The possible strategies for mitigating methane emission from rice cultivation can be made by altering water management, particularly promoting mid-season aeration by short-term drainage. Improving organic matter management by promoting aerobic degradation through composting or incorporating into soil during off-season drained period, is another promising technique. Organic amendments to flooded soils increase methane production and emission. However, application of fermented manure, like biogas slurry, reduces the emission (Debnath et al. 1996). In addition, nitrification inhibitors have been shown to inhibit methane emission. Another mitigation option may be selection of low CH4 emitting rice cultivars, as cultivars grown in similar conditions show pronounced variations in methane emission (Mitra 2000). Screening of rice cultivars with few unproductive tillers, small root system, high root oxidative activity and high harvest index are ideal for mitigating methane emission from rice fields.
Combined with a package of technologies, methane emission can best be reduced by (a) the practice of mid-season drainage instead of continuous flooding, (b) direct crop establishment like dry seeded rice and (c) use of low C: N organic manure and biogas slurry.
Appropriate crop management practices, which lead to increase N use efficiency and yield, hold the key to reduce nitrous oxide emission. Application of nitrate (NO3-N) fertilizers e.g. calcium ammonium nitrate (CAN), in crops with aerobic conditions and ammonium (NH4-N) fertilizers e.g., ammonium sulphate, urea, in wetland crops also help reducing the nitrous oxide emission (Pathak and Ned-well 2001). Curtailing the nitrification process by the use of nitrification inhibitor may further decrease the N2O emission from soil. There are some plant-derived organics, such as neem oil and neem cake, which can also act as nitrification inhibitors. These are being experimented in fields to reduce the emission of nitrous oxide and increase the fertilizer use efficiency. Other biocidal inhibitors, such as karanja seed extract, have been found to retard nitrification by 60-70% (Majumdar et al. 2000). The efficacy of various mitigation technologies, however, needs to be tested in farmers' fields. Moreover, such technologies need to be also assessed for non-target effects and economic feasibility.
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