Radon (222Rn) and carbon dioxide have been monitored simultaneously in soil air at a depth of 30 and 100 cm under a cool-temperate deciduous stand on the campus of Hokkaido University, Sapporo, Japan since October in 2007. Air samples were collected periodically from the hole of 1 m in depth, and also from the air in the woods at 2 m in height from the ground.
Carbon isotopic (12C, 13C, 14C) analyses of the CO2 samples were performed by AMS at Mutsu Facility of the Japan Atomic Energy Agency (JAEA-AMS-MUTSU). These are intended to elucidate possible sources of soil air components using 222Rn and CO2 (and its carbon isotopes) as natural tracers, especially in winter months when the ground is completely covered with snow. Among various components of soil air, radon as an inert gas may be suitable for tracing transportation of underground air mass. In contrast, release and uptake of carbon dioxide in soil air are affected by various factors including geological, meteorological and biological activities. Stable and radio-carbon isotopic ratios of CO2 (δ13C, Δ14C) are quite useful for estimating its origin(s) in soil environment.
The results showed that soil radon levels decreasing with decreasing temperature in November were finally kept low with little fluctuations after lingering snow, since the soil temperature, a predominant factor controlling the soil radon level, became virtually constant at 0 °C in the depth of 30 cm. Concentration of CO2 in soil air also decreased in winter when temperature and thus biological activities decreased. The CO2 concentration was found to be consistently higher at deeper depth (100 cm). Carbon isotopic analyses of CO2 suggested that CO2 in the woods would result from mixing of atmospheric air and soil components of several origins, such as CO2 from contemporary soil organic matter and old carbon from the deep source to a varying degree depending on meteorological and biological conditions.
This study is still going on for elucidating possible relation between radon anomalies sometimes observed in lingering snow and a release of CO2 containing old carbon independently not simultaneously observed in February 2007.