Absorption and emission of CO2 by ponded water of a paddy field

In the daytime, the CO₂ concentration in the air close to the water surface of a ponded paddy field was lowest and it increased with the distance above the water surface, while an inverse relation was observed in the nighttime. On the other hand, the pH of the ponded water changed significantly throughout a day and was expected to affect atmospheric CO₂ in the vicinity of the water surface, because the solubility of CO₂ in water depends on the pH. In this study, we investigated the relationship between the changes in the pH of the ponded water and the response of the CO₂ concentration in the air above the water. The pH of the ponded water of the paddy field increased in the daytime and decreased in the nighttime, so that the water was alkaline in the daytime and weakly acidic in the nighttime. We found that the daily changes in the atmospheric CO₂ concentration gradient almost corresponded to the daily changes in pH. The increase of the pH of the ponded water in the daytime was due to the absorption of dissolved CO₂ by photosynthetic bacteria and micro-algae within the ponded water. Furthermore, we compared the pH with RpH, defined as the pH at which the CO₂ concentration of the water is in equilibrium with that of the air, to determine whether CO₂ was absorbed by or emitted from the ponded water. In the daytime, the pH value of the ponded water was higher than that of the RpH, and the water could therefore absorb CO₂ , whereas during the nighttime, since the pH value of the ponded water was lower than that of the RpH, the water was expected to emit CO₂. These results show that the ponded water absorbed CO₂ from the air above the water surface in the daytime and emitted CO₂ in the nighttime.