Gas Phase in the Layer of the Open System Percolation at the Paddy Field in Volcanic Ash Soil

S tudies on the influence of percolation pattern in seepage water on several phenomena in a layer (V)

Abstract

We observed concentrations of gas phase component (oxygen, carbon dioxide) in the layer (the subsoil layer, umber loam) of the open system dropping percolation at paddy fields of volcanic ash for three years. Soil air extracted at 60 cm depth were selected from the interface of levee between the paddy fields to the central part of paddy fields (5.5 m, 12.7 m, 22.5 m from the levee).
Following results were obtained in this study.
1) Oxygen concentrations in the layer of the open system percolation decreased in irrigation period (the minimum value was 10 % mark at the point of 5.5 m, 9% mark at the point of 12.7 m and 8-9 % at the point of 22.5 m), whereas increased in non-irrigation period and soon became the fixed value (the maximum value was 16 % mark at the point of 5.5 m, 15 % mark at the point of 12.7 m and 12 % mark at the point of 22.5 m) It was thought that these annual changes depended upon soil temperature, gas diffusion and mass flow of gas accompanied with moisture content change caused by.drying and drainage.
2) Carbon dioxide concentrations increased slowly after ponding and became the maximum value early in September (about 9% at each three point in paddy fields), then decreased to the stable value (about 5% at the each point) last December.
3) It was recognized that the oxygen concentrations lowered with the horizontal distance from the levee to the center, whereas the carbon dioxide concentrations had the fixed value with the distance. Formation of carbon dioxide was approximately equal to oxygen consumption.