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Methane and [N.sub.2]O are gases that are several times more radiatively active than C[O.sub.2]. It is well known that flooded rice (Oryza sativa L.) soils are a globally important source of atmospheric C[H.sub.4]. Mitigation strategies for C[H.sub.4] flux, such as mid-season drainage, might have the opposite effect on [N.sub.2]O emissions. An automated chamber system at the International Rice Research Institute in the Philippines measured C[H.sub.4] and [N.sub.2]O 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 [N.sub.2]O-N [m.sup.-2] [d.sup.-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.sup.-1], dry) or green manure (GM; Sesbania rostrata L.; 12 t [ha.sup.-1], wet) stimulated C[H.sub.4] flux severalfold. Seasonal C[H.sub.4] flux with ammonium sulfate (AS) was one-fourth to one-third the flux with urea. During the DS, however, the seasonal [N.sub.2]O 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 C[H.sub.4] flux up to 60%. However, [N.sub.2]O flux increased sharply during the drainage period at mid-tillering until reflooding, when it dropped back to near zero.

Methane and [N.sub.2]O are radiatively important gases that are emitted from waterlogged soils. Automated chamber measurements of C[H.sub.4] and [N.sub.2]O fluxes were carried out at the International Rice Research Institute in the Philippines in flooded rice (Oryza sativa L.) and fallow rice fields 24 h a day between December 1992 and April 1994. This period included three 5- to 11-wk rainfed fallow periods. During the first two fallows, the soil was generally aerobic, and moderate amounts of N[O.sub.3] accumulated (7-20 kg N[O.sub.3]-N [ha.sup.-1]). Moderately high, continuous [N.sub.2]O fluxes were evident during these two fallow periods. This [N.sub.2]O was apparently emitted during nitrification of mineralized organic N in the topsoil and possibly from denitrification in the wet subsoil. Nitrous oxide fluxes were highest (up to 80 mg [N.sub.2]O-N [m.sup.-2] [d.sup.-1]) immediately after rainfalls >20 mm, and following the establishment of flooding for rice at the end of the fallows. Acetylene inhibition in intact cores at these times showed that more [N.sub.2] was produced than [N.sub.2]O. Denitrification of accumulated N[O.sub.3] was therefore occurring after the wetting events. Methane emissions were generally absent during the fallow periods. Two exceptions were immediately after rice harvest and 1 to 2 wk after the establishment of the permanent flood. Following flooding and green manure (GM; Sesbania rostrata L.) incorporation, C[H.sub.4] fluxes appeared within 7 d. During the third fallow period, which was unusually wet, no N[O.sub.3] accumulated in the soil. Nitrous oxide emissions were not significant, and low levels of C[H.sub.4] fluxes persisted throughout this fallow period. This study demonstrates that rice soils in the fallow periods can be significant sources of [N.sub.2]O, and during wet fallow seasons, important sources of C[H.sub.4] as well.