Cycling of carbon in a paddy field. I. Carbon dioxide exchange between the surface of a paddy field and the atmosphere

Abstract

Co2-exchange between the surface of a paddy field and the atmosphere was examined with special reference to its diurnal and seasonal changes. The results obtained are as follows: 1. Throughout the rice growing period, CO2 evolution from the flooding water was observed at night. After sunrise, the CO2 evolution rate declined with time. At the early stages of rice growth, the flooding water began to absorb CO2 from the atmosphere at a time in the morning. CO2 absorption rate increased with time and attained a maximum in the afternoon and then turned to decline. After sunset, the flooding water began to evolve CO2 again. The time course of CO2-exchange was similar to that of solar radiation, although the former varied always behind the latter. Thus, the COa absorption proceeded even after sunset for a while, sometimes until midnight. The maximum CO2 absorption rate in the daytime diminished with rice growth and after the middle stage, CO2 absorption by the flooding water was not observed any more. In the latter half of rice growing season, CO2 evolution occurred throughout a day, although the CO2 evolution rate lowered in the daytime. The amplitude in the diurnal cycle of CO2-exchange rate was large in the early season and got smaller with rice growth. 2. The concentration of total CO2 dissolved in the flooding water (DIC) increased at night and decreased in the daytime. pH value of the flooding water decreased at night and increased in the daytime. The partial pressure of free CO2 (pCO2) in the flooding water which was theoretically derived from DIC and pH exhibited a diurnal pattern similar to that of DIC. The amplitude of diurnal cycle of pCO2 was much larger than that for DIC. The amplitudes of diurnal cycles for DIC, pH and pCO2 were all large in the early season and became smaller as the leaf canopy of rice plants closed. The diurnal variations in DIC, pH and pCO2 as described above are considered to be caused by CO2 uptake through algal photosynthesis and CO2 production through respiration of algae, micro-organisms and roots of rice plants. 3. There was a definite quantitative relationship between pCO2 in the flooding water and the rate of CO2 exchange between the flooding water and the atmosphere. This fact suggests that the CO2 exchange rate is able to be estimated on the basis of DIC and pH of the flooding water. 4. In the fallow period when the field was drained, CO2 evolved from the soil surface throughout a day. The CO2 evolution rate was low at night and high in the daytime. The time course was closely related with that of soil temperature. Comparing the CO2 evolution rates under the same soil temperature in a diurnal cycle, the rate under ascending temperature was higher than that under descending temperature in most cases. A close positive correlation was detected between daily mean soil temperature and logalithm of the daily mean CO2 evolution rate. © 1980, CROP SCIENCE SOCIETY OF JAPAN. All rights reserved.