**3. Methane chemistry**

CH4 is a very special kind of molecule. CH4 is an end product of the organic carbon decomposition under anoxic conditions and the simplest organic compound and member of the paraffin series of hydrocarbons [27]. It is colorless, odorless gas that occurs abundantly in nature and as a product of anthropogenic activities. Its chemical formula is CH4 (**Figure 2**). CH4 is lighter than air, having a specific gravity of 0.554. It is slightly soluble gas in water and burns readily in air, forming carbon dioxide and water vapor; the flame is pale, luminous and very hot. The boiling point of CH4 is −162°C and the melting point is −182.5°C. Basically, CH4 is very stable, but mixtures of CH4 and air, with the content between 5 and 14% by volume, are explosive [28].

*Reactions of Methane*

	- a.Complete oxidation: CH4 + 2 O2, flame or spark → CO2 + H2O + heat

b.Partial oxidation: 6 CH4 + O2, 1500°C → CO + H2 + H2C2 CH4 + H2O, 850o , Ni → CO + H2

2.Halogenation CH4 + X2, heat → CH3X + HX requires heat or light. Cl2 > Br2 no reaction with I2.

**Figure 2.** *Chemical formula of methane.*

## **4. Biogeochemical mechanisms of methane production of paddy fields**

The production of CH4 is a microbiological process, which is predominantly controlled by the absence of oxygen and the amount of easily degradable actions [29]. Methanogens produce CH4 under anaerobic conditions [30, 31]. Methanogens are prokaryotic microorganisms and belong to the domain of archaea. They are living in an anaerobic environment (e.g., soil or water) or in the intestines of animals. Methanogens mainly use acetate (contributes about 80% to CH4 production) as a carbon substrate but another substrate like H2/CO2 and formats also contribute 10–30% to CH4 production [32]. Acetate and hydrogen are formed by fermentation from hydrolyzed organic matter [29]. However, flooding of rice fields cuts off oxygen supply from the atmosphere to the soil, which leads to anaerobic fermentation of organic matter in the soil, resulting in the production of CH4 [33]. And thereafter much of it escapes from the soil into the atmosphere via gas spaces in the rice roots and stems, and the remainder CH4 bubbles up from the soil and/or diffuses slowly through the soil and overlying flood water (**Figure 1**). In flooded rice paddies, straw incorporation usually stimulates CH4 production [34]. Root exudates and degrading roots are also important sources of CH4 production, especially at the later growth stages of paddy. There are two major pathways of CH4 production (e.g., acetoclastic and hydrogenotrophic). Acetoclastic methanogens use ATP to convert acetate to acetyl phosphate and then remove the phosphate ion via a reaction catalyzed by coenzyme A [12]. CH4 is formed gradually by processes involving oxidized ferredoxin, tetrahydrosarcinapterin, coenzyme M, and coenzymes B. Taking account of all, CH4 emissions from paddy soil are the net result of CH4 production, oxidation and transportation. The complete CH4 production process can be expressed as reduction and oxidation of two molecules of a simple hydrocarbon, one of which is reduced to CH4 and the other of which is oxidized to CO2: 2CH2O → CO2 + CH4 [15].

#### **5. Methane transportation from paddy soil to atmosphere**

The total CH4 emission process consists of three ways from soil to atmosphere e.g., transport via rice plants; bubble ebullition and molecular diffusion through

**75**

*Methane Cycling in Paddy Field: A Global Warming Issue*

the paddy water (see **Figure 1**). CH4 can escape from the rice paddy soil via aerenchyma in the plant (90%), ebullition (10%), and diffusion through the soil and water layer (1%). CH4 transports via the plant starts in the roots; CH4 enters by diffusion through the epidermis and during the water uptake. It is likely that dissolved CH4 is directly gasified in the root cortex and further diffuses upwards to the root-shoot transition zone traveling through intercellular spaces and aerenchyma. The aerenchyma system is developed by the plant to transport the oxygen necessary for respiration from leaves towards the roots. Just like CH4 diffuses from the soil into the root system, oxygen diffuses from the root into the soil, creating a relative oxygen-rich zone in the rhizosphere. CH4 is partly oxidized in the rhizosphere to CO2 by methanotrophic bacteria. Methanogenesis in the rhizosphere itself is suppressed by oxygen. The transport of CH4 to the atmosphere depends on the properties of the rice plant. The flux of gases in the aerenchyma depends on permeability coefficients, concentration gradients of roots and the internal structure of the aerenchyma. The number of tillers m−2, root mass, rooting pattern, total biomass, and metabolic activity also influence

**6. Factors effecting the methane production in paddy fields**

**6.2 Effects of soil pH and Eh on methane production in paddy field**

obtained a decrease in CH4 production in paddy soil.

Soil pH is another influential factor in CH4 production. The pH effects on CH4 production in a flooded rice soil. Methanogenic bacteria are acid sensitive. Generally, the optimum pH for methanogenesis is 7.0. Introducing the acidic materials frequently results in a decrease in CH4 production [39]. A slight decrease in soil pH can cause decreases in CH4 production. A slight increase in soil pH (about 0.2 unit higher than the natural soil suspension pH) resulted in an enhancement of CH4 production by 11 to 20% and 24 to 25% at controlled Eh of −250 and − 200 mV, respectively [40]. These results suggested that a small reduction of soil pH could be

Temperature is one of the major determining factors on the biological process

(e.g., within the soil), which controls the CH4 production. Previous studies showed that increased soil temperature leads to an increase in CH4 production [35]. There is a lot of qualitative evidence showing that CH4 production from rice field increase with the increasing temperature [35]. A laboratory experiment regarding CH4 production from two rice cultures incubated at temperatures between 20 and 38°C showed Eα values of 41 and 53 kJ mol−1. CH4 production in anoxic paddy soil suspensions incubated between 7 and 43°C showed Eα values between 53 and 132 kJ mol−1 with an average value of 85 kJ mol−1 [36]. The paddy soil temperature could control the amount of CH4 production and there is a positive and strong correlation in both soil temperature and CH4 production pattern [37]. The effect of temperature on CH4 production in paddy soil was investigated by [38], they found that in continuously flooded soil, the temperature optimum for CH4 production was 40°C, however, this shifted to 45°C during a period of intermittent irrigation accompanied by a marked decrease in activity. The optimum temperature during the non-cropping

**6.1 Effects of temperature on methane production**

season was also 45°C (**Figure 3**).

*DOI: http://dx.doi.org/10.5772/intechopen.94200*

gas fluxes.

the paddy water (see **Figure 1**). CH4 can escape from the rice paddy soil via aerenchyma in the plant (90%), ebullition (10%), and diffusion through the soil and water layer (1%). CH4 transports via the plant starts in the roots; CH4 enters by diffusion through the epidermis and during the water uptake. It is likely that dissolved CH4 is directly gasified in the root cortex and further diffuses upwards to the root-shoot transition zone traveling through intercellular spaces and aerenchyma. The aerenchyma system is developed by the plant to transport the oxygen necessary for respiration from leaves towards the roots. Just like CH4 diffuses from the soil into the root system, oxygen diffuses from the root into the soil, creating a relative oxygen-rich zone in the rhizosphere. CH4 is partly oxidized in the rhizosphere to CO2 by methanotrophic bacteria. Methanogenesis in the rhizosphere itself is suppressed by oxygen. The transport of CH4 to the atmosphere depends on the properties of the rice plant. The flux of gases in the aerenchyma depends on permeability coefficients, concentration gradients of roots and the internal structure of the aerenchyma. The number of tillers m−2, root mass, rooting pattern, total biomass, and metabolic activity also influence gas fluxes.
