Methane

Methane contributes about 20% of the estimated anthropogenic radiative forcing, second to CO2 which contributes 60% (Lassey 2007). Enteric fermentation by ruminants, rice cultivation, anaerobic waste processing and manure managements are the principal sources of CH4 from agriculture. US-EPA (2007) estimated that about 28% of CH4 emission was the result of livestock products. However, a certain amount of CH4 is also consumed by microorganisms in aerobic soils by oxidation, the only known biological sink.

a. Rice cultivation: Methane emission from rice fields to the atmosphere is controlled by CH4 production (methanogenesis), CH4 oxidation (methanotrophy) and CH4 transport process (Aulakh et al. 2000a; Kruger et al. 2001). Thus, strategies to reduce CH4 emission from rice cultivation may be targeted at (i) reducing CH4 production, (ii) increasing CH4 oxidation and (iii) reducing CH4 transport through plants. CH4 production from flooded paddy is closely associated with rice growing and different cultural practices, commonly used in rice cultivation, can influence CH4 production and its emission. Rice cultivation, considered as one of the most important anthropogenic sources of CH4 emission, also appears to be the most suitable candidate for reducing CH4 in the atmosphere because of the possibility of controlling emissions by selected agronomic practices (Neue and Roger 1993; Wassmann et al. 1993, 2004). Worldwide researches indicated that water management (Mishra et al. 1997), organic amendment (Rath et al. 1999; Bharati et al. 2000), fertilizer management (Adhya et al. 1998; Rath et al. 1999; Babu et al. 2006) and rice cultivars (Adhya et al. 1994; Shalini et al. 1997; Satpathy et al. 1998; Mitra et al. 1999) affect the flux of CH4 from this economically important agro-ecosystem and can be suitably manipulated for mitigation of CH4 emission from flooded rice fields (Table 15.3). With the current technology, improvements in management of water and nutrients and other cultural practices could substantially reduce CH4 emission from rice cultivation. The total potential for reducing CH4 emissions from agriculture amounts to 24-92 Tgyr-1 (15-65% of current levels) with potential reduction from rice cultivation amounting to 8-35 Tgyr-1, depending on effectiveness of proposed options and degree of implementations (Cole et al. 1997). Potential technologies include water management, fertilizer management, cropping pattern, vari-etal/cultivar selection and the use of selective inhibitors (Table 15.4). b. Manure management: CH4 is produced from anaerobic decomposition of animal manure in slurry pits, solid manure piles and from moist soil following incorporation of manure (Lassey 2007). Methane emission from swine lagoons from North Carolina was 8-62KgCH4ha-1 (Sharpe et al. 2002). In a series of four lagoons designed to successively purify water from a swine production facility, the gas flux from Lagoon I, directly receiving animal waste was dominated by CH4 (79% of total gas flux), while the gas flux in subsequent lagoons were dom-

Table 15.3 Impact of cultural practices on CH4 emission from rice fields (Data from field experiments conducted at Central Rice Research Institute. Cuttack. India) Cultivation Seasonal flux of Grain yield

practice

Rice variety

Treatment

CH4 (kg.ha )

(Mg.ha )

% Change

Stand

Gayatri

Transplanted

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