Absorption and Emission of CO_2 by Ponded Water of a Paddy Field (Soil Physics)

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In the daytime, the CO_2 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 CO_2 in the vicinity of the water surface, because the solubility of CO_2 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 CO_2 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 night-time, so that the water was alkaline in the daytime and weakly acidic in the nighttime. We found that the daily changes in the atmospheric CO_2 concentration gradient almost corresponded to the daily changes in pH. The increase of the pH of the ponded water in the day-time was due to the absorption of dissolved CO_2 by photosynthetic bacteria and micro-algae within the ponded water. Furthermore, we compared the pH with RpH, defined as the pH at which the CO_2 concentration of the water is in equilibrium with that of the air, to deter-mine whether CO_2 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 there-fore absorb CO_2, 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 CO_2. These results show that the ponded water absorbed CO_2 from the air above the water surface in the daytime and emitted CO_2 in the nighttime.
社団法人日本土壌肥料学会の論文

Absorption and Emission of CO_2 by Ponded Water of a Paddy Field (Soil Physics)

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