CO2 reduction are dominant in these environments (Hines et al., 2001). However, acetate is still produced, and accrues in large concentrations, whereby the acetate then diffuses into the oxic layers of the peat and is degraded into CO2(Hines et al., 2001). In contrast, other studies have shown that acetate is a substrate used in CH4 production, but it has not been ruled out that some acetate could be degraded into CO2(Ström et al., 2003; Ström et al., 2005). Hines et al. (2008) found that the amount of acetate produced by the anoxic decay of plant matter varied depending on the plant species; a dominance of Sphagnum mosses resulted in 67 % of the carbon produced through decay being acetate, compared to only 13 % in areas without any Sphagnum cover.
Plants deliver a range of labile carbon compounds down to anoxic peat layers through their roots. These compounds can then act as substrates, readily available for methanogenic archaea to utilise (Ström et al., 2003), alongside acetate, H2and CO2(Kotsyurbenko et al., 2004), because at the depths where methanogenesis occurs the organic matter is frequently resistant to decomposition (Ström et al., 2003). However, O2can also be transported down to the anoxic layer via plant roots which can hinder CH4production (Tuittila et al., 2000), whilst root decay can contribute to CH4production (Segers, 1998). Methanogens which ferment acetate are likely to be more active in the summer months when there is a greater supply of labile organic carbon (Gauci et al., 2004). Different vegetation assemblages can result in different methanogenic communities. Galand et al. (2003) showed that hummocks were populated with the Methanomicrobiales community, whereas
Eriophorum lawns were populated with the Methanosarcinales community.
Rooney-Varga et al. (2007) found that vegetation composition was the best explanatory variable the differences in methanogenic communities in two North-
American peatlands, followed by temperature. Vegetation cover can also have a further influence over CH4production. Lai et al. (2014) found evidence for the quick turnaround of photosynthates into CH4production in Eriophorum species with a lag of 9-12 hours. Levy et al. (2012) examined the data from multiple studies of peatland CH4fluxes and discovered that plant species composition was the most accurate indicator of CH4emissions; however, the link with vegetation cover may not be limited to CH4production, but could also be caused by effects on CH4 transport or oxidation.
There are many other reported controls on CH4production. These include: the extent of the anoxic zone (Baird et al., 2009) which is determined by the position of the water table (MacDonald et al., 1998), as shown in Figure 2.1; the size of the methanogenic population (Segers, 1998) and the amount and quality of substrate available to them (Bergman et al., 2000); temperature and pH (Valentine et al., 1994); and the amount of rival electron acceptors present (MacDonald et al., 1998).
The quality of the substrate, as well as the quantity available to methanogenic archaea can affect CH4production, where quality is defined as the chemical
availability of carbon for decomposition (Valentine et al., 1994). A higher quality of substrate leads to greater CH4production rates (Bergman et al., 2000; Granberg et al., 1997; MacDonald et al., 1998; Waddington and Day, 2007). The less
decomposed the substrate is by the time it reaches the anoxic zone, the higher quality it is, which suggests that a lower water table would result in less CH4 production (Granberg et al., 1997; Sundh et al., 1995). Methane production is exponentially linked to temperature (Dunfield et al., 1993). The Q10values for CH4 production in peatlands are generally higher than those for CO2production with an average value of 4.1 (Blodau, 2002). A pH of 7 is suggested as best for
methanogens (Segers, 1998; Williams and Crawford, 1984); however, pH in blanket peatlands and raised bogs is acidic (Gore, 1961; Charman, 2002). Electron
acceptors are used by organisms to release energy from organic matter (Baird et al., 2009). Rival electron acceptors (for example; nitrate (NO3-), sulphate (SO42-), or ferric iron (Fe3+)) can hinder methanogenesis (MacDonald et al., 1998; Valentine et al., 1994). In terms of electron acceptors, SO42-is preferred over CO2for
fermenting organic substrates and H+(Baird et al., 2009) because SO42-provides more energy (Segers, 1998). Therefore, the atmospheric depositions of sulphur dioxide over peatlands from industrial incinerations that swiftly increased over the 20thcentury may have resulted in lowered substrate availability for methanogenic archaea (Baird et al., 2009). The substrates are instead used by sulphate-reducing bacteria which transport electrons to SO42-in order to create hydrogen sulphide, and therefore, through a reduction in methanogenesis there is a reduction in CH4 flux to the atmosphere (Baird et al., 2009).