□: very effective; o: effective/applicable; A: case by case; •: not applicable/require long time; ?: no information; f : increase; decrease; about equal to previous situation; +: positive; -: negative

□: very effective; o: effective/applicable; A: case by case; •: not applicable/require long time; ?: no information; f : increase; decrease; about equal to previous situation; +: positive; -: negative improving organic matter management, changing tillage, rotation, and selecting rice varieties (Table 16.5).

16.7.1 Water Management

A primary switch turning on the production and emission of methane, is the absence of oxygen in the rhizosphere environment. Removal of oxygen from the rhizosphere is normally through consumption by soil bacteria. The presence of the floodwater impedes the diffusion of oxygen from the atmosphere into the soil and thus keeps it anoxic. Hence, water management is one of the most effective options in decreasing CH4 emission, because it prevents the development of soil reductive conditions. Water management, such as mid-season drainage and intermittent irrigation, is commonly used in irrigated paddy fields to control surplus tillering and to supply rice roots with molecular O2 for preventing sulfide toxicity (Kanno et al. 1997). However, the reduction in CH4 emission achieved through intermittent irrigation varies in a broad range from 7 to 80% and imply a number of constraints due to an inverse effect on N2O emission, the third most important greenhouse gas. Changes in the soil moisture regime stimulate nitrification (through soil dying) and denitrification (through soil wetting) and thus, enhance emissions of N2O (Bronson et al. 1997; Zheng et al. 1997 and Abao et al. 2000).

The extent of mitigation is likely to vary appreciably depending on different factors, e.g. soil texture, percolation rate, frequency of drainage, duration of dry period, etc. Field drying at mid-tillering stage has been shown to reduce CH4 emission by 15-80%, as compared to continuous flooding, without a significant effect on grain yield. The net impact of mid-tillering drainage was diminished when (i) rainfall was strong during the drainage period and (ii) emissions were suppressed by very low levels of organic substrate in the soil (Wassmann et al. 2000). Mid-Season Drainage

Reduction in CH4 emission by mid-season drainage (aeration) was first observed in the early field measurement in Japan (Yagi and Minami 1990). Mid-season drainage, supplies oxygen into soil, resulting in a reduction of CH4 production. This practice encompasses a distinct period of one week when irrigation is interrupted. Rate of methane efflux shows a short-term peak at the beginning of soil aeration due to the release of soil-entrapped methane and then followed by persistently low emissions even when the fields are flooded again. A properly timed mid-season drainage appears as a promising option to achieve net-gains in greenhouse gas reduction when the baseline of CH4 emissions is very high (Wassmann et al. 2000). Single mid-season drainage may reduce seasonal CH4 emission rates by about 50%, which can be explained by the influx of oxygen into the soil (Kimura 1992; Sass etal. 1992).

CH4 emission is very sensitive to duration and timing of the drainage period, hence management practice may further be improved to reduce emission, especially in areas where the general concept of mid-season drainage is hardly known as a successful agronomic practice by some farmers. This mitigation practice is, however, not feasible during periods of heavy rainfall and when excess water is not available to flood it again. Therefore, in case of non-availability of water for reflooding, it has limited applicability in time and space. Mid-season drainage supplies oxygen into soil, resulting in a reduced of CH4 production and induced CH4 oxidation in soil. Alternate Flooding/Drying

The water regime refers to alternate flooding and aeration (drying) of the soil throughout the vegetation period. Methane emission from this water regime in northern India, are generally very low, but N2O emissions from this regime vary in a broad range. In contrast to mid-season drainage, the time intervals between wet and dry conditions appear to be too short to facilitate the shift from aerobic to anaerobic soil conditions. Yagi et al. (1994) observed CH4 emission reduction of approximately 50% in intermittent drying plots.

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