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: email@example.com)
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.