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

European Journal of Soil Science, June 2010, 61, 319–332 doi: 10.1111/j.1365-2389.2010.01238.x

Geochemical characterization of placic horizonsin subtropical montane forest soils, northeasternTaiwan

S . - H . J i e na , Z . - Y . H s e ub , Y . I i z u k ac , T . - H . C h e nd & C . - Y . C h i ua

aBiodiversity Research Center, Academia Sinica, Taipei 115-29, Taiwan, bDepartment of Environmental Science and Engineering,National Pingtung University of Science and Technology, Pingtung 912-01, Taiwan, cInstitute of Earth Sciences, Academia Sinica, Taipei115-29, Taiwan, and dDivision of Silviculture, Taiwan Forestry Research Institute, Council of Agriculture, Executive Yuan, Taipei 100-66,Taiwan

Summary

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 microanaly-sis (EPMA), differential X-ray diffraction (DXRD) and chemical extractions. The placic horizons, ranging from3- to 17-mm thick, always occurred above argillic horizons with abrupt changes in pH and texture between thetwo horizons. When fully developed, the placic horizons were clearly differentiated between upper and lowersub-horizons. EDS and chemical extractions revealed that the cementing materials in both were predominantlyinorganic Fe oxides. However, contents of aluminosilicates and organically complexed Fe and Al were greaterin the lower than in the upper placic sub-horizon. Results of EPMA indicated that interstitial fine materials inthe upper placic sub-horizon were composed mainly of Fe oxides, whereas Fe oxides were codominant withilluvial clay in the lower sub-horizon. These analyses identified the migration of Fe and clay as major formationprocesses in both sub-horizons. We hypothesize that there is a pedogenic sequence that starts with clay illuvia-tion, followed by podzolization. The resultant textural and permeability differentiation reinforces the tendencyto profile episaturation that is already inherent from the heavy rainfall and clayey surface soils. Topsoil Fe istherefore reduced and mobilized, and then illuviated with clay and organically complexed Fe/Al to initiate thelower placic sub-horizon. The poor permeability of this layer reinforces the moisture conditions in the surfacesoils, and the further reduction, illuviation and deposition of inorganic Fe to form the upper placic sub-horizon.

Introduction

The definition of a placic horizon in USDA Soil Taxonomy is athin, black to dark-reddish pan cemented by Fe (or Fe and Mn)and organic matter (Soil Survey Staff, 2006). The placic horizonis hard, laterally continuous, impermeable and impenetrable. Itretards vertical leaching of water, inhibits the growth of roots andmay cause plant mortality (Lapen & Wang, 1999; Wu & Chen,2005). The wetness of surface layers above a placic horizon cancreate difficulties in land use (Conry et al., 1996), especially forforestry management. Placic horizons occur in a wide range oflatitudes, from temperate to tropical, but are always associatedwith udic or perudic soil moisture regimes (McKeague et al.,1983; Hseu et al., 1999; Pinheiro et al., 2004; Schawe et al., 2007;

Correspondence: C.-Y. Chiu. Email: [email protected]

Received 3 August 2009; revised version accepted 9 February 2010

Jien et al., 2010). They are mainly associated with sandy to sandyloam soils but are also found in some Spodosols or other podzolicsoils with large clay contents in subtropical regions (Hseu et al.,1999; Wu & Chen, 2005).

Spodic and ortstein horizons are formed by the accumulation oforganic Fe/Al complexes (Do Nascimento et al., 2008), whereasthe accumulation of Fe/Mn in placic horizons is predominantlyinorganic (McKeague et al., 1967, 1968; De Coninck & Righi,1983; Clayden et al., 1990; Conry et al., 1996). Organic matter inthe placic horizon is considered to be adsorbed onto the Fe oxides(McKeague et al., 1968; Lapen & Wang, 1999; Pinheiro et al.,2004). Several hypotheses have been proposed for the genesisof placic horizons, including the translocation and precipitationof organic Fe complexes (Crompton, 1952; Clayden et al., 1990;Conry et al., 1996), and the mobilization of inorganic, reducedFe in surface soils, with subsequent oxidation and precipitation in

© 2010 The AuthorsJournal compilation © 2010 British Society of Soil Science 319

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320 S.-H. Jien et al.

Figure 1 Location of the study area and sampling sites.

the subsoil (Crampton, 1963; Fitzpatrick, 1988; Lapen & Wang,1999; Cunningham et al., 2001; Pinheiro et al., 2004).

