The mechanism of CH4 formation involves either reduction of CO2 with hydrogen ions, where fatty acids and alcohols act as hydrogen donor or transmethylation of acetic acid or methyl alcohol, which does not produce CO2 as an intermediate (Takai 1970; Vogels et al. 1988). Under flooded paddy soils, after the trapped molecular O2 is used, sequential reduction of the soil oxidants (NO3-, Mn4+, Fe3+, SO42- and CO2) progresses in accordance with the thermodynamic theory.
5CH2O + 4NO3- —> 4HCO3- + CO2 + 3H2O + N2 - 448kj
CH2O + 2MnO2 + 3CO2 + H2O —> 2Mn++ + 4HCO3- - 349kj
CH2O + 4Fe (OH)3 + 7CO2 —> 4Fe++ + 8HCO3- + 3H2O - 114kj
The production of methane is preceded by the production of volatile acids. Hydrogen evolution follows the disappearance of oxygen following flooding of the soil into water. In this sequence, carbon dioxide evolution is followed by methane formation and emission (Takai et al. 1956). The main electron-acceptors in submerged soils include dissolved oxygen (O2), NO-, Fe (III), SO4-, and CO2. The final products of reduction in submerged soils are Fe (II) from Fe (III), H2S from SO4- and CH4 from CO2.
Carbon substrates for methanogenesis are supplied from applied organic matter, exudates and sloughed tissues of rice plants, and soil organic matter (Watanable et al. 1999; Kimura et al. 2004). A major part of the CH4 produced is consumed by methanotrophs under oxidative conditions in the rhizosphere of rice plants and in a layer of soil interfacing with the surface water. Besides, CH4 generation in the sediment is regulated by various edaphic factors like temperature, redox potential, and moisture regime, length of water logging, sulphate and pH (Granfeld and Brix 1999; Singh 2001).
In fact, net CH4 emission is the difference between production and consumption. Processes that regulate the CH4 efflux from the site of generation to the atmosphere include ebullition, molecular diffusion and vascular transport by plants. Ebullition of CH4 gas generally occurs when the partial pressure of the entrapped CH4 within the sediment results in an upward surge of the gas into the atmosphere. While in molecular diffusion, dissolved CH4 diffuses according to the concentration gradients through the sediment-water and air-water interfaces (Bartlett et al. 1985) and the diffusion of gases in water is approximately 10,000 times slower than in air (Wang et al. 1995). The actual diffusion of CH4 from rice fields is a function of CH4 supply to the floodwater, CH4 concentration in the floodwater and prevailing wind speed (Sebacher et al. 1983). Diffusion through floodwater is usually less than 1% of the total flux (Conard 1993). Most of the CH4 emitted to the atmosphere is transported through aerenchyma of rice plants by molecular diffusion or by high-low pressure induced flow. The aerenchyma tissue of aquatic plants helps in transport of atmospheric O2 to the rhizosphere for the root respiration (Nouchi et al. 1990; Schutz et al. 1991 and Nouchi and Mariko 1993) and also facilitates the reverse transport of CH4 produced in the anaerobic zone of flooded soil to the atmosphere. The path of CH4 through the rice plants includes diffusion into the root, conversion into gaseous CH4 in the root cortex, diffusion through cortex and aerenchyma and release to the atmosphere through micropores in the leaf sheath (Nouchi and Mariko 1993; Aulakh et al. 2000).
In the temperate rice fields, more than 90% of CH4 is emitted through plants (Schutz et al. 1989), while from the tropical rice fields, significant amounts of CH4 may evolve by ebullition, particularly during the early part of the season and in the case of high organic input (Wassmann et al. 1998). Thus, rice plants influence the methane dynamics in paddy soils by (1) providing substrate in the form of root exudates to methanogens to enhance the production of CH4; (2) transporting CH4 from soil to atmosphere (conduit effect), and (3) creating aerobic microhabitat in rhizosphere, which is suitable for growth and multiplication of methanotropic bacteria responsible for CH4 consumption (Dubey et al. 2000).
Soils, that support production of CH4 in the anoxic condition, also act as a sink for the same in presence of oxygen. Methane oxidation in aerobic condition is carried out by methanotrophs or methane oxidizers. The enzyme responsible for the initial step in CH4 oxidation is methane monooxygenase (MMO) enzyme, which requires molecular O2 to perform its activity. Methane monooxygenase (MMO) has two isoenzymes i.e. soluble MMO (sMMO) and particulate MMO (pMMO), which require Cu and Fe metals for their activity (Dalton et al. 1993). The overall reaction for methane oxidation given by Dalton and Hocknall (1990) is as follows:
CH4 + 2O2 —> CO2 + 2H2O (CG0 = -780 kj mole-1)
During different stages of CH4 oxidation, Whalen et al. (1990) found that a small fraction (19.1±2.3%) of formaldehyde was assimilated by the plants into cellular biomass. Rhizosphere, being oxic zone, works as a major sink for methane in the sediment. The decrease in CH4 concentration in the rhizospheric zone of some aquatic plants has been reported by Wagatsuma et al. (1992).
Yagi (1997) categorized the factors controlling CH4 emission into four scales (Fig. 16.2). From microbial to global scales, soil properties, rice plant activity, field management and climate are the factors, which modulate CH4 efflux from paddy fields.
Global scale ft
Temperature Water regime
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