Abstract
As a consequence of atmospheric pollution and related strong acid deposition, soil acidification has been considered to be a major environmental problem in the forest regions adjacent to densely populated cities. In this study, we assess acid neutralization potential of two types of soil; soils derived from granite parent material (tonalitic soils) and soils derived from volcanic ash (loam soils), in Tanzawa mountain area. The Tanzawa Mountains is located about 50 km west of the Metropolitan Tokyo and has received significant amounts of pollutants through atmospheric deposition. In addition, this area has received considerably amounts of SO2 emitted from Mt. Miyake volcano since August 2000.
The study area is located in the southern part of Oomuroyama watershed (~25 km2) and covered by mixed hardwood forest (elevation 525-1600 m). The bedrock of the study area is Miocene granitic (tonalitic) rock with minor amounts of gabbro. The area is partly blanketed by the Kanto loam. We conducted the fieldwork in every one month (from Apr 2003 to Nov 2004), and collected tonalitic soil and loam soil samples (Ao, A, AB, BC, C, and R horizon; 30 to 300 cm total thicknesses) from 77 sampling points. Average pH in the soil solutions in the A horizon is 5.96 in the tonalitic soils and 5.04 in the loam soils. The loam soils have lower pH values compared with the tonalitic soil. In the B-horizon, the soil solutions of loam soil exhibit a wide range of pH variation ranging from 5.61 to 7.39 (average 6.57). This average value is slightly higher than that in the tonalitic soil (6.18), suggesting that higher potentiality of acid neutralization in the loam soil.The tonalitic soils show a wide range of chemical variation from C to Ao horizon. From bottom to up, SiO2, Al2O3, CaO, K2O, Na2O systematically decrease, while TiO2, P2O5, total C (TC) and total N (TN) markedly increase. The concentrations of analyzed elements are normalized to the Ti content in the same sample to evaluate quantity of elemental lost during weathering. We take the chemical composition of the bottom horizon (C) as the initial composition of parent material. The data suggest that the tonalitic soils have need extensively weathered and considerable mounts of elements leached out from the parent material. The present results indicate that 25 wt.% SiO2 in the parental material has leached out. The estimated amounts of other elements lost from the parent material are: 5wt.% Al2O3, 0.8wt.% Fe2O3, 0.4wt.% MgO, 1.6%wt.% CaO, 1.2wt.% Na2O:, and 0.3wt.% K2O. SiO2 exhibit well defined correlation with Al2O3, Na2O, K2O, CaO, and Fe2O3 in the tonalitic soil. Al2O3 has good correlation with CaO and Fe2O3. The present observation suggests that chemical changes in the soil are associated with chemical weathering of plagioclase, hornblende and biotite, and formation of clay minerals. The loam soils exhibit narrow stratigraphic variation.These show comparable composition from the top to the bottom horizon. The present data indicate that the loam soils were not extensively modified by weathering and have higher potential in acid neutralization than the tonalitic soil.
