Automated chamber measurements of methane and nitrous oxide flux in a flooded rice soil: I. Residue, nitrogen, and water management

Methane and N 2 O are gases that are several times more radiatively active than CO 2 . It is well known that flooded rice (Oryza saliva L.) soils are a globally important source of atmospheric CH 4 . Mitigation strategies for CH 4 flux, such as mid-season drainage, might have the opposite effect on N 2 O emissions. An automated chamber system at the International Rice Research Institute in the Philippines measured CH 4 and N 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 2 O-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 CH 4 flux severalfold. Seasonal CH 4 flux with ammonium sulfate (AS) was one-fourth to one-third the flux with urea. During the DS, however, the seasonal N 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 CH 4 flux up to 60%. However, N 2 O flux increased sharply during the drainage period at mid-tillering until reflooding, when it dropped back to near zero.