Inorganic forms and organic complexes of illuviated Fe and Alcan be differentiated by selective extractions from disturbed bulksamples. However, these destructive methods are time-consumingand cannot determine the micro-distribution of elements in undis-turbed soil aggregates. Microscopic quantitative tools have beenwidely used to describe the spatial micro-heterogeneity of ele-ments in undisturbed soils. Similar studies of placic horizonshave used polarized microscopy (Clayden et al., 1990; Hseu et al.,1999; Pinheiro et al., 2004; Wu & Chen, 2005). X-ray spectralanalyses in energy-dispersive spectrometry (EDS) and electronprobe microanalysis (EPMA) have enabled progressive examina-tion of the constituents of cementing materials and the spatialdistribution of chemical elements in placic horizons. Previouspedogenetic studies of spodic and ortstein horizons have used non-destructive microanalysis techniques (McKeague & Wang, 1980;Lee et al., 1988; Horbe et al., 2004; Kaczorek et al., 2004), butthere have been few applications of these techniques to the for-mation of placic horizons (Brewer et al., 1973; Righi et al., 1982;Breuning-Madsen et al., 2000); all of these studies concentratedon placic horizons in temperate sandy soils. Few studies havelooked at placic horizons in tropical soils with high clay contents.

Taiwan has tropical and subtropical climates. More than 70%of the land is mountainous and forested. In the subalpine andalpine forest ecosystem at higher altitudes, soils show clear illuvialprocesses and strong chemical weathering because of high annualrainfalls (≥3000 mm). Hseu et al. (1999, 2004), Wu & Chen(2005) and Jien et al. (2010) found podzolic soils and soils withplacic horizons in the subalpine cloud forests of northern Taiwan.Ultisols with a placic horizon or Spodosols with large silt and Fecontents have attracted attention elsewhere in Asia (Baillie et al.,1982, 2004; Nakao et al., 2009a,b). Baillie et al. (2004) foundsilty subalpine Podzols in Bhutan, eastern Himalayas, and that

study indicates that podzolization occurs not only in sandy soilsbut also in fine loams.

In the present study we characterize the geochemistry ofplacic horizons to elucidate the formation processes under humidconditions in a subtropical subalpine ecosystem in Taiwan, usingSEM, EDS EPMA, and chemical extractions of bulk soil.

Materials and methods

Environmental setting and soil characteristics

The study was conducted on Taiping Mountain (24◦32′N,121◦56′E) in northeastern Taiwan (Figure 1). The profiles studiedwere located from 1800 to 2100 m above sea level near asubalpine lake, Lake Tsuifeng. Taiping Mountain was formed bytectonic uplift in the Miocene and is mainly composed of shaleand slate (Ho, 1988). Mean annual air temperature at the studysite is approximately 10◦C (monthly maximum temperature isapproximately 24◦C in August and monthly minimum is 4◦Cin February), and total annual precipitation is approximately3200 mm (Wu & Chen, 2005). Heavy rainfall in the monsoonseason (July to November) can lead to high water levels in the lakeand even inundate the footslope site in the study area. Subalpineforests with Taiwan red cypress (Chamaecyparis formosensisMatsum), Taiwan Chinese fir (Taiwania cryptomerioides Hay.)and Willow fir (Cryptomeria japonica Hassk.) are the dominantvegetation. Silver grass (Miscanthus transmorrisonensis Hay.) andaquatic plants (Schoenoplectus mucronatus) are dominant in theshore areas of the lake.

We investigated five profiles with placic horizons (Table 1,Figure 1). Pits were excavated for describing macromorphologicalfeatures and for collecting soil samples according to standardprocedures (Soil Survey Staff, 1993). The profiles were classifiedaccording to the USDA Soil Taxonomy (Soil Survey Staff,2006) and the World Reference Base for Soil Resources system(FAO, 2006).

