Measurements of CO₂ Concentration in Forest Soils by a Diaphragm-glass Electrode Method

Abstract

A diaphragm-glass electrode method was applied to measure CO₂ concentration in soils in a Japanese cedar forest at Isehara City. A continuous measurement throughout a year at a fixed depth of 80 cm showed that the seasonal variation of CO₂ concentration was generally attributable to that of ambient air temperature inside the forest. In summer with water famine, however, CO₂ concentration decreased dramatically from 4.75% in July to 1.84% in September. This was probably caused by the expansion of air pathways to the atmosphere as well as the decline of biological activity. Spatial variation of CO₂ concentration was relatively large. Even at a constant depth (50 cm), CO₂ concentration ranged from 0.71 to 2.53% at 5 points allocated at 2-4 m intervals. CO₂ concentration was higher in deeper soil horizons throughout a year. The primary advantages of the method are (1) it enables an automatically continuous measurement of CO₂ concentration in soil air, and (2) it consumes a quite small amount of CO₂ and hence it enables a pinpoint CO₂ measurement. The disadvantage of the method is related to a bias from actual CO₂ concentration . This may be caused by (1) the turbulence of soil horizons in the observation hole due to a periodical calibration of the electrode, and (2) the difference between air temperature on the ground, where calibration is carried out, and that in the soil of interest. The percentage of the bias in the latter case was estimated to be lower than roughly 15% .