Soil CO2 concentration and microclimatic parameters were measured at a primary forest and grassland in dry season (August to September 2003) and rainy season (March 2004) in central Amazonia in order to investigate the effects of infiltration of rainfall and evaporation of soil water upon the changes in soil CO2. The CO2 temporarily increased with infiltration, and decreased after soil water descended downward. Then, the increase and decrease moved to deeper soil. This phenomenon was found at both a primary forest and a grassland in the rainy season, and was found in the dry season as well at primary forest. At the grassland in dry season, the soil moisture change was positive during daytime, while the CO2 concentration change was negative. During nighttime, in contrast, the soil moisture change was negative and the concentration change was positive. It was estimated that the fluctuation of radiation between day and night reversed the relative position of soil water and CO2 in the soil. In the rainy season, there was not a large difference in the surface soil concentration (10-30 cm: 20,000-25,000 ppm) between the sites, while in dry season the concentration at the grassland was quite low (2,500-2,600 ppm). From these findings, it was estimated that the soil CO2 was emitted or the root respiration was quitted due to the withering of herbs on the grassland in the dry season. In either case, it is thought that the decrease in soil CO2 was brought about by the exposure of naked land due to the disappearance of forest canopy.
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