Carbon dioxide and methane fluxes from an intermittently flooded paddy field

Abstract

To assess the role of floodwater in controlling the exchanges of CO 2 and CH 4 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 CO 2. The flux-gradient method was used to determine CH 4 fluxes from measured profiles of CH 4 concentrations, with the required eddy diffusivity estimated using a modified aerodynamic approach or CO 2 as a reference scalar. When the paddy was drained, net CO 2 uptake from the atmosphere during daytime was 23% less, and nighttime CO 2 emissions were almost twice as great, than when the paddy was flooded. The mean daily CO 2 uptake on the drained days was 14.5 g m −2, <50% of the mean for the flooded days. These differences in the CO 2 budget were mainly due to increased CO 2 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 CO 2 budget and could be explained by sensitivity of stomata to humidity saturation deficit. The CH 4 flux for the drained paddy showed distinct diurnal variation with a maximum of ∼1.3 μg CH 4 m −2 s −1 in the afternoon, but after reflooding the peak flux decreased to <0.9 μg CH 4 m −2 s −1. Mean daily CH 4 emissions were 28% larger for the drained paddy than when it was flooded. As with the CO 2 flux, the larger CH 4 flux on the drained days can be attributed to reduced resistance of CH 4 transfer from the soil to air by removal of the floodwater.