© 2010 The AuthorsJournal compilation © 2010 British Society of Soil Science, European Journal of Soil Science, 61, 319–332

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https://www.researchgate.net/publication/249430149_Regolith_and_soils_in_Bhutan_Eastern_Himalaya?el=1_x_8&enrichId=rgreq-78e1649b-9e72-4913-8b4a-8903b1379ccf&enrichSource=Y292ZXJQYWdlOzI0OTQzMDIyNDtBUzoxMDEyOTI4OTQ1ODg5MjhAMTQwMTE2MTUxNjMzMg==

Placic horizons in subtropical soils 321

Table 1 Environmental conditions and international classification of profiles

Profile Sites Altitude / m Slope (%) Parent materials USDA Soil Taxonomy (2006)World Reference Basefor Soil Resources (2006)

TP-1 Summit 2078 5 Shale and slate Hapludults Albic Stagnic AlisolTP-2 Shoulder 2021 15 Shale and slate Hapludults Albic AlisolTP-3 Backslope 1960 10 Shale and slate Hapludults Albic Stagnic AlisolTP-4 Footslope 1887 13 Shale and slate Hapludults Albic AlisolTP-5 Footslope 1858 5 Shale and slate Haplaquults Albic Stagnic Alisol

Soil analysis

Soil samples were collected from each horizon of the five profilesfor physical and chemical analysis. Soil samples were air-driedand ground to pass through a 2-mm sieve. Particle size was deter-mined by the pipette method (Gee & Bauder, 1986). Soil pH wasdetermined in 1:2.5 soil:water suspension (McLean, 1982). Totalcarbon content was measured with an elemental analyser (FisionNA 1500, Fison Instruments SpA, Milan, Italy). Cation exchangecapacity (CEC) and exchangeable bases were determined byleaching with neutral ammonium acetate (Thomas, 1982). Thedithionite-citrate-bicarbonate (DCB) method was used to deter-mine the content of free Fe and Al oxides (Fed, Ald) (Mehra &Jackson, 1960). The content of amorphous Fe and Al oxides (Feo,Alo) was estimated by extraction with ammonium oxalate at pH3.0 (McKeague & Day, 1966), and organically complexed Fe andAl (Fep, Alp) by extraction with sodium pyrophosphate at pH 10(Loveland & Digby, 1984). The total content of major metal ele-ments in soils was determined by X-ray fluorescence spectrometry(XRF) (ZSX Primus II, Rigaku Corp., Osaka, Japan) with an Rhtarget and a beam voltage of 20 kV. The XRF data were cali-brated against a standard reference material (NIST-2709) with anelemental composition certified by the US National Institute ofStandards and Technology.

Differential X-ray diffraction (DXRD)

The contents of the crystalline Fe oxides in the placic horizonswere determined by DXRD (Schulze, 1981) using a RigakuD/max-2200/PC (Rigaku Corp.) diffractometer set at: CuKα, Nifilter; 30 kV, and 10 mA. The XRD patterns were recorded over arange of 32 to 48◦ 2θ with a scanning speed of 1.0◦ 2θ /min. TheDXRD patterns were obtained by comparison of the step-scannedpowder diffractograms of the samples before and after DCBextraction. The basal phyllosilicate lines tended to intensify afterDCB treatment relative to the quartz and other lines (Campbell &Schwertmann, 1984).

Soil micromorphology and elemental micro probe mapping

Kubiena boxes were used to collect undisturbed soil blocks inthe field for making thin sections. After air-drying, verticallyoriented thin sections of 5 × 8 cm and 30-μm thick wereprepared by Spectrum Petrographics (Winston, OR, USA). The

soil block samples were placed in a vacuum chamber and resinwas poured in along a glass rod to avoid disturbing the samples.The chamber was then sealed and evacuated to approximately20 mm of mercury (Hg) and left overnight to fully impregnatethe soil samples with polyester resin (Crystic 17449). The thinsections for all soil horizons were observed by both plain-and cross-polarized microscopy (AFX-II Type, Nikon PrecisionInstruments, Belmont, CA, USA). We also made polished slidesfor the soil placic sections by mounting them in cold-mountingepoxy resin (EpoFix; Struers Co., Copenhagen, Denmark) in a2.54-cm diameter mould at room temperature overnight, grindingwith SiC and then polishing with alumina paste (up to 0.3 μm).Each polished sample was initially observed by optical microscopywith reflected light, and then by scanning electron microscopy(SEM; JEOL JSM-6360LV, JEOL Ltd, Tokyo, Japan) to observethe micro-texture. Back-scattered electron images, which indicatemean atomic abundances when contrasted with the black-and-white image, were observed from the surface of the polishedsection. Mineral phases were identified with an EDS (INCA-300;Oxford Instruments Ltd, Oxford, UK) equipped with SEM, withan acceleration beam voltage of 15 kV and current of 180 pA in avacuum condition of 25 Pa without carbon coating. The analysedpoints were selected with the back-scattered electron images toavoid damaging samples.

