Biogeochemistry of methane production

Methane production in natural wetlands, and in rice paddies, is a process occurring in strictly reduced (anoxic) conditions (see also Chapter 2). The creation of these conditions is controlled by both chemical and microbiological soil properties (Conrad, 1989a, 1993; Neue and Roger, 1993). Aerobic, drained soils become completely anoxic after flooding because of the barrier to entry of atmospheric oxygen presented by the water layer: the rate of diffusion of oxygen and other gases through water is about 10-4 times that in the gas phase. The only regions where anoxia does not prevail are at the soil-floodwater interface and the zones around plant roots (Conrad, 1993). Respiration by microorganisms and plant roots rapidly depletes the remaining oxygen in the system and then other chemical species - nitrate, manganese(IV), iron(III), sulphate and carbon dioxide - act in turn as alternative electron acceptors, and are consequently reduced by microbial activity (for example Ponnamperuma, 1972, 1981; Peters and Conrad, 1996). Organic compounds such as humic acids may also act as electron acceptors (Lovley et al, 1996).

In most paddy soils the concentration of sulphate in the pore water increases for a few days before sulphate reduction starts (Ponnamperuma, 1981; Yao et al, 1999). According to Yao et al (1999), the increase is due to the release of sulphate adsorbed onto ferric iron minerals such as goethite either by reduction of bicarbonate or Fe(III). In acidic soils, clays and hydrous oxides of aluminium strongly sorb sulphate and release it when the pH increases after flooding.

The sequential reduction of the various electron acceptors takes place in the order indicated by their redox potentials, i.e. as predicted by thermodynamic theory (Ponnamperuma, 1972, 1981; Zehnder and Stumm, 1988). After their reduction, methanogenesis becomes possible. Hydrogen is a key substrate for this process. It is produced from the degradation of organic substances (Conrad, 1996a) and is rapidly consumed as an electron donor in various redox reactions, so that the turnover time of H2 is very short (Conrad et al, 1989b; Yao et al 1999). At the end of these reduction processes, the H2 partial pressure increases, allowing hydrogenotrophic methanogenesis to begin. Yao et al (1999) reported that this took place at partial pressures between 1 and 23Pa. A steady-state concentration of H2 is determined by the relative rates of H2 production and H2 consumption. The thermodynamic conditions for hydrogenotrophic methanogenesis are described in detail by Yao and Conrad (1999).

Acetate is the other compound acting as a major driver of methanogenesis. Like H2, it is produced from organic substances. Yao et al (1999) found that acetate turnover time in paddy soil was much longer than that of H2, typically in the range of hours or even days, and that CH4 production was initiated at concentrations of 30-8000^M, and at pH values between 6.0 and 7.5 and a redox potential (Eh value) of between -80 and +250mV. These Eh values at the onset of methanogenesis were higher than the -150mV reported by Wang et al (1993) and Masscheleyn et al (1993), but experiments with cultures of methanogenic bacteria show that O2 has a much more adverse effect on methanogenic activity than high redox potentials and that methanogens are able to initiate CH4 production in the absence of O2 at higher Eh values: up to +420mV (Fetzer and Conrad, 1993) and 0 to +100 mV (Garcia et al, 1974; Peters and Conrad, 1996; Ratering and Conrad, 1998). Yao et al (1999)

Figure 8.3 Production, oxidation and emission of methane in rice paddy fields

Source: Yagi et al (1997)

Figure 8.3 Production, oxidation and emission of methane in rice paddy fields

Source: Yagi et al (1997)

concluded that redox potential is not a good indicator for the onset of soil methanogenesis, and should only be used as an indicator when the soil and its CH4 production behaviour have been carefully characterized (for example Yagi et al, 1996; Sigren et al, 1997).

The emission pathways of CH4 which are accumulated in flooded paddy soils are: diffusion into the floodwater, loss through ebullition, and diffusive transport through the aerenchyma system of rice plants, as shown in Figure 8.3 (Yagi et al, 1997). In temperate rice fields, more than 90 per cent of CH4 emissions take place through plants (Sch├╝tz et al, 1989). In tropical rice fields, by contrast, significant amounts of CH4 may evolve by ebullition, in particular during the early part of the season and when organic inputs are high (Denier van der Gon and Neue, 1995).

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