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

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.