Characteristics and genesis of volcanic soils along a toposequence under a subtropical climate in Taiwan

摘要

This study investigated the pedogenesis of consolidated volcanic rocks along a southeastern aspect of a toposequence in the subtropical region of Taiwan and evaluated the effects of topography on their genesis. Three pedons were selected representing the following positions in the toposequence: summit at 970 m elevation with 12% slope, backslope at 890 m elevation with 25% slope, and footslope at 800 m elevation and 10% slope. The soils formed on andesitic pyroclastic rocks of relatively the same geochemistry and age and under the same subtropical climate, with a mean annual precipitation of 4800 mm and an average temperature of 16.6deg C. Results showed that the soils were in their middle stage (<300 000 years) of development and have characteristics that were closely comparable except for depth of solum. They exhibited andic properties and were strongly acidic, with high Al saturation. Aluminum toxicity and P deficiency contributed to humus accumulation. The high porosity of the soils was attributed to high contents of humus and noncrystalline materials as well as to isovolumetric weathering. The noncrystalline forms of Fe and Al were associated with humus, which is related to the little or no allophane formation in A horizons. Kaolinite, halloysite, gibbsite, and quartz dominated the clay mineral assemblage, with minor amounts of 2:1 silicate clays. Kaolinite, halloysite and gibbsite formed from the weathering of felspar, whereas the layer silicates may have formed in cracks and pores in rockswhere weathering solutions are held for long periods. Quartz is thought to have come from pedogenic processes and inheritance from parent material. Goethite formation was more likely than haematite. Solum thickness was related to the position in the toposequence. More lateral water flow and surfacial runoff occurred in backslope soils with higher slopes.