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is also controlled by flooding. The followings are known as the reasons to inhibit the injury by continuous cropping in flooded paddy fields: the supplement of micro-nutrients, the wash away of harmful substances, and the decrease in insect pest.

However, the development of reductive conditions in the flooded soil may cause the root rot of rice plants by sulfide toxicity. Furthermore, CH4, a greenhouse gas (GHG) is produced by methanogenic bacteria (methanogens) in the reduced soil, and emitted to the atmosphere (Fig. 14.2a). In the flooded paddy soil, after the trapped molecular O2 is quickly consumed, sequential reduction of the following soil oxidants progresses according to the thermodynamic theory: NO-, Mn4+, Fe3+, SO42-, and CO2. Methane is the end product of this process.

Generally, the oxidative-reductive conditions in the soil are indicated by soil redox potential (soil Eh). There are also other indicators, such as the terminal electron-accepting processes and the oxidative capacity (Gao et al. 2002). Soil Eh is a comprehensive measure of soil chemical and biological redox processes. With the development of the reductive conditions, soil Eh changes from +600 to < -200 mV. Generally, CH4 production begins at approximately — 150mV, and gets stimulated at lower range.

The major carbon substrates for CH4 production are H2 + CO2, formate, acetate, methyl alcohol, and methylamine. The following two reactions are dominant in the

Fig. 14.2 Major GHG exchanges in a paddy field during: (a) flooded period, (b) drainage, and (c) fallow period. Dissolved GHGs in water include CO2, CH4 and N2O. Light color arrow indicates the limited flow (slight amount, temporal, or conditional). Weeds and algae are excluded a) Rainfall C02 N20

Irrigation a) Rainfall C02 N20

Irrigation

Surface m drainage

Percolation b) Rainfall C02 N20 CH4

Surface m drainage

Percolation b) Rainfall C02 N20 CH4

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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