Elemental distributions were mapped with a five-wavelengthdispersive X-ray spectrometer on a field-emission type EPMA(JEOL EPMA JXA-8500F, JEOL Ltd). The acceleration beam was15 kV and 20 nA with a focused beam and a diffracting crystalthallium acid phthalate for Na, Mg, Si and Al, a penta erithrotolcrystal for P, K, Ca, Ti and Mn, and a lithium fluoride crystalfor Fe.

Results and discussion

Morphological characteristics of the studied soils

All five profiles exhibited a similar sequence of horizons, the mainfeatures of which are: grey mottled eluvial horizons (AE and E)about 10 cm thick; over dark red placic (Bsm) horizons 3- to 17-mm thick; over a reddish-brownish yellow argillic (Bt) horizon(Table 2). Illuvial clay occured as distinct and continuous coatingsin the Bsm (Figure 2a,b) and Bt horizons (Figure 2c,d).

The placic Bsm horizons had colours ranging from 2.5 YR 2.5/1to 7.5 YR 5/8 and an abrupt and wavy upper and a gradual lower



Table 2 Macromorphological characteristics of the studied soil profiles

Profile Horizon Depth / cmMunsellcolour Mottlesa Textureb Structurec Consistencyd Roote

Clayskinf Boundaryg

TP-1 O 5–0 10 YR 2/2 — — Hemic — mvf&f&m — gsA 0–7 10 YR 2/2 — SiC 2vf&fabk ss&sp mvf&f&m — cwE 7–21 10 YR 7/2 10 YR 6/8 (5%)

10 YR 4/2 (5%)SIC Massive s&p mvf&f — as

Bsm1 (upper) 21–21.7 5 YR2 5/1 2.5 YR 4/8 (30%) — Platy fm — — asBsm2 (lower) 21.7–22.7 2.5 YR 3/6 — — — fri — — gsBt1 22.7–45 7.5 Y 6/8 — C 2vf&fabk s&p fvf&f 2dpf dBt2 45–65 7.5 Y 6/8 — C 2vf&fabk s&p fvf&f 2dpf d

TP-2 O 3–0 — — — Hemic — mvf&f — gsA 0–5 10 YR 3/2 — SiCL 2vf&fgr ss&sp mvf&f, cm — cwE 5–22 10 YR 6/2 — SiCL Massive ss&sp mvf&f, cm — cwBsm (simple) 22–25 5 YR 4/6 — — 2vf&fabk fri — — gsBt 25–42 7.5 YR 5/8 7.5 YR 3/2 (10%) C 2vf&fabk vs&vp — 2dpf dBC >42 — — — — — — — —

TP-3 O/A 0–10 10 YR 3/2 — L 2vf&fgr ss&sp mvf&f&m — ciE 10–20 10 YR 6/1 10 YR 6/8 (20%) SiCL Massive ss&sp cvf&f — ciEB 20–40 10 YR 5/8 7.5 YR 3/4 (20%) SiC 2vf&fabk ss&sp cvf&f — asBsm1 (upper) 40–40.5 2.5 YR 3/2 — — Platy fm — — asBsm2 (lower) 40.5–41 5 YR 4/6 — — — fri — — gsBt1 41–65 10 YR 5/8 7.5 YR4/4 (40%)

5 YR 5/8 (15%)C 2vf&fabk s&p cvf&f 2dpf d

Bt2 65–100 10 YR 5/8 7.5 YR 4/4 (30%)5 YR 5/8 (10%)

C 2vf&fabk s&p cvf&f 2dpf d

TP-4 O 10–0 10 YR 3/1 — — Hemic — — — cwA 0–22 10 YR 3/2 — CL 2vf&fgr ss&sp mvf&f&m — csE 22–30 2.5 Y 7/1 — SiCL Massive s&p mvf&f — csBsm (simple) 30–30.3 5 YR 4/6 — — Platy fri cvf&f — asBt1 30.3–46 7.5 YR 5/8 — — 2vf&fabk s&p cvf&f 2dpf —BC >46 10 YR 5/8 — C — — — — —

TP-5 O 30–0 10 YR 3/1 — — Hemic — mvf&f&m — gwA 0–10 10 YR 3/2 — SiCL 2vf&fgr ss&sp mvf&f&m — gwE 10–22 2.5 Y 6/3 10 YR 4/2 (15%) SiC Massive ss&sp mvf&f&m — gwEB 22–55 2.5 Y 7/3 10 YR 4/2 (15%)

