slow release (Coated urea) and fast release (Compound fertilizer) of N sources (Sugii et al. 1999).

16.7.3 Pesticides

In modern rice culture, pesticides are being increasingly used. Most pesticides at field-recommended doses are considered to be harmless to beneficial soil microorganisms and their activities. However, some pesticides may effect inhibition or stimulation of certain microbial transformations in the rice fields. There is an evidence for the inhibition of nitrification (Ray et al. 1980) and nitrogenase activity (Patnaik et al. 1994) by an organochlorine insecticide, hexachlorocyclohexane (HCH), and stimulation of autotrophic nitrification of NH+ by a carbamate insecticide, carbofuran (Ramakrishna and Sethunathan 1982). However, there is less information available on the effects of pesticides on bacteria involved in the production or consumption of CH4. The organochlorine insecticide DDT is known to inhibit CH4 production as well as CH4 oxidation in the culture media (McBridge and Wolfe 1971). Topp (1993) found that the pesticides, bromoxynil and methomyl as well as nitrification inhibitor, nitrapyrin, were inhibitory to CH4 oxidation at 50 ^gl"1, and the inhibitory effect lasted for <3 weeks. None of these agrochemicals were inhibitory to CH4 oxidation when applied at a concentration of 5 ^gl"1. Arif et al. (1996) reported a negative effect of the herbicide, 2, 4-dichlorophenoxyacetic acid (2, 4-D) on CH4 oxidation in an arable soil. The degree of inhibition of CH4 oxidation increased with increasing levels of 2, 4-D, with 100 ^g 2, 4-D g"1 soil being completely inhibitory. Satpathy et al. (1997) found that the application of a commercial formulation of a widely used organochlorine insecticide, HCH to flooded rice soils, or its technical-grade isomers (a", p", and 8"), to laboratory-incubated flooded soils, retarded the production and emission of CH4, even at a field application rate of 1-2 kg active ingredient (a.i.) ha"1 to control insect pests. In contrast, HCH inhibited CH4 oxidation significantly at 5 ^g g"1 soil, and almost completely at 10 ^gg"1 soil. Similarly, a commercial formulation of carbofuran, a carbamate insecticide, when applied at rates of 2 kg and 12 kg a.i. ha"1 to flooded field planted to rice, resulted in a significant inhibition of CH4 emissions. Interestingly, in laboratory incubation studies, carbofuran inhibited net CH4 production when applied at low rates (5 and 10 mg g"1 soil), but stimulated when applied at a rate of 100 mgg"1 soil. The oxidation of CH4 was, however, stimulated by carbofuran when applied at low rates, and inhibited when carbofuran was applied at a rate of 100 ^g g"1 soils (Kumaraswamy et al. 1997).

16.7.4 Chemical Inhibitors and Electron Acceptors

Oxygen, alternate acceptors and some chemicals inhibit methanogenesis by different mechanisms. A central electron carrier (co-enzyme F420) is irreversibly disso ciated by oxygen (Vogels et al. 1988). Exposure to low levels of O2 (a few ppm) lowers the adenylate charge of methanogens and cause their death (Robertson and Wolfe 1970). Alternate electron acceptors other than O2, inhibit methanogensis in mixed microbial ecosystem by channelizing electron flow to microorganisms that are thermodynamically more efficient than methanogens (e.g. denitrifers or sulphate reducers).

Chemical inhibitors have also been used to lower the emission of CH4 from agricultural fields. 2-bromoethane sulphonic acid, because of working as structural analogue of co-enzyme M of methanogenesis, inhibits methane formation in the soil. Chlorinated CH4 (e.g. chloroform and methyl chloride) is also found to be competitive inhibitor of CH4 formation (Cicerone and Oremland 1988).

Methanogenesis may also be inhibited by some metals, like copper and cadmium (Drauschke and Neumann 1992). Inubushi et al. (1990) also studied the effect of cadmium (Cd) on CH4 emission from the rice soil in both laboratory and pot experiment and reported lower CH4 emission.

