13 Characterization of methane emissions from rice fields in Asia. II. Differences among irrigated, rainfed, and deepwater rice R. Wassmann 1,2 , H.U. Neue 1,3 , R.S. Lantin 1 , K. Makarim 4 , N. Chareonsilp 5 , L.V. Buendia 1 & H. Rennenberg 2 1 International Rice Research Institute (IRRI), P. O. Box 3127, Makati City 1271, Philippines; 2 Fraunhofer Institute for Atmospheric Environmental Research (IFU), Garmisch-Partenkirchen, Germany; 3 UfZ-Centre for Environmental Research Leipzig-Halle, Department of Soil Sciences, Halle, Germany; 4 Central Research Institute for Food Crops, Bogor, Indonesia; 5 Prachinburi Rice Research Institute, Prachinburi, Thailand Key words: water regime, soil aeration, mineral fertilizer, rainfall, acid sulfate soil, soil pH, Indonesia, Thailand, Philippines, mitigation options Abstract Methane (CH 4 ) emission rates were recorded automatically using the closed chamber technique in major rice- growing areas of Southeast Asia. The three experimental sites covered different ecosystems of wetland rice— irrigated, rainfed, and deepwater rice—using only mineral fertilizers (for this comparison). In Jakenan (Indone- sia), the local water regime in rainfed rice encompassed a gradual increase (wet season) and a gradual decrease (dry season) in floodwater levels. Emission rates accumulated to 52 and 91 kg CH 4 ha -1 season -1 corresponding to approximately 40% of emissions from irrigated rice in each season. Distinct drainage periods within the season can drastically reduce CH 4 emissions to less than 30 kg CH 4 ha -1 season -1 as shown in Los Baños (Philippines). The reduction effect of this water regime as compared with irrigated rice varied from 20% to 80% from season to season. Methane fluxes from deepwater rice in Prachinburi (Thailand) were lower than from irrigated rice but accumulated to equally high seasonal values, i.e., about 99 kg CH 4 ha -1 season -1 , due to longer seasons and assured periods of flooding. Rice ecosystems with continuous flooding were characterized by anaerobic conditions in the soil. These conditions commonly found in irrigated and deepwater rice favored CH 4 emissions. Temporary aera- tion of flooded rice soils, which is generic in rainfed rice, reduced emission rates due to low CH 4 production and high CH 4 oxidation. Based on these findings and the global distribution of rice area, irrigated rice accounts glo- bally for 70-80% of CH 4 from the global rice area. Rainfed rice (about 15%) and deepwater rice (about 10%) have much lower shares. In turn, irrigated rice represents the most promising target for mitigation strategies. Proper water management could reduce CH 4 emission without affecting yields. Introduction The human population continues to increase by 85 mil- lion people a year; the developing world will add an- other 2 billion people over the next three decades. In- tensification of rice cultivation to meet the demand for rice by the increasing human population is imperative, especially in Asia where approximately 90% of the rice is grown and consumed (IRRI, 1993a). Given the ex- pected doubling in rice production in Asia, research on improving rice yield should focus on strategies that do not harm the environment. Rice fields represent glo- bally one of the main sources of the greenhouse gas methane (CH 4 ) (GEIA, 1993; IPCC, 1996), but the glo- bal source strength of rice cultivation remains uncer- tain. The diversified conditions in crop management and environments for growing rice are not sufficiently characterized for accurate estimates (Sass et al., 1990; Rennenberg et al., 1992; Neue & Roger, 1994; Yagi et al., 1994; Byrnes et al., 1995; Wassmann et al., 1998). Nutrient Cycling in Agroecosystems 58: 13–22, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.