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Source: UNFCCC (2005) and Nojiri et al. (2007) Calculated by CO2 equivalent considering the GWP. Land-use change and forestry are excluded.

Source: UNFCCC (2005) and Nojiri et al. (2007) Calculated by CO2 equivalent considering the GWP. Land-use change and forestry are excluded.

GHG emission. Therefore, the mitigation of CH4 emission from paddy fields will have significant impact on the total mitigation of GHG emission in Asia.

Well then, does the mitigation have the cost economy? Originally, paddy fields are the place for rice production, and thus its productivity is given the highest priority. An appropriate combination of field management is practiced to obtain suitable rice yield, but these practices have various effects on the GHG emission. Of course, some management practices can be win-win options that sustain rice yield and decrease CH4 emission. However, farmers generally aim at economic rice production only. Although the consideration of environmental issues on various scales (e.g. GHG emission and water pollution) has some advantages in the developed countries, however, environmental concerns are not comparable to economic one in the present socio-economic situation. In the near future, mitigation of GHG emission will be profitable through some environmental taxes and the Clean Development Mechanism of the Kyoto Protocol all over the world. Therefore, researchers should look ahead into the future, and have to clarify the efficiency of mitigation options quantitatively. Furthermore, even in disregard of profitability, researchers should prepare the mitigation options as a approach to contain global warming.

14.2.2 How Can We Mitigate the Total GHG Emission from An Irrigated Paddy Field Most Effectively?

14.2.2.1 Concrete Method of Mitigation

The factors controlling GHG emission range from microbial to a global scale. The global warming occurs on the largest scale, while microbial GHG production and consumption occur on the smallest scale. As reviewed by Minamikawa et al. (2006a), present mitigation options are mostly on the field scale, and conducted as one part of field management. At the present research level of the GHG emission from agro-ecosystems, the field-scale mitigation is the most feasible method that we can do it now.

All of the mitigation options essentially affect the ecology of microorganisms. However, the insights into the microbial ecology are often disregarded in the field-scale mitigation. Conrad (1996) has reviewed the significance of soil microorganisms as controllers of GHG emission. As suggested by Kumaraswamy et al. (2000) and Schimel (2000), further microbial studies will enable us to predict the ecosystem behavior and the ecological significance of diversity and community structure. In other words, further studies on the microbial scale will give us additional clues to the mitigation of GHG emission from paddy fields.

14.2.2.2 Which Gas and Pathway is the Main Target for the Mitigation?

It is very difficult to answer this question completely at the present research level. Then, what have the previous GHG studies clarified up to now?

Tsuruta et al. (1998) evaluated the direct GHG emission from Japanese paddy fields, considering the GWP. The CH4, CO2, and N2O emission accounted for 78.2,

16.0, and 5.8% of the GWP, respectively. As for CO2 emission (i.e. soil carbon budget), it is difficult to quantify because both the emission and consumption occur simultaneously, while CH4 and N2O have little consumption. It will take a little longer to evaluate CO2 emission as accurate as CH4 and N2O emission. On the other hand, simultaneous measurement of CH4 and N2O emission has demonstrated the priority of CH4 emission as the main mitigation target (Nishimura et al. 2004; Zou et al. 2005). Therefore, the decrease in CH4 emission will be the best way to mitigate the direct GHG emission from paddy fields.

Next, how many are the GHGs emitted as dissolved into groundwater? Sawamoto et al. (2003) reported the result of simultaneous measurement of the direct and dissolved GHG emission at an upland field. As compared to the direct emission of CO2, CH4, and N2O, 2.5, 58, and 4.6% were emitted to a subsurface drain, respectively. On the other hand, at a lysimeter paddy field, Minamikawa et al. (2007a) simultaneously measured the direct and dissolved GHG emission. The annual amount of dissolved N2O emission to the percolating water was 30.6% of the direct one, and dissolved CO2 was 34.2 g C m-2 y-1. Between upland and irrigated paddy, soil redox conditions and amount of percolating water are largely different. Anyway, dissolved GHG emission would be lower than the direct one in an irrigated paddy field, but further studies are needed to understand the mechanism and amount of dissolved GHG emission.

The most difficult problem is the evaluation of soil carbon sequestration. The latest IPCC report suggests that soil carbon sequestration is one of the most effective options in mitigating CO2 emission from agriculture (Smith et al. 2007). Of the total mitigation potential, the contribution of soil carbon sequestration, CH4 emission, and N2O emission was estimated to be 89, 9, and 2%, respectively. Its quantitative significance would defeat the mitigation of minor GHGs. However, there have been only a few reports on soil carbon sequestration in a paddy field (Witt et al. 2000; Ramesh and Chandrasekaran 2004). As reported by Sain and Broadbent (1977) and Devevre and Horwath (2000), flooding would delay the decomposition of soil organic carbon. Paddy fields may have a potential to store soil organic carbon more than upland fields.

There have been a lot of studies either related to GHG emission or soil carbon sequestration only. Now-a-days, we know that it is necessary to measure GHG emission simultaneously with soil carbon storage (soil carbon budget). Simultaneous measurement, considering the time scale, will give us the best answer to the optimal management practice for mitigating the total GHG emission from irrigated paddy fields.

14.3 Mitigation by Conventional Water Management 14.3.1 Different Aim Depending on the Water Availability

Water management is the most effective option in decreasing CH4 emission from an irrigated paddy field, because it prevents the development of soil reductive conditions. From the statistical analysis of available data, Yan et al. (2005) reported that multiple drainage decreases CH4 emission by 48% as compared to continuous flooding. This value has been adopted to the default emission factor by the IPCC report (2006).

The original aim of water management is different depending on the water availability. Where irrigation water is enough supplied as in the temperate region, water management is for the sound rice growth. Where irrigation water is not ensured as in the tropic region, water management has other aim, saving irrigation water. In the former region, mid-season drainage and intermittent irrigation are usually practiced by farmers. On the other hand, the alternate wetting and drying (AWD) is now being suggested to farmers in the later region.

14.3.2 Mid-Season Drainage and Intermittent Irrigation 14.3.2.1 Role for Sound Rice Growth

In the temperate region, water management is practiced to control surplus tillering and supply rice roots with molecular O2 for preventing sulfide toxicity. Figure 14.4 mgpF4 shows a typical method of water management for an irrigated paddy field in Japan. Mid-season drainage is conducted at the panicle formation stage of rice plants for 1-2 weeks, aiming at the crack formation on the soil surface. Intermittent irrigation follows the mid-season drainage, and continues until several weeks before rice harvest. Intermittent irrigation is conducted at the several-day intervals of flooding and drainage. Generally, mid-season drainage and intermittent irrigation are conducted together.

Rice plant is especially sensitive to drought at the heading stage, and the rooting of rice seedling is stabilized by flooding. Therefore, the field during these periods should be flooded for sound rice growth. Additionally, control of surface water depth is conducted as occasion demands before mid-season drainage, in order to avoid the cool weather damage with the deep flooding of higher water temperature, and in order to stimulate the tillering of rice plants with the shallow flooding of lower water temperature. Although there remain some ambiguities in the timing and duration of drainage, appropriate water management certainly hastens rice growth and hence increases rice yield.

Flooding

Fig. 14.4 Typical water management in an irrigated paddy field in Japan

Transplanting

Midseason drainage

I Intermittent ^ irrigation

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