Geochemical characterization of placic horizons in subtropical montane forest soils, northeastern Taiwan

Abstract

Well-developed placic horizons have been found in subalpine forest soils with large clay contents in Taiwan. We investigated their formation processes in five profiles in a subalpine ecosystem of northeastern Taiwan, using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), electron probe microanalysis (EPMA), differential X-ray diffraction (DXRD) and chemical extractions. The placic horizons, ranging from 3-to 17-mm thick, always occurred above argillic horizons with abrupt changes in pH and texture between the two horizons. When fully developed, the placic horizons were clearly differentiated between upper and lower sub-horizons. EDS and chemical extractions revealed that the cementing materials in both were predominantly inorganic Fe oxides. However, contents of aluminosilicates and organically complexed Fe and Al were greater in the lower than in the upper placic sub-horizon. Results of EPMA indicated that interstitial fine materials in the upper placic sub-horizon were composed mainly of Fe oxides, whereas Fe oxides were codominant with illuvial clay in the lower sub-horizon. These analyses identified the migration of Fe and clay as major formation processes in both sub-horizons. We hypothesize that there is a pedogenic sequence that starts with clay illuviation, followed by podzolization. The resultant textural and permeability differentiation reinforces the tendency to profile episaturation that is already inherent from the heavy rainfall and clayey surface soils. Topsoil Fe is therefore reduced and mobilized, and then illuviated with clay and organically complexed Fe/Al to initiate the lower placic sub-horizon. The poor permeability of this layer reinforces the moisture conditions in the surface soils, and the further reduction, illuviation and deposition of inorganic Fe to form the upper placic sub-horizon.

Distribution and Genesis of Ortstein and Placic Horizons in Soils of the USA: A Review

“…The existence of ferrihydrite and goethite, along with some lepidocrocite, has been verified with differential XRD, SEM, EDS, micromorphology, and selective dissolution techniques (Campbell and Schwertmann, 1984;Jien et al, 2010). These data suggest that placic horizons are cemented by amorphous or weakly crystalline Fe minerals such as ferrihydrite, goethite, and lepidocrocite.…”

“…One ofthe soils in the present study is derived from volcanic ash; three are developed from basaltic or andesitic residuum. Jien et al (2010) reported an argillic horizon below the placic horizon and suggested that clay migration leads to restricted drainage and the development of a placic horizon. In the present study six of seven pedons were underlain by either saprolite or dense till.…”

“…These layers cause perching of the water table which enables the Fe to become temporarily reduced; with drying the Fe is mobilized and reoxidized at the textural break. Some investigators suggest that clay eluviation followed by podzolization creates the textural discontinuity that leads to episaturation (Jien et al, 2010). The precipitation of Fe may be due to higher oxidation potential and pH at depth in the profile (Lapen and Wang, 1999).…”

Characterization of Phosphorus in a Toposequence of Subtropical Perhumid Forest Soils Facing a Subalpine Lake

“…The high Fe o , Fe d , Al o and Al d contents in the B horizons implied that iron moved downwards and accumulated in the subsoil. Because high soil moisture and rich organic matter decreased soil redox potential in the surface layer, reduced Fe and Al ions moving from surface to the bottom layer were re-oxidized in the B horizons [49,50]. This formation of accumulated Fe, which may due to the redoximorphic process [51,52], created a placic-like horizon and resulted in slow permeability of P in such perhumid forest soil [49].…”

Characterization of Phosphorus in a Toposequence of Subtropical Perhumid Forest Soils Facing a Subalpine Lake

“…The high Fe o , Fe d , Al o and Al d contents in the B horizons implied that iron moved downwards and accumulated in the subsoil. Because high soil moisture and rich organic matter decreased soil redox potential in the surface layer, reduced Fe and Al ions moving from surface to the bottom layer were re-oxidized in the B horizons [47,48]. This formation of accumulated Fe, which may due to the redoximorphic process [49,50], created a placic-like horizon and resulted in slow permeability of P in such perhumid forest soil [47].…”