Yan, Xiaoyuan (Frontier Research Center for Global Change, Japan, Frontier Research Center for Global Change, ShaowaYokohama, 236-0001, Japan; Phone: 81-45-778-5725; Fax: 81-45-778-5496; Email: yanxy@jamstec.go.jp)

Factors Affecting Methane and Nitrous Oxide Emissions from Croplands in Asia and Potential Mitigation Options

X. Yan *, H. Akiyama, K. Yagi

East, Southeast, and South Asia are among the most densely populated regions in the world. To meet the requirement for food, 54% of the world total chemical nitrogen fertilizer is consumed and 89% of the world rice cultivation area is located there. Thus, the emissions of methane (CH₄) and nitrous oxide (N₂O) from croplands in these regions have been a great concern of scientists and field measurements have been widely conducted. We have collected available field measurement results of CH₄ and N₂O emission from croplands in Asia and have made statistical analysis on the factors affecting the emissions. For CH₄ emission from rice fields, the top influencing factors are organic amendment and water regime, both during the rice-growing season and before the rice-growing season. Soil pH and organic carbon content, and climate also affect CH₄ emission. These six factors together can explain 68% of the variability in observed CH₄ fluxes. In contrast to the previously reported optimum soil pH of around neutrality, an optimum soil pH of 5.0¨C5.5 for CH₄ emission was found through the analysis. The average CH₄ fluxes from rice fields with single and multiple drainage are 60% and 52% of that from continuously flooded rice fields. The flux from fields that were flooded in the previous season is 2.8 times that from fields previously drained for a long season and 1.9 times that from fields previously drained for a short season. Application of rice straw at 6 t ha ^C1 before rice transplanting can increase CH₄ emission by 2.1 times; when applied in the previous season, however, it increases CH₄ emission by only 0.8 times. These results indicate that CH₄ emission from rice fields can be reduced by appropriate water management before and during rice season and by appropriate use of organic materials. Field measurement data of N₂O emission from croplands in Asia are rather disorderly, but the fertilizer-induced N₂O emission factors are generally smaller than the IPCC value of 1.25%. On average, fertilizer-induced N₂O emission factor is 0.44% for upland crops and 0.30% for rice fields. Application of nitrification inhibitors may reduce 24% of the N₂O emission, while the effect of slow release fertilizer is less significant. The effect of organic materials varies, depending on the degree of decomposition or the C/N ratio of the materials. Direct application of straw tends to reduce N₂O emission. Decomposed organic materials increase N₂O emission, but not as effective as inorganic fertilizer at equal nitrogen basis.