Characterization of Placic Horizons in Two Subalpine Forest Inceptisols

Two Inceptisols (Typic Placaquept and Typic Epiaquept) with placic horizons were selected from the subalpine forest soils in northern Taiwan to evaluate the characteristics and genesis of the placic horizons commonly found in these soils. The placic horizon in the Placaquept had sharp upper and diffuse lower boundaries and was only 10 mm thick. The placic horizon in the Epiaquept was thought to have formed originally as a Spodosol. It had sharp upper and lower boundaries and a thickness of up to 40 mm. The upper hard iron pan and the lower dense plasma constituted the placic horizon in the Placaquept, while the multilayered band with darker color and the interwoven dense plasma with lighter color characterized the placic horizon in the Epiaquepts. The microfabric of the placic horizon was isotropic due to precipitated Fe–organic matter complexes, a pedogenic material indicated by the high ratio of oxalate/dithionite‐citrate‐bicarbonate extractable Fe (Fe o /Fe d ). The maximum contents of Fe occurred in the placic horizons of these two Inceptisols. The formation of the placic horizon in the Placaquept was largely the result of redox processes that were enhanced by perched water. Under the saturated condition, Fe was reduced then translocated vertically down through the E horizon where it was subsequently reoxidized and precipitated to form the placic horizon. The characteristics of the placic horizon in the original soil of the Epiaquept met the criteria of a spodic horizon. The genesis model is related to cycles of landscape evolution. The original soil of this Epiaquept may have had a surface horizon prior to truncation and emplacement of a younger, more deeply deposited, new soil materials. The original soil of Epiaquept might have formed a cemented spodic horizon below the E horizon.