16.7.5 Cultural Practices and Crop Diversification

Mechanical disturbances of flooded soils caused by cultural practices (e.g. land preparation, transplanting, weeding, fertilization and harvest) increase ebullition of soil-entrapped CH4 (Neue et al. 1994). Less cultural disturbances of reduced soils and shorter flooding period in direct-seeded rice, also lower CH4 emissions. The adoption of direct seedling (wet and dry seedling), instead of transplanting, is highly dependent on the ability to manage water regimes. Often rice fields cannot be drained in the wet season and in rain-fed areas, by the farmers because of uncertain rainfall.

Crop diversification is a feasible option to reduce total CH4 emissions in line with economic benefits. In rice growing areas with year around irrigation, production can be shifted by adopting a rice upland cropping system (i.e. sequential cropping of one upland crop before or after one or two crops of rice). Such cultural practices would drastically cut CH4 emission from paddy fields.

16.7.6 Soil Amendments

Soil amendment is essential for intensive cultivation. Nitrogen is an element that is fundamental to crops and strongly affects growth and yield of crops. However, the application of large amounts of nitrogen can cause over-luxuriant growth, spindly growth, lodging of rice plants, and N2O emission (Cai et al. 1997; Hua et al. 1997). Nitrogen fertilizers are also known to influence the activities of many groups of microorganisms. The source, mode and rate of application of nitrogen fertilizers influence CH4 production and emission from flooded rice paddies. Even under the best possible fertilization practices, substantial amounts of the N applied to the field are emitted to the atmosphere. In irrigated rice, gaseous losses of N may account for up to 48% of the N applied (Reddy and Patrick 1980).

Adding any electron acceptor to the soil, can arrest soil reduction. The main electron-acceptors in submerged soils include dissolved oxygen (O2), NO3-, Fe (III), SO^- and CO2. The final products of reduction in submerged soils are Fe (II) from Fe (III), H2S from SO^- and CH4 from CO2. SO4- is a plant nutrient and can supply the much-needed nutrient in S deficient soil. Sulphate aided fertilizers are known to decrease CH4 emission because of the competition between sulphate-reducing bacteria and methanogens for the substrates, hydrogen and acetate (Hori et al. 1990, 1993). Methane emission has been significantly reduced by 20% by the application of sulphate through ammonium sulphate (Schutz et al. 1989; Lindau et al. 1993), sodium sulphate (28-35%) (Lindau et al. 1993) and gypsum (5570%) (Denier van der Gon and Neue 1994), while a non-significant reduction with potassium sulphate was reported by Wassmann et al. (1993). In ammonium sulfate, although ammonium can reduce CH4 oxidation and thereby, increase CH4 emission (Hutsch et al. 1993; Willison et al. 1995), but the presence of sulphate overwhelms this effect and reduce overall CH4 emission. In a Beijing rice field, an organic amendment plus (NH4)2SO4 as topdressing, applied in different amounts and at different growth stages, reduced methane emission by about 58% and increased rice yield by top dressing (Kumar et al. 2000). Minamikawa et al. (2005) reported that increases in the application rate of ammonium in the form of ammonium sulphate from 45 to 135 kg N/ha (compared to a convential rate of 90 kg N/ha) decreased CH4 emission and increased rice yield under field condition (Table 16.7).

Addition of single super phosphate (which also contains sulphur) as phosphorus (P) source to flooded rice soils, not only supplied P to wetland rice, but also inhibited methane production. On the other hand, application of P as K2HPO4 was found to stimulate methane production in a P-deficient soil. Also application of rock phosphates, which contained sulphur, retarded methane production, while rock phosphate sources, that did not contain sulphur, stimulated methane production. It was concluded that the application of single super, phosphate to wetland rice could be used as a methane mitigation strategy in P-deficient soils where addition of P is necessary for sustaining increase rice production and productivity (Adhya et al. 1997). Lindau et al. (1994) evaluated the efficacy of application of calcium sulphate for mitigating methane evolution in irrigated rice field. Calcium sulphate was applied at 0, 1000, and 2000 kg ha-1 to rice plots treated with urea (128kgNha-1). It was

Table 16.7 Total CH4 emission and rice yield at different application rate of ammonium sulphate (Minamikawa et al. 2005)

Application rate

CH4 emission

Grain yield

Straw yield

(kg N/ha)

(g CH4/m2)

(g DW/nr)

(g DW/nr)

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