Nitrification Inhibitors

Nitrification inhibitors (NI) prevent the conversion of NH+-N into NO--N (Bron-son et al. 1991), thereby reducing emissions of N2O via nitrification directly (Majumdar et al. 2000; Kumar et al. 2000) and then conserve by reducing the availability of NO- for denitrification (Aulakh et al. 1984; Bronson et al. 1992). Nitrification inhibitors are also increasingly recommended for rice agriculture to minimize fertilizer N losses (Prasad and Power 1995) by limiting the formation of nitrate from ammonium.

In addition to their role in controlling various processes of N losses, nitrification inhibitors have been also reported to inhibit CH4 emission from flooded soil planted with rice. Besides, artificially synthesized materials, some plant products have been also found suitable to mitigate N2O from rice fields (Majumdar et al. 2000).

Nitrification inhibitors, such as dicyanamide (DCD), acetylene and nitrapyrin, mitigated emission of CH4 gas from rice fields by inhibiting the growth of nitrifers, methanogens and methanotrophs (McCarty 1999). In a study at Cuttack, application of nitrification inhibitor i.e. dicyanamide (DCD), inhibited CH4 emission, while nimin (a plant product), another nitrification inhibitor, stimulated its emission from flooded rice field. Besides DCD, calcium carbide, ammonium thio sulphate and thiourea have also been shown to inhibit CH4 emission from flooded soil planted to rice (Bronson and Mosier 1991 and Keerthisinghe et al. 1993). Bronson and Mosier (1991) reported a significant reduction in CH4 emissions from rice paddies, following the application of urea in combination with encapsulated calcium carbide. Lindau et al. (1993) found that CH4 emissions from rice fields were decreased by 35 and 14%, following the application of encapsulated calcium carbide and dicyandimide, respectively. According to Keerthisinghe et al. (1993), wax coated calcium carbide and nitrapyrin effected a significant reduction in CH4 emission from rice paddies. The decrease in CH4 emissions in plots treated with wax-coated calcium carbide was attributed directly to the slow release of acetylene, a known inhibitor of methanogenesis (Bronson and Mosier 1991).

In a rice field study, Rath et al. (1999) observed that CH4 emissions were lowest in plots treated with a mixture of prilled urea and Nimin, a nitrification inhibitor which inhibits the autotrophic oxidation of NH+ to NO- (Sahrawat and Parmar 1975). However, the mechanism of the inhibitory action of Nimin on CH4 emission is not known. On the contrary, nitrifications inhibitors are also known to inhibit CH4 oxidation (Bronson and Mosier 1994). Kumaraswamy et al. (1997) showed that nitrification inhibitors (thiourea, sodium thiosulphate and dicyandi-amide) inhibited the CH4-oxidizing activity of flooded rice field soil samples. Both surface soil and rhizospheric samples, which harboured higher populations of CH4-oxidizing bacteria than the sub-surface soil, showed inhibition of CH4 oxidation.

Several benzene-ring compounds (Patel and Roth 1977) and N-containing compounds (Bollag and Czlonkowski 1973) are also known to suppress methanogens-esis in pure cultures and in soils. Chemicals, known to inhibit CH4 production as well as CH4 oxidation, include DDT (2, 2-dichlorodiphenyltrichloroethane) (McBridge and Wolfe 1971) and the nitrification inhibitor, acetylene (Sprott et al. 1982). Availability of these specific as well as general inhibitors of microorganisms holds a promise for their use along with chemical fertilizers or other agro-chemicals to mitigate CH4 emission from rice soils. Bharati et al. (2000) also studied impact of 6 nitrification inhibitors on CH4 emission and reported reduction in CH4 emission in the decreasing order: Sodium azide > DCD > Pyridine > aminopurine > ammonium thiosulphate> thiourea (Table 16.6).

Sodium azide, a potent NI, is also microbial respiratory inhibitor, causing a reduction in methane production by 75% over that of unamended control (Aulakh and Rennie 1985). HQ (Hydroquinone) is also found to decrease N2O as well as CH4 emission via inhibition of the methanogenic fermentation of acetate (Wang etal. 1991).

Nimin, a plant product, also mitigated methane emission when added to rice fields with prilled urea as compared to pilled urea alone (Rath et al. 2000). Although slow release fertilizers mitigated N2O emission (Majumdar et al. 2000; Abao et al. 2000), but no significant difference in methane emission was found between

Table 16.6 Effects of various nitrification inhibitors on CH4 production (^gkg 1) in an alluvial soil under flooded conditions (Bharati et al. 2000) Days after flooding

Treatments

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