Diurnal changes of carbon dioxide flux from bare Soil in agricultural field in Japan

We measured diurnal changes of CO₂ flux from bare soil in agricultural field in central Japan. Measurements of soil CO₂ flux were made by the open-flow IRGA method. Temperature and ambient CO₂ concentration, which are highly variable in time, were also monitored during the measurements. The soil CO₂ flux showed significant diurnal changes, and these patterns were highly correlated with the soil surface temperatures. The measured values ranged from 2.52 × 10^-2 to 8.54 × 10^-2 mg CO₂ m^-2 s^-1 in August and from -4.95 × 10^-3 to 1.13 × 10^-2 mg CO₂ m^-2 s^-1 in February. Moreover, a negative linear relationship between soil CO₂ flux and ambient CO₂ concentration was found in diurnal fluctuations. To explore the mechanism of these daily cycles, we estimated soil CO₂ flux with a coupled model of soil microbial activity and gas diffusion. The parameter functions used in our calculation were obtained based on the measured values during the same time of soil respiration measurements. The soil CO₂ production rates were obtained by laboratory experiments using the undisturbed soil core samples, which were taken from the field measuring soil respiration. The calculated soil CO₂ flux ranged from 4.41 × 10^-2 to 7.78 × 10^-2 mg CO₂ m^-2 s^-1 in August and 1.14 × 10^-3 to 5.03 × 10^-3 mg CO₂ m^-2 s^-1 in February. These patterns of diurnal variability were similar for the measured ones, and the daily amounts estimated by integrating the model outputs were almost equal to those from the measured values. However, the amplitude of the measured respiration rates was larger than that of the estimated ones. The differences between measured and estimated values were related with thermal gradient around soil surface. The calculation suggested that the soil horizon above 10 cm depth would be a major CO₂ source of soil respiration at about 70% in August and 40% in February. The depth of soil horizon in which soil microbial activity was significantly higher was not settled and would have a seasonal variation.