Increases in pH of Soil Solution Samples Due to CO_2 Degassing

Abstract

It has been pointed out that a conventional method for collecting soil solution using ceramic porous cups causes C0_2 degassing in the container (flask), which results in an increase in the pH (HCO_3-4+H^+ CO_2 ↑ +H_2O) of the sample. Thus, in order to collect soil solution samples that are little subjected to CO_2 degassing, we improved the conventional method, based on the method of Suarez, 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, we collected soil solution samples twice a month from June 1997 to May 1998 in a Japanese cedar (Cryptomeria japonica) forest located in Gunma Prefecture, and compared the pH of the samples collected in the tube (pH(tube)) with that of 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 CO_2 partial pressure in the field. The results obtained were follows: 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 CO_2 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 CO_2 degassing. 2) However, the pH(tube) was higher than the pH(cal). This implies that CO_2 degassing also occurred in the tube samples. The difference between the two was larger in deeper soil horizons with higher CO_2 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 CO_2 loss during tube sample collection was about 4% and that the resulting pH increase was as low as 0.02 pH units. This result suggests that CO_2 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.