Changes in the concentrations and fluxes of CO2 were monitored to a depth of 100 cm in an Andisol, under both fallow and soybean crop for a period of one year. Gas flux was calculated by the diffusion equation. Diurnal concentrations at depths less than 40 cm followed a sinusoidal pattern similar to that of the soil temperature, with the highest value being recorded in the daytime. Heavy rainfall which closes air voids open to the atmosphere resulted in a higher CO2 concentration in the shallow soil layers. The subsequent decrease in the concentration in the shallow layers by the recovery of diffusion paths to the atmosphere was accompanied by an increase in concentrations in the deeper soil layers. CO2 concentration profile under soybean showed a peak at depths that increased gradually from 20 to 80 cm with the growth of the roots. Upward CO2 fluxes decreased with depth in both fields, and the fluxes in the soil profile were high in summer and low in winter. CO2 fluxes from the soil surface calculated by the diffusion equation and measured by the closed chamber method were fairly well correlated. Annual CO2 fluxes were 3,522 g m-2 under fallow and 4,975 g m-2 under soybean. The CO2 movement in soil was simulated by use of the diffusion first law combined with the mass conservation equation. This mechanistic model enabled to analyze phenomena occurring under field conditions, suggesting that soil aeration is controlled by gas diffusion.