Emitted CH4 from the paddy fields enhances global warming on the largest scale, but the microbiological processes of CH4 production and oxidation occur on the smallest scale. In terms of environmental factors, temperature (Dunfield et al. 1993; Fey and Conrad 2003) and water regime (Mishra et al. 1997; Rath et al. 1999) determine the potential for CH4 emission. On the microbial and plant scales, the amount of CH4 emitted is determined by the population and activity of microorganisms (Mayer and Conrad 1990; Adachi 2001; Eller and Frenzel 2001; Hadi et al. 2001), the physical conditions and chemical properties of the soil (Yao and Zhang 1996; Van den pol-van Dasselar and Oenema 1999; Wang et al. 1999; Watanable and Kimura 1999; Yao et al. 1999), the biomass and activity of rice plants (Chanton et al. 1997) and the kinds and amounts of substrates (Watanable et al. 1999; Kimura et al. 2004).

From the farmer's viewpoint, management on a field scale is the only way to produce rice and increase the rice yield (Minamikawa et al. 2006). Such field management also has physical, chemical and biological effects on CH4 emission. Statistically during growing period, average methane efflux was found to be significantly affected by water management, organic matter application, and content of soil organic carbon, soil pH, pre-season water status and climate. Therefore, ecological problems on different scales from different view points i.e. CH4 emission (global warming) and rice production, need to be considered simultaneously.

Irrigated paddy fields were estimated to account for 70-80% of global CH4 whereas rainfed (about 15%) and deepwater (about 10%) fields had much lower shares (Wassmann et al. 2000).

As evaluated by Wassmann et al. (1999), irrigated rice offers the most options for modifying field management to mitigate CH4 emission. On the other hand, mitigation options for rain fed and deepwater fields are very limited in both number and potential gain (Wassmann et al. 1999). Therefore, it is certain that irrigated paddy fields are the most promising targets for applying mitigating options.

16.7 Options for Mitigating CH4 Emission

Reduction in the CH4 efflux from rice fields can be made either by controlling the production, oxidation or transport processes of CH4. Since methanogens require highly reducing conditions for their activity (Zeikus 1977; Cicerone and Orem-land 1988), arresting the development of soil reduction is one of the most effective ways of decreasing CH4 production rate in soils. This can be accomplished by aerating soils during the flooding period with altering water management, or by inhibiting the progress of the sequential redox reactions with adding chemicals. These options may enhance the CH4 oxidation rate in soils. Reducing the amount of labile organic matter in soils by composting organic fertilizer or promoting aerobic decomposition of biomass is another effective way of controlling CH4 production in soils. Since, rice plants also have a significant contribution on production/oxidation of CH4 in the rhizosphere and its transport to the atmosphere, selection of rice variety that emits small amount of CH4 may also mitigate CH4 emission significantly from rice paddy fields. In summary, the options, that agree with the above strategies, includes altering water management, using application of mineral fertilizer or soil amendments,

Table 16.5 Evaluation of the mitigation options for methane emission form paddy fields Problem

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