Not determined



The same letters in each column are not significantly different at p = 0.05 by Tukey's HSD test

The same letters in each column are not significantly different at p = 0.05 by Tukey's HSD test found that calcium sulphate application reduced methane evolution by 29 and 46% compared to control (no calcium sulphate applied) at the low (1000 kg ha-1) and high rate (2000 kg ha-1), respectively, over the 70 days sampling.

Application of salts to wetland rice soils can affect methane production and emission. For example, addition of sodium chloride at relatively high concentration (0.18 M) retards methane production (Patel and Roth 1977). Application of seawater also retards methane formation at relatively low rates of salt concentrations, because of sulphate content in the seawater (Koyama et al. 1970). The presence of sulphate provides the opportunity for sulphate reducers to compete with methanogens for hydrogen. Methane emission from wetland rice fields on saline, were lower than CH4 emission from otherwise comparable non-saline rice fields (Denier van der Gon and Neue 1995).

Urea [(NH2)2CO] is the most used nitrogen fertilizer in the world, accounting for 47% of the total as of 2002 (FAO 2005). The CH4 emission from rice fields was decreased by 18% with the shallow incorporation (5 cm deep) of urea (100 or 200 kgNha-1) compared to that of the control plots, and by about 40% with the deep incorporation (20 cm deep) of urea (200 kg N ha-1) compared to that of shallow incorporation (Schutz et al. 1989). But CH4 emissions increased by about 19% with the surface application of urea (200 kg N ha-1) compared to those of untreated plots. In a laboratory incubation study, addition of urea (100,200 and 300 mg N kg-1 soil) to flooded soil stimulated CH4 production (Wang et al. 1992). Kimura et al. (1992) suggested that foliar applications of N fertilizers could suppress CH4 fluxes from flooded paddy soils. In a recent study, Rath et al. (1999) found that the sub-surface application of urea super granules was marginally effective in reducing the CH4 flux relative to that in the untreated control. Compared to the effects of ammonium sulphate, the effects of urea on CH4 emission are very complex, bringing conflicting results among different fields and soils. Many researchers have reported that CH4 emission increased with increasing application of urea as compared to no application under field conditions (Lindau and Bollich 1993; Matsumoto et al. 2002; Dubey 2003). This result was attributed to an increase in soil pH caused by the hydrolysis of urea (Wang et al. 1992), an increase in rice biomass (Banik et al. 1996) and the inhibition of methanotrophs by ammonium (Conrad and Rothfuss 1991; Kumaraswamy et al. 1997; Dubey 2003).

In principle, three different reasons have been suggested for the inhibitory effect of nitrogenous fertilizers, especially NH4-N fertilizers on CH4 oxidation: (1) an intermediate inhibition of the methanotrophic enzyme system (Bedard and Knowles 1989); (2) secondary inhibition through the NO- production from methan-otrophic NH+ oxidation (Megraw and Knowles 1989); and (3) dynamic alterations of microbial communities of soil (Powlson et al. 1997).

Addition of other oxidants may also influence CH4 emission from rice paddy fields as they delay the development of soil reductive conditions. There have been several reports on the effects of Fe-containing materials on decreasing CH4 emission (Furukawa et al. 2001; Qu et al. 2004). Addition of Fe-containing slag increased the soil Eh and pH (Yoshiba et al. 1996; Nozoe et al. 1999). Kitada et al. (1993) showed that irrigation water containing 40mgL-1 nitrate decreased CH4 emission by 23%

as compared to normal irrigation water (less than 1 mgL-1). Addition of iron containing materials as bauxite, iron ore and residues of iron manufacture, probably reduces CH4 emission. This option, as well as mid-season draining practice, have been adopted to prevent Aki-ochi, a disorder in Japanese rice attributed to hydrogen sulphide toxicity due to low soil redox potential (Table 16.8). In this case, the effect depends on the contents of free iron oxide in the soil dressing. Yoshiba et al. (1996) also reported impact of Mn++ and SO4- on reducing CH4 emission. Since excessive use of oxidants would damage the physiology of rice plants, these substances could be used judiciously as a mitigating option.

16.7.7 Organic Matter Application

The application of organic matter conserves soil fertility for sustainable rice production. In Japan, organic matter is applied together with chemical fertilizer. Because of the slow release of organic nitrogen during decomposition, the use of organic matter prevents temporal and spatial variations in the rice yield. Since, the organic matter supplies substrates for the development of soil reductive conditions and CH4 production, the kind, rate, timing and degree of maturation affect the magnitude of CH4 emission.

Methane emissions are generally enhanced by organic inputs in to the soil, such as straw or manure amendment. The increment in CH4 emission following organic inputs depends on quantity, quality and timing of the application (Yagi and Minami 1990; Sass et al. 1991). Rice straw and manure are typically applied before transplanting, resulting in an emission peak during the first half of the growing season. High temperature in the weeks, following the incorporation of these material, results in a pronounced emission peak, whereas low temperature during this period diminishes this peak (Wassmann et al. 2000). Incorporation of organic material also creates a pool of readily available N and therefore, often stimulates N2O emission (Rolston et al. 1982; Flessa and Beese 1995). However, the observed increments in N2O emission were not as pronounced as for CH4 emission. Fermentation of Manure

Several field studies have compared different types of organic amendments with regard to CH4 emissions. While the differences between fresh materials either straw, animal manure or green manure, have been relatively small, field records showed a big disparity between emissions triggered by fresh pre-fermented material (Yagi and Minami 1990; Wassmann et al. 1993; Corton et al. 2000). During fermentation processes, the incorporation of fermented material into the soil entails a lower emission potential. Applying residue from a biogas generator could reduce emission by approximately 60% as compared to fresh organic amendments and 52%, compared to the locally practiced combination of urea and organic amendments (Wassmann et al. 1993). The combustion of biogas will also save fossil fuel consumption, hence this mitigation option could be considered as a win-win solution.

Table 16.8 Effects of various options against Aki-ochi on the yield of rice (Yanagisawa 1978)

Distribution of yield index a . , . No. of _l_ Average yield

Table 16.8 Effects of various options against Aki-ochi on the yield of rice (Yanagisawa 1978)

Distribution of yield index a . , . No. of _l_ Average yield



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