Characteristics and genesis of Inceptisols with placic horizons in the subalpine forest soils of Taiwan

Three pedons of Inceptisols with placic horizons were selected to study the characteristics and genesis of subalpine forest soils located in Shang-Hsing Mountain in Ilan county, northeastern Taiwan. The selected pedons were elevated from 1900 to 2100 m and derived from slate formed in the late Quaternary age. The vegetation types are dominated by red cypress (Chamaecyparis formosensis Matsum) and Taiwan Chinese fir (Taiwania cryptomerioides Hay). The annual rainfall is about 3000 mm and mostly falls from April to October. The soil moisture regime is perudic, and the soil temperature regime is mesic. Three Inceptisols can be classified as Lithic Dystrudept and Oxyaquic Dystrudept based on USDA Soil Taxonomy or Cambisol based on World reference base system. Clear illuvial pedogenic processes were observed by the bleached albic (E) horizon and the underlain placic (Bs) horizons, which were characterized on the changes of soil morphology, micromorphology, soil texture and chemical compositions. The placic horizon appeared to be dominated by amorphous iron and free iron oxides, with lesser amounts of amorphous aluminum and free aluminum oxides. The genesis of the placic horizons in the study area was proposed as two stages. In the first stage, the perched water was located between albic horizon and cambic horizon after a storm owing to the texture changes of these two horizons. In the second stage, Fe of the upper part of the pedon was reduced, then be mobilized and reoxidized it in the boundary of E and Bw horizon. Therefore, the boundary of placic horizon has a wavy or irregular boundary. In addition, the genesis of placic horizon may also relate to the precipitation of mobile Fe at higher soil pH value and oxidizing soil contacts below the mineral soil surface. Although we do not find the argillic horizon formed below the placic horizon, Ultisols are a major soil order formed in the level landscape position of the study area.