AUTOMATED CHAMBER MEASUREMENTS OF METHANE AND NITROUS-OXIDE FLUX IN AFLOODED RICE SOIL .1. RESIDUE, NITROGEN, AND WATER MANAGEMENT

Methane and N2O are gases that are several times more radiatively acti ve than CO2. It is well known that flooded rice (Oryza sativa L.) soil s are a globally important source of atmospheric CH4. Mitigation strat egies for CH4 flux, such as mid-season drainage, might have the opposi te effect on NIO emissions. An automated chamber system at the Interna tional Rice Research Institute in the Philippines measured CH4 and N2O nu?res from flooded rice and fallow rice fields essentially 24 h a da y between December 1992 and April 1994. This period included two irrig ated dry rite-growing seasons (DS) and one wet rice-growing season (WS ). Nitrous oxide fluxes were generally barely detectable during the gr owing seasons, but small peaks (maximum 3.5 mg N2O-N m(-2) d(-1)) appe ared after N fertilizer applications. Methane fluxes, on the other han d, were evident throughout the rice-growing seasons. Organic matter ad ditions as straw (5.5 t ha(-1), dry) or green manure (GM; Sesbania ros trata L.; 12 t ha(-1), wet) stimulated CHI flux severalfold. Seasonal CH4 flux with ammonium sulfate (AS) was one-fourth to one-third the fl ux with urea. During tile DS, however, the seasonal N2O flux was 2.5 t imes 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 d uring the drainage period at mid-tillering until reflooding, when it d ropped back to near zero.