Statistical analysis of the major variables controlling methane emission from rice fields

Rice cultivation is an important anthropogenic source of atmospheric methane (CH4), the emission of which is affected by management practices. Many field measurements have been conducted in major rice-producing countries in Asia. We compiled a database of CH4 emissions from rice fields in Asia from peer-reviewed journals. We developed a statistical model to relate CH4 flux in the rice-growing season to soil properties, water regime in the rice-growing season, water status in the previous season, organic amendment and climate. The statistical results showed that all these variables significantly affected CH4 flux, and explained 68% of the variability. Organic amendment and water regime in the rice-growing season were the top two controlling variables; climate was the least critical variable. The average CH4 fluxes from rice fields with single and multiple drainages were 60% and 52% of that from continuously flooded rice fields. The flux from fields that were flooded in the previous season was 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. In contrast to the previously reported optimum soil pH of around neutrality, soils with pH of 5.0-5.5 gave the maximum CH4 emission. The model results demonstrate that application of rice straw at 6 t ha(-1) 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. Default emission factors and scaling factors for different water regimes and organic amendments derived from this work can be used to develop national or regional emission inventories.