Carbon dioxide and methane fluxes from an intermittently flooded paddy field

To assess the role of floodwater in controlling the exchanges of CO2 and CH4 from soil, floodwater and the canopy in intermittently flooded rice paddies, an intensive field campaign (IREX96) was conducted in Japan during August 1996. Eddy covariance was employed to measure fluxes of heat, water vapor and CO2. The flux-gradient method was used to determine CH4 fluxes from measured profiles of CH4 concentrations, with the required eddy diffusivity estimated using a modified aerodynamic approach or CO2 as a reference scalar. When the paddy was drained, net CO2 uptake from the atmosphere during daytime was 23% less, and nighttime CO2 emissions were almost twice as great, than when the paddy was flooded. The mean daily CO2 uptake on the drained days was 14.5 g m−2, <50% of the mean for the flooded days. These differences in the CO2 budget were mainly due to increased CO2 emissions from the soil surface under drained conditions resulting from the removal of the diffusion barrier caused by the floodwater. Small changes in canopy photosynthesis observed between flooded and drained paddies had little influence on the CO2 budget and could be explained by sensitivity of stomata to humidity saturation deficit. The CH4 flux for the drained paddy showed distinct diurnal variation with a maximum of ∼1.3 μg CH4 m−2 s−1 in the afternoon, but after reflooding the peak flux decreased to <0.9 μg CH4 m−2 s−1. Mean daily CH4 emissions were 28% larger for the drained paddy than when it was flooded. As with the CO2 flux, the larger CH4 flux on the drained days can be attributed to reduced resistance of CH4 transfer from the soil to air by removal of the floodwater.