The carbon dioxide concentration (pCO2) in irrigation water in a rice field was measured from August 9 to 11, 1996. The pCO2 showed low values of a few μatm in the daytime, and high values over 2,500μatm(=253.3 Pa) in the early morning. This large diel change of pCO2 is associated with three factors
water temperature change, gas exchange across air-water interface, and biological consumption of carbon dioxide in water through photosynthesis. The effect of water temperature change results in the modification by 60μatm in pCO2 (2% of the diurnal change) taking into account the diurnal change of water temperature(ca.6°C). The carbon dioxide exchange across the air-water interface varied from -2.7 to 28.4μgm-2s-1, with a daily mean of 5.5μgm-2s-1, causing avariation of about 609μatm per day (24% of the diurnal change). Dissolved carbon dioxide [CO2] and oxygen [O2] data showed the photosynthetic (respiration) relationship: [CO2]/[O2]=-106/138. This suggests the possibility that the photosynthetic (respiration) activity of algae is responsible for the residual fraction (ca.73%) of the diurnal change of pCO2. It is also noted that there was a measurable increase in the downward carbon dioxide flux(ca.0.3mgm-2s-1) from the atmosphere when the paddy field was flooded compared to when it was drained. The increment of the downward carbon dioxide flux was compatible with the carbon dioxide emission rates at the soil surface. The carbon dioxide exchange rates at the water surface were smaller by a factor of 1,000 compared with fluxes over a rice stand and smaller by a factor of 100 than the carbon dioxide emission rates at the soil surface. © 2001, The Society of Agricultural Meteorology of Japan. All rights reserved.