FLUXES AND POOLS OF METHANE IN WETLAND RICE SOILS WITH VARYING ORGANIC INPUTS

Measurements of methane emission rates and concentrations in the soil were made during four growing seasons at the International Rice Resear ch Institute in the Philippines, on plots receiving different levels o f organic input. Fluxes were measured using the automated closed chamb ers system (total emission) and small chambers installed between plant s (water surface Bur). Concentrations of methane in the soil were meas ured by collecting soil cores including the gas phase (soil-entrapped methane) and by sampling soil solution in situ (dissolved methane). Th ere was much variability between seasons, but total fluxes from plots receiving high organic inputs (16-24 g CH4 m(-2)) always exceeded thos e from the low input plots (3-9 g CH4 m(-2)). The fraction of the tota l emission emerging from the surface water (presumably dominated by eb ullition) was greater during the first part of the season, and greater from the high organic input plots (35-62%) than from the low input pl ots (15-23%). Concentrations of dissolved and entrapped methane in the low organic input plots increased gradually throughout the season; in the high input plots there was an early-season peak which was also se en in emissions. On both treatments, periods of high methane concentra tions in the soil coincided with high rates of water surface flux wher eas low concentrations of methane were generally associated with low f lux rates.