Methane emission from irrigated and intensively managed rice fields in Central Luzon (Philippines)

Methane (CH4) emissions were measured with an automated system in Central L uzon, the major rice producing area of the Philippines. Emission records co vered nine consecutive seasons from 1994 to 1998 and showed a distinct seas onal pattern: an early flush of CH4 before transplanting, an increasing tre nd in emission rates reaching maximum toward grain ripening, and a second f lush after water is withdrawn prior to harvesting. The local practice of cr op management, which consists of continuous flooding and urea application, resulted in 79-184 mg CH4 m(-2) d(-1) in the dry season (DS) and 269-503 mg CH4 m(-2) d(-1) in the wet season (WS). The higher emission in the WS may be attributed to more labile carbon accumulation during the dry fallow peri od before the WS cropping as shown by higher % organic C. Incorporation of sulfate into the soil reduced CH4 emission rates. The use of ammonium sulfa te as N fertilizer in place of urea resulted in a 25-36% reduction in CH4 e missions. Phosphogypsum reduced CH4 emissions by 72% when applied in combin ation with urea fertilizer. Midseason drainage reduced CH4 emission by 43%, which can be explained by the influx of oxygen into the soil. The practice of direct seeding instead of transplanting resulted in a 16-54% reduction in CH4 emission, but the mechanisms for the reducing effect are not clear. Addition of rice straw compost increased CH4 emission by only 23-30% as com pared with the 162-250% increase in emissions with the use of fresh rice st raw. Chicken manure combined with urea did not increase CH4 emission. Fresh rice straw has wider C/N (25 to 45) while rice straw compost has C/N = 6 t o 10 and chicken manure has C/N = 5 to 8. Modifications in inorganic and or ganic fertilizer management and water regime did not adversely affect grain yield and are therefore potential mitigation options. Direct seeding has a lower yield potential than transplanting but is getting increasingly popul ar among farmers due to labor savings. Combined with a package of technolog ies, CH4 emission can best be reduced by (1) the practice of midseason drai nage instead of continuous flooding, (2) the use of sulfate-containing fert ilizers such as ammonium sulfate and phosphogypsum combined with urea; (3) direct seeding crop establishment; and (4) use of low C/N organic fertilize r such as chicken manure and rice straw compost.