Carbon dioxide concentration in soil air was observed using gas detection device under coniferous (Red pine, ERC site) and deciduous (Oak, KEB site) forests from late May, 1993 to early December, 1994. Soil temperature and soil water content were also measured to discuss the relationships between carbon dioxide concentration in soil air and these environmental factors.
Results and conclusion of the study are summarized as follows:
1) Carbon dioxide concentration in soil air was always higher than in the atmosphere and ranged from 0.1% at the minimum to 0.75-0.85% at the maximum.
2) Carbon dioxide concentration in soil air increased from spring to summer and decreased from autumn to winter. In the summer of 1994, however, increase of carbon dioxide concentration was inhibited or even decreased because of an extremely low suction condition of soil. This seasonal change of the concentration reflects a similar seasonal change of carbon dioxide production in the soil which is mainly controlled by soil temperature and soil water content.
3) Carbon dioxide concentration in soil air was relatively low in shallow depth and increased with depths. In the summer of 1993, a peak of carbon dioxide concentration was observed at the depths of 50-70cm at KEB site. These concentration profiles are not consistent with general profiles of carbon dioxide production, that indicates the importance of a process of carbon dioxide transport.
4) Carbon dioxide concentration in soil air increased exponentially with soil temperature of the same depth. In a very dry condition (pF >2.4) at ERC site, however, this correlation reduced and carbon dioxide concentrations became lower than in a normal moisture condition. Carbon dioxide concentration in soil air was estimated using the relationship and soil temperature data. The estimated value agreed well with the observed value except when the soil was under extremely wet or dry condition.
