Soil Morphological Variations on Pleistocene Multi-step Terraces in the Taoyuan-Chungli Area, Northwestern Taiwan

To study the complex factors controlling soil development on multi-step terraces, and discuss the possibility of developing a soil chronosequence, this study examines the variations of soil morphology in the Taoyuan-Chungli area, originally created as an alluvial fan by the paleo-Tahan River. We adopted the method of soil map reclassification, which has proved useful in clarifying the effects of topographic and hydrological conditions on soil formation on single terrace surface (Y. S. Lin et al., Geomorphology 69: 138-151). Based on morphological characteristics such as soil thickness, color, mottling, soil depth to the Fe-Mn concentration, and plinthite, the soil series on the soil map can be divided into nine types, including three homogeneous groups, two mottled groups, one gley group, and three re-worked groups. We further collected eleven hand-core samples in the field to study the morphology in detail. Our results indicate that topography, hydrology, human activities, and time play dominant roles in soil morphological variation in different temporal-spatial scales. In the younger and lower terraces, the soil toposequence described by Lin et al. (2005) is evident from the terrace fore-edge to the back-edge. Such a pattern is superimposed by patches of homogeneous red soils in local topographic high areas, which are generated by anticlines and faults. In the coastal area, intensive agricultural activities and fluvial processes disturb the development of homogeneous soils even in the fore-edge area. The influence of time becomes evident in the regional scale, with older and higher terraces bearing higher proportion of homogeneous soils than the younger ones. This work suggests that the knowledge of soil morphological variations on each terrace should be a prerequisite to construct a soil chronosequence in multi-step terraces.