脱炭酸にともなう土壌溶液資料のpH上昇  [in Japanese] Increases in pH of Soil Solution Samples Due to CO_2 Degassing  [in Japanese]

ポーラスカップを用いる従来の土壌溶液採取方法では,採取容器(フラスコ)内で脱炭酸が起こり,pHが上昇することが指摘されていた。そこでSuarez^<10>)の方法に準拠して,ポーラスカップとフラスコの間に低容量の試験管をつなぎ,土壌溶液をフラスコ側に数回溢れさせることで極力脱炭酸が起こらない土壌溶液を採取できるように改良した。そして,この改良法および従来法を群馬県のスギ林土壌に適用し,両方法で1年間に採取した各22試料のpH(pH(tube)およびpH(flask))を比較した。さらに,土壌空気のC0_2分圧の観測値を用いて土壌溶液のpH(pH(cal))を推算し,pH(tube)との異同を調べた。その結果,次のことが明らかとなった。1)pH(flask)はpH(tube)よりも平均で0.29,最高で0.92高い値を示した。この差異は,溶存無機炭素の分析結果から,フラスコ内での脱炭酸に起因することがわかった。この結果から,改良法には脱炭酸にともなうpH上昇を抑制する効果があることが確認された。 2)しかし,pH(tube)はpH(cal)よりも高くなったことから,試験管試料におし・ても脱炭酸によるpH上昇が起こったと推察された。両者の差はC0_2分圧が高い深部ほど大きくなり,深さ50cmや100 cmではpH(flask)とpH(tube)の差に匹敵する値であった。3)試験管試料の採取過程における脱炭酸の割合を試算した結果,脱炭酸は4%程度であり,これによるpH上昇は0.02程度ときわめて小さいことが推察された。この結果から,試験管試料における脱炭酸は,主として採取後の試験管の取り外しやpH測定の過程で起こると考えられた。

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.