Rice plant ^ Surface water Q Soil methanogenesis: the reduction of CO2 using H2 and the transmethylation of acetate (Takai 1970; Schutz et al. 1989). These substrates are supplied from soil organic matter, exudates and sloughed tissues of rice plants, and applied organic matter (Watanabe et al. 1999; Kimura et al. 2004). A part of produced CH4 is consumed by methanotrophic bacteria (methanotrophs) in the oxidative zones as the rhizo-sphere of rice plants and thin soil layer interfacing with the surface water. During the flooded period, produced CH4 is emitted to the atmosphere mainly through the aerenchyma of rice plants, and other pathways are ebullition and diffusion through the surface water (Schutz et al. 1991; Butterbach-Bahl et al. 1997). On the other hand, during drainage, CH4 trapped in the soil is emitted by the direct diffusion (Fig. 14.2b). During the fallow period, a little amount of CH4 is consumed in the oxidative soil by methanotrophs (Fig. 14.2c).

From the recent estimates (Cao et al. 1998; Mosier et al. 1998; Neue and Sass 1998; Yan et al. 2003), the total CH4 emission from the paddy fields in the world accounts for 4.2-9.0% of the global CH4 emission (600 Tg CH4 y-1). Worldwide, rice area is slightly increasing year by year (IRRI2007), and this trend will not turn around due to growing human population. Therefore, paddy fields will continue to be a major CH4 source of the world from now onwards.

14.1.3 Other GHG Exchanges

The CO2 and N2O are also emitted and/or consumed through various pathways (Fig. 14.2). Flooding and drainage by water management drastically change the pattern of GHG exchanges. Other kinds of field management, such as ploughing, straw incorporation, and nitrogen fertilization, also affect the GHG exchanges.

Between the soil and the atmosphere, GHG exchanges occur by the ecologies of microorganisms and plants. Carbon dioxide is exchanged by the photosynthesis and respiration of rice and weed plants, and the respiration of heterotrophs in the soil (i.e. the decomposition of soil organic matter). Carbon fixed by rice plants is partly removed from the field by harvesting, and the rest is incorporated to the soil by ploughing. Nitrous oxide is produced as a by-product of nitrification under the oxidative conditions and an intermediate product of denitrification under the semi-reductive conditions. Under the strict reductive conditions where CH4 production occurs, N2O produced by denitrification is further denitrified to N2, the end product of this process. The nitrogen substrates for N2O production are mainly supplied from fertilizer, soil organic matter, and applied organic matter.

The other pathway for GHG exchanges is the dissolved emission to soil percolating water or surface drainage (Fig. 14.2). As reported by Mosier et al. (1998b), the indirect N2O emission through the groundwater and surface flow has a quantitative significance. The N2O detected as the indirect emission is produced mainly by denitrification, using NO3- as the sole substrate, in the soil and the subsequent groundwater. Although it is not clear about the contribution of N2O produced in the sub-surface soil layer of the field to the total indirect emission, non-negligible amount of dissolved N2O was detected in the percolating water at a lysimeter experiment (Minamikawa et al. 2007a). The CO2 and CH4 produced in the soil are also, more or less, dissolved in the water, and then indirectly emitted to the atmosphere.

Soil organic carbon has various existing forms of itself. Although this is not a flow, but a stock, decomposed organic matter is used as a major substrate for CO2 and CH4 production. The amount of soil carbon in the topsoil is more than several thousands g C m-2 depending on the soil type. Moreover, a huge amount of carbon is stored in the soil including the sub-soil and the subsequent layers. Therefore, its decomposition, even if a little amount, has a strong impact on CO2 emission. The conservation of soil carbon is known as 'soil carbon sequestration'.

14.2 Mitigation of GHG Emission from Irrigated Paddy Fields

14.2.1 Quantitative Significance of Mitigating GHG Emission from Paddy Fields in Asia

How much can agriculture contribute to the mitigation of GHG emission? Figure 14.3 shows the inventories of GHG emitted from major anthropogenic sources in the selected Asian countries. Generally, the distribution of agriculture

Fig. 14.3 The GHG

inventories for rice producing countries in Asia (UNFCCC 2005; Nojiri et al. 2007). The value indicates the distribution of agriculture sector. Land-use change and forestry are excluded

Fig. 14.3 The GHG

inventories for rice producing countries in Asia (UNFCCC 2005; Nojiri et al. 2007). The value indicates the distribution of agriculture sector. Land-use change and forestry are excluded

China (1994) 4057 Mt C02-eq

Japan(2005) 1329 Mt C02-eq

India (1994) 1214 Mt C02-eq

Indonesia (1994) 323 Mt C02-eq

Thailand (1994) 224 Mt C02-eq

II Industrial Processes

III Agriculture ^ Waste

Philippines (1994) 101 Mt C02-eq

II Industrial Processes

III Agriculture ^ Waste

Vietnam (1994) 84 Mt CO2-eq

Bangladesh (1994) 56 Mt CO2-eq sector is diverse and reaches one third to two third of the national total in the developing countries. Although the percentage of agriculture will fall with the development of socio-economic conditions in the developing countries, the decrease in GHG emission from agriculture is effective in mitigating the total GHG emission (i.e. the total CO2-equivalent emission considering the Global Warming Potential, GWP) under the present conditions.

As for the distribution of each GHG emission within a country, those of CH4 and N2O, that mainly emitted from agriculture, are relatively high in the developing countries (Table 14.2). In these countries, rice production is also high (Table 14.1), and thus CH4 emitted from paddy fields has a large contribution to the national

Table 14.2 Distribution of GHG emission for rice producing countries in Asia (%)






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