Other natural factors

If only diffusion is considered, methano-trophs are expected to live in surface soil, where they are in closest proximity to the atmospheric methane source. However, in many soils surface organic-rich horizons lack appreciable activity, and there is a pronounced subsurface maximum for methane oxidation (Crill, 1991; Adamsen and King, 1993; Koschorreck and Conrad, 1993; Yavitt et al., 1993; Bender and Conrad, 1994b; Schnell and King, 1994; Prieme and

Water stress Diffusion limitation Methanogenesis

% Water-filled pore space

Fig. 10.1. Conceptual model of the effect of water on soil methane flux. The dotted line represents the relationship expected due to diffusion alone. The solid line is the pattern commonly observed, with a zone of water stress and a zone of diffusion limitation. The dashed line indicates that methanogenesis may occur at high water contents.

% Water-filled pore space

Fig. 10.1. Conceptual model of the effect of water on soil methane flux. The dotted line represents the relationship expected due to diffusion alone. The solid line is the pattern commonly observed, with a zone of water stress and a zone of diffusion limitation. The dashed line indicates that methanogenesis may occur at high water contents.

Christensen, 1997; Ishizuka et al., 2000). The growth of methanotrophic bacteria in many surface soils must therefore be limited, perhaps by inhibitory substances. Potential inhibitors include ammonium (Adamsen and King, 1993; Schnell and King, 1994), ethylene (Sexstone and Mains, 1990; Jackel et al., 2004) and monoterpenes (Amaral and Knowles, 1998). Desiccation of surface soils could also prevent establishment of methanotrophs, as populations may recover very slowly from water stress (Nesbit and Breitenbeck, 1992; Whalen et al., 1992).

Other natural factors affecting the soil methane sink include tree species composition and fires. Soils under different forest compositions support different methane uptake rates (Borken et al., 2003; Menyailo and Hungate, 2003; Reay et al., 2005). Fires may either increase or decrease the methane sink, and the effects may not be immediate (Burke et al., 1997; Prieme and Christensen, 1999; Jaatinen et al., 2004). Tree species and fire effects are probably mediated through soil chemistry, moisture and microbiology, but the precise mechanisms are complex and not yet clearly understood.

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