Relationships Between Soil Mass Movement and Relief in Humid Subtropical Low-Elevation Mountains

Abstract

Well-developed and undeveloped soils are present in tropical and subtropical mountainous landscape. The soil types and mass movement were determined by geomorphological processes. We studied the relationships between soil mass wasting and soil distribution and relief in a subtropical low-elevation mountain in central Taiwan. Five soil profiles located on different landscape sites with 5% to 30% slopes along two transects were selected to evaluate soil mass wasting episodes by estimating absolute soil ages according to predictive equations. From the estimated absolute soil age, the soils in this mountain may have formed in the early Pleistocene (ca. 1200∼1500 ka). However, the soils of this era were mass wasted during mid-Pleistocene (ca. 200∼900 ka) and were preserved only in the footslope areas as buried soils. The geomorphological surface of the mid-Pleistocene was covered with colluvial material in the backslope areas and exposed at the footslope sites. In recent era, soils on the ridge were mass wasted and accumulated on the backslope areas, which led to a new soil genesis direction; therefore, soils located on the ridge and backslope are classified as Inceptisols or Cambisols and those on the footslope area as Typic Hapludults or Acric Nitisols. The variety of soil development and distribution based on relief in this mountain forest in Taiwan show the key features of unstable K-cycle phases and are best illustrated in the alternating (backslope) and accreting (footslope) zones of the K-cycle. This study also presents that the activity ratio (Feo/Fed) and the crystallinity ratio of free iron oxides (Fed − Feo/Fet) are good indices to infer or predict soil ages in this area.