Being produced from anaerobic decomposition of organic material in waterlogged anaerobic parts of the soil, wetland environments have for a long time been known to be significant contributors to atmospheric CH4 (Ehhalt, 1974; Fung et al, 1991; Bartlett and Harriss, 1993). In these wet anaerobic environments, CH4 is formed through the microbial process of methanogenesis (see also Chapter 2). Methane formation follows from a complex set of ecosystem processes that begins with the primary fermentation of organic macromolecules to acetic acid, other carboxylic acids, alcohols, CO2 and hydrogen. This is then followed by the secondary fermentation of the alcohols and carboxylic acids to acetate, H2 and CO2, which are fully converted to CH4 by methanogenic bacteria (Cicerone and Oremland, 1988; Conrad, 1996). The controls on this sequence of events span a range of factors, most notably temperature, the persistence of anaerobic conditions, gas transport by vascular plants as well as supply of labile organic substrates (Whalen and Reebugh, 1992; Davidson and Schimel, 1995; Joabsson and Christensen, 2001; Ström et al, 2003). Figure 3.1 shows the variety of controls on CH4 formation rates at different spatial and temporal scales.
Methane is, however, not only being produced, but also consumed in aerobic parts of the soil. This takes place through the microbial process of methanotrophy, which can even take place in dry soils with the bacteria living off the atmospheric concentration of CH4 (Whalen and Reeburgh, 1992; Moosavi and Crill, 1997; Christensen et al, 1999). Methanotrophy is responsible for the oxidation of an estimated 50 per cent of the CH4 produced at depth in the soil (Reeburgh et al, 1994) and, as such, is as important a process for net CH4 emissions as is the methanogenesis. The anaerobic process of methanogenesis is much more responsive to temperature than CH4 oxidation. The mechanistic basis for this difference is not clear, but the ecosystem consequences are rather straightforward: soil warming in the absence of any other changes will accelerate emission (which is the difference between production and consumption), in spite of the simultaneous stimulation of the two opposing processes (Ridgwell et al, 1999). There may be a buffering effect of temperature changes at greater soil depths, where the methanogenesis mainly takes place. But, in the absence of other changes, warming still favours increasing production and net emission of CH4.
Controls on methanogenesis
■ Level of regulation
Figure 3.1 Major controls on the pathways to methane formation
Note: Distal and proximal controlling parameters are indicated as well as hierarchy of importance in a complex ecosystem context.
Source: Based on Schimel (2004)
The controls on CH4 emissions are, hence, a rather complex set of processes working in opposing directions. Early empirical models of wetland CH4 exchanges suggested sensitivity to climate change (Roulet et al, 1992; Harriss et al, 1993). A simple mechanistic model of tundra CH4 emissions including the combined effects of temperature, moisture and active layer depth also suggested significant changes in CH4 emissions as a result of climate change (Christensen and Cox, 1995). Wetland CH4 emission models have grown in complexity (Panikov, 1995; Christensen et al, 1996; Cao et al, 1996; Walter and Heimann, 2000; Granberg et al, 2001; Wania, 2007) as the mechanistic understanding of the most important processes controlling CH4 fluxes have improved. Autumn and winter processes have also been found to have a strong influence on net annual emissions of CH4 (Panikov and Dedysh, 2000; Mastepanov et al, 2008). Variations in CH4 emissions at the large regional-global scale have been found to be driven largely by temperature (Crill et al, 1992; Harriss et al, 1993), but with important modulating effects of vascular plant species composition superimposed (Christensen et al, 2003a; Ström et al, 2003). From the perspective of empirical studies, then, an initial warming is expected to lead to increased CH4 emissions, but the scale of this increase depends on associated changes in soil moisture conditions, and the secondary effects of changes in vegetation composition.
The highest emissions are generally associated with stagnant constant high water table levels combined with highly organic soils (often peat). Plant productivity can further amplify the source strength of CH4 production, and this interaction has been studied at scales ranging from below-ground microbial investigations (Panikov, 1995; Thomas et al, 1996; Joabsson et al, 1999) to large-scale vegetation models linked to CH4 parameterizations (Cao et al, 1996; Christensen et al, 1996; Walter and Heimann, 2000; Zhuang et al, 2004; Sitch et al, 2007). Various studies have attributed the relationship to different mechanisms such as:
1 stimulation of methanogenesis by increasing C-substrate availability (input of organic substances to soil through root exudation and litter production);
2 build-up of plant-derived peat deposits that retain water and provide an anoxic soil environment;
3 removal of mineral plant nutrients such as nitrate and sulphate, which are competitive inhibitors of methanogenesis (competitive electron acceptors);
4 enhancement of gas transport from methanogenic soil layers to the atmosphere via root aerenchyma acting as gas conduits that bypass zones of potential CH4 oxidation in the soil.
Note: The processes associated with the vascular plants play a pivotal role as discussed in the text Source: Figure modified from Joabsson and Christensen (2001)
In addition to these stimulatory effects on net CH4 emissions, certain plants may also reduce emissions through actively oxidizing the root vicinity (rhizospheric oxidation). Figure 3.2 summarizes the ways in which plants may affect CH4 emissions from wetlands.
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