7.5 YR 6/8 (10%)10 YR 6/8 (15%)

C 2vf&fabk ss&sp mvf&f&m 2dpf gi

Bsm1 (upper) 55–55.5 5 YR 3/3 — — Platy fm — — aiBsm2 (lower) 55.5–56.5 7.5 YR 5/8 — — — fri — — ciBC 57–100 — 2.5 YR 7/2 (30%) CL 2vf&fabk ss&sp — — —

aThe value in parentheses is the proportion of redoximorphic mottles.bSiCL = silty clay loam; SiC = silty clay; C = clay; CL = clay loam.c1 = weak; 2 = moderate; vf = very fine; f = fine; m = medium; gr = granular; abk = angular blocky.dfm = firm; fri = friable; s = sticky; p = plastic; np = non-plastic; ss = slightly sticky; sp = slightly plastic; vs = very sticky; vp = very plastic.ec = common; m = many; f = few; c = coarse; m = medium; f = fine; vf = very fine.f1 = few; 2 = common; 3 = many; f = faint; d = distinct; pf = ped surface.ga = abrupt; c = clear; s = smooth; g = gradual; d = diffuse; w = wavy; i = irregular.

boundary. There are distinct upper and lower sub-horizons in themore developed Bsm horizons (profiles TP-1, TP-3 and TP-5 inTable 2). Parallel bands of dark red (5 YR 2.5/1, 2.5 YR 3/2, and5 YR 3/3) and lighter matrix colours (2.5 YR 3/6, 5 YR 4/6, and7.5 YR 5/8) alternated in the Bsm horizons (Table 2, Figure 3).These observations agreed with the results of McKeague et al.(1968), FitzPatrick (1988), Hseu et al. (1999), Cunningham et al.

(2001) and Wu & Chen (2005). The upper Bsm sub-horizon wasdistinctly hard and dense, and had a stratified lamellar structure,whereas the lower Bsm sub-horizon graded into the reddish-brownish yellow and less cemented Bt horizons. Non-layeredincipient Bsm horizons in profiles TP-2 and TP-4 exhibited clearsmooth or wavy upper and gradual wavy lower boundaries. Themorphological features of these incipient Bsm horizons resembled



Figure 2 Thin-section micrographs of clayilluviation in profile TP-1: (a) accumulationof illuviated clay above the Bsm horizon,with plain polarized light (PPL) and (b) cross-polarized light (XPL); and (c) illuviation ofclay in the Bt2 horizon with PPL and (d) XPL.

those of lower Bsm sub-horizons in profiles TP-1, TP-3 andTP-5.

Physical and chemical characteristics

We found clear micromorphological evidence of clay luviationfrom the E to Bt horizons (Figure 2a,b). Field textural andparticle size differences between surface and subsurface horizons(Tables 2 and 3) met the criteria for the argillic horizon, as definedby Soil Taxonomy (Soil Survey Staff, 2006). We also foundincreases of at least 0.35 pH units from the E to the Bt horizons(Table 3). As in previous studies (Clayden et al., 1990; Lapen &Wang, 1999; Hseu et al., 1999; Wu & Chen, 2005), we foundthat the marked textural difference between the E and Bt horizonwas associated with accumulation of illuviated Fe or Al. The pHgradient between these two horizons enhanced oxidation of Fe attheir interface (Lapen & Wang, 1999).

We found a sharp increase of organic carbon (OC) content fromthe E to the Bsm (Table 3) horizons. CEC values correspondedwell with the distribution of OC in the soil profile and showeda similar increase. The largest contents of all forms of Fe werefound in the Bsm horizons, as shown by the contents of XRFtotal Fe (Fet), DCB-extractable free Fe (Fed), oxalate-extractableamorphous Fe (Feo), and pyrophosphate-extractable organic Fe(Fep) (Table 4). The Fe contents of the Bsm horizons were verylarge, comprising approximately one-third of the fine earth. TheFe in the Bsm horizon was almost entirely in easily extractableforms, with Fed accounting for virtually all of the Fet. In thelower Bsm sub-horizon of profile TP-3 and upper Bsm sub-horizonof profile TP-5, the contents of Fed were almost equivalent toFet, indicating that the Fe in the Bsm horizon was entirely in

free form and that structural Fe was virtually zero. In the otherhorizons, Fet > Fed, which indicates that some of the Fe was stillstructural.

