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.

Recent anthropogenic emissions of key atmospheric trace gases (e.g. CO2 and CH4) which absorb infra‐red radiation may lead to an increase in mean surface temperatures and potential changes in climate. Although sources of each gas have been evaluated independently, little attention has focused on potential interactions between gases which could influence emission rates. In the current experiment, the effect of enhanced CO2 (300 μL L–1 above ambient) and/or air temperature (4 °C above ambient) on methane generation and emission were determined for the irrigated tropical paddy rice system over 3 consecutive field seasons (1995 wet and dry seasons 1996 dry season). For all three seasons, elevated CO2 concentration resulted in a significant increase in dissolved soil methane relative to the ambient control. Consistent with the observed increases in soil methane, measurements of methane flux per unit surface area during the 1995 wet and 1996 dry seasons also showed a significant increase at elevated carbon dioxide concentration relative to the ambient CO2 condition (+49 and 60% for each season, respectively). Growth of rice at both increasing CO2 concentration and air temperature did not result in additional stimulation of either dissolved or emitted methane compared to growth at elevated CO2 alone. The observed increase in methane emissions were associated with a large, consistent, CO2‐induced stimulation of root growth. Results from this experiment suggest that as atmospheric CO2 concentration increases, methane emissions from tropical paddy rice could increase above current projections.

Methane and N2O are radiatively important gases that are emitted from waterlogged soils. Automated chamber measurements of CH4 and N2O 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 NO3 accumulated (7–20 kg NO3‐N ha‐1). Moderately high, continuous N2O fluxes were evident during these two fallow periods. This N2O 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 N2O‐N m‐2 d‐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 N2 was produced than N2O. Denitrification of accumulated NO3 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, CH4 fluxes appeared within 7 d. During the third fallow period, which was unusually wet, no NO3 accumulated in the soil. Nitrous oxide emissions were not significant, and low levels of CH4 fluxes persisted throughout this fallow period. This study demonstrates that rice soils in the fallow periods can be significant sources of N2O, and during wet fallow seasons, important sources of CH4 as well.

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