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Methane and N2O are gases that are several times more radiatively active than CO2. It is well known that flooded rice (Oryza sativa L.) soils are a globally important source of atmospheric CH4. Mitigation strategies for CH4 flux, such as mid‐season drainage, might have the opposite effect on N2O emissions. An automated chamber system at the International Rice Research Institute in the Philippines measured CH4 and N2O fluxes from flooded rice and fallow rice fields essentially 24 h a day between December 1992 and April 1994. This period included two irrigated dry rice‐growing seasons (DS) and one wet rice‐growing season (WS). Nitrous oxide fluxes were generally barely detectable during the growing seasons, but small peaks (maximum 3.5 mg N2O‐N m‐2 d‐1) appeared after N fertilizer applications. Methane fluxes, on the other hand, were evident throughout the rice‐growing seasons. Organic matter additions as straw (5.5 t ha‐1, dry) or green manure (GM; Sesbania rostrata L.; 12 t ha‐1, wet) stimulated CH4 flux severalfold. Seasonal CH4 flux with ammonium sulfate (AS) was one‐fourth to one‐third the flux with urea. During the DS, however, the seasonal N2O flux was 2.5 times higher with AS than with urea. Mid‐season drainage (2‐wk duration) at either mid‐tillering or panicle initiation was very successful in suppressing CH4 flux up to 60%. However, N2O flux increased sharply during the drainage period at mid‐tillering until reflooding, when it dropped back to near zero.

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