森林樹木と土壌間における雨水の挙動: 現地測定と浸出・透過実験

金沢大学理工研究域地球社会基盤学系雨水が樹冠を通過することにより水質がどのように変化するのかを明らかにするため,スギ,コナラ,モウソウチタの林外雨,林内雨および樹幹流の水質測定と溶存イオンをED-XRFにより分析した.雨水の採取や測定は2004年2-3月金沢大学角間キャンパスで行った.また,雨水が森林の樹木を伝わり土壌に行く物質循環を明らかにするため,スギ,コナラ,モウソウチクの樹皮の浸出実験と土壌の透過実験を行った.その結果,スギ,コナラの樹皮の浸出液のpHは酸性化し,モウソウチクは弱酸性化した.このpHの変化は溶出したK,Caなどのイオンによって生じる.樹皮の浸出実験の前と後をSEM-EDXで観察したところ,浸出後の表面は平滑になっていた.また,3種の樹木の樹幹流を土壌に透過させるとpHは4-6の一定の値に収束した.これは土壌中に存在する粘土鉱物のイオン交換能を示唆している.さらに,土壌は植生によって溶脱するイオンの種類と量が異なる.以上の結果は,雨水が樹幹を伝わることにより大きく水質および化学成分を変化させることを明らかにした.この結果は森林環境保全を行う上で重要である.Field and laboratory studies were conducted to investigate the successive changes in the chemical and physical characteristics of rainwater through stemflow as well as soil in three different forest ecosystems. That is, Sugi (Cryptomeria japonica), Konara (Quercus serrata), and Mousoutiku (Phyllostachys heterocycla) forests which are located in the Kakuma campus of Kanazawa University, Ishikawa Prefecture, Japan. Rainwater outside those forests was also sampled as control precipitation. For field investigation, the rainwater samples as throughfall and stemflow were collected from February 25 to March 18, 2004. For laboratory investigation, the batch leaching experiment using control rainwater was performed over a period of one week, and the continuous filtration experiment using downflow soil bed system with the stemflow of those trees was undertaken as well. XRD analyses of bulk soil and < 2 μ m fraction of soil samples collected from those forests showed similar mineral compositions composed of quartz, feldspars, cristobalite, gibbsite, chlorite, vermiculite, and kaolin minerals. The leaching experimental data showed similar mineral decreases in the rainwater pH of Sugi forest (pH〜4), while little change in the rainwater pH of Konara forest was observed. Conversely, the rainwater pH of Mousoutiku forest tended to increase. These pH changes may be the result of dissolution of K and Ca contained in the bark of those trees in agreement with the experimental data of chemical composition of rainwater analyzed by ED-XRF. Scanning electron microscopic observations of tree bark showed that the surfaces of bark after leaching became smoother than those before leaching, and their EDX spectra showed that there were decreases in the K and Ca contents after leaching. The continuous filtration experiment demonstrated that there were significant changes in the pH, EC, and Eh of the effluent rainwater after filtering through a soil filter bed with several kinds of minerals, indicating that minerals serve as a natural buffer. These results indicate that the type of plants and mineralogical composition of soils in forest ecosystems have effects on the successive changes in the chemical and physical characteristics of rainwater