Increases in pH of soil solution samples due to CO2 degassing

摘要

A conventional method for collecting soil solution using ceramic porous cups causes CO2 degassing in the container (flask), which results in an increase in the pH (HCO3- + H+CO2 + H2O) of the sample. To collect soil solution samples that are little subjected to CO2 degassing, the conventional method, based on the method of Suarez, was improved by setting a tube with a low volume (30 ml) between a porous cup and a flask to flush several volumes of soil solution from the tube to the flask. Using this setup, soil solution samples were collected twice a month from June 1997 to May 1998 in a Japanese cedar (Cryptomeria japonica) forest in Gunma Prefecture, Japan, and compared the pH of the samples collected in the tube (pH(tube)) with thatof samples in the flask (pH(flask)) using 22 samples. In addition, we compared the pH(tube) with the pH calculated (pH(cal)) when the solution was assumed to be in equilibrium with CO2 partial pressure in the field. The results showed: (1) the pH(flask)was always higher than the pH(tube). The difference between the two was 0.29 pH units in average with a maximum value of 0.92. This was thought to be caused by CO2 degassing in the flask because the concentration of dissolved inorganic carbon in the flask sample was always lower than that of the tube sample. This clearly shows that the improved method can suppress the increase in pH caused by CO2 degassing; (2) the pH(tube) was higher than the pH(cal). This implies that CO2 degassing also occurred in the tube samples. The difference between the two was larger in deeper soil horizons with higher CO2 partial pressures. This corresponded to the difference between the pH(flask) and the pH(tube) at depths of 50 and 100 cm; (3) it was estimated that the percentage of CO2 loss during tube sample collection was～4% and that the resulting pH increase was as low as 0.02 pH units. This result suggests that CO2 degassing of the tube samples mainly occurred during the processes of removing the tube sample from the collector and/or measuring the pH of the sample.