The DCB-extractable free Al (Ald) content revealed a slightaccumulation in the Bsm horizons, but no accumulation trendsof other forms of extractable Al were found. Extremely smallcontents of free Mn (Mnd) were found throughout the soils andin Bsm horizons in particular. The slightly larger Mnd content inthe Bt/BC horizons may be derived from original Mn of parentmaterials. However, the illuviation of Mn may be involved in thecontribution of placic horizon formation, because Mn contentswere relatively small in E horizons and increased with increasingdepth. There was more manganese in the lower than the upperBsm sub-horizon in the three profiles that were examined.

The contents of Fep and Alp in the Bt horizons were greater thanthose in the Bsm horizons, suggesting that illuviation of organic Feand Al complexes may have occurred in the Bt horizons prior tothe formation of the Bsm horizon. The Feo/Fed ratio ranged from0.24 to 0.53 for the Bsm horizons, which was consistent withpublished data for Fe pans (McKeague et al., 1967; De Coninck,1980; Righi et al., 1982; Hseu et al., 1999; Pinheiro et al., 2004;Wu & Chen, 2005).

Clay mineral identification in the placic horizons

The Feo/Fed ratios of Bsm horizons ranged from 0.33 to 0.53(Table 4), which indicated that much of the Fe in Bsm horizonswas in crystalline mineral forms. These ratios were slightly greaterin the upper Bsm sub-horizons in the TP-1, TP-3 and TP-5profiles than in the lower Bsm sub-horizons and Bt horizons,suggesting that there were more amorphous Fe oxides in the upper



Figure 3 DXRD of soil samples from placic horizons. G,goethite; L, lepidocrocite; F, ferrihydrite. Values in nm ared spacings of Fe oxides.

sub-horizons. These indications were supported by our DXRDresults (Figure 3), as the upper Bsm sub-horizons were dominatedby goethite, lepidocrocite and ferrihydrite.. The DXRD spectralpattern (Figure 3) showed obvious peaks at 0.269, 0.258, 0.252,0.245, 0.225, 0.219, 0.201 and 0.192 nm identified as goethiteand at 0.236, 0.209 and 0.194 nm identified as lepidocrocite inthese upper Bsm sub-horizons. The formation of lepidocrocite andgoethite is favoured by wet moisture regimes, and large contentsof organic matter. They are commonly found in redoximorphic

soils in temperate and subtropical regions (Fitzpatrick et al.,1985; Chiang et al., 1999). The presence of ferrihydrite, as thepeak identified at 0.240 nm, in our Bsm horizons is attributedto the large contents of organic matter in these horizons.These results agree with those from Campbell & Schwertmann(1984), who studied the Fe oxide mineralogy of 15 placichorizons in Europe, and Chiang et al. (1999), who identifiedthese Fe oxides of the Bsm horizons in subalpine forest soilsin Taiwan.



Table 3 Physical and chemical properties

Profile Horizon Depth Sand Silt Clay pH (H2O)Organiccarbon (OC)

Cation exchangecapacity (CEC)

Base saturationpercentage (BS)

/ cm – – – – –/ g kg−1 – – – – – / % /cmol(+) kg−1 / %

TP-1 O 5–0 — — — 3.65 35.7 — —

A 0–7 21.1 510 469 3.67 25.1 76.4 7.05

E 7–21 29.6 475 495 3.16 1.31 26.7 1.12

Bsm1 (upper) 21–21.7 — — — 3.87 3.24 34.2 0.44

Bsm2 (lower) 21.7–22.7 — — — 3.89 2.44 32.4 0.55

Bt1 22.7–45 36.7 379 584 4.96 0.91 18.0 1.05

Bt2 45–65 61.3 397 541 4.83 0.99 14.9 1.07

TP-2 O 3–0 — — — 3.85 20.1 — 5.32

A 0–5 57.5 592 350 3.96 10.3 66.8 4.36

E 5–22 164 533 303 3.43 1.47 21.7 1.86

Bsm (simple) 22–25 — — — 3.78 2.18 6.10 0.95

Bt 25–42 161 276 563 3.81 1.87 15.2 0.92

TP-3 O/A 0–10 483 347 170 4.45 3.77 14.7 4.30

E 10–20 67.7 657 276 4.00 0.80 11.2 4.02

EB 20–40 111 483 407 4.09 0.68 20.8 0.74

Bsm1 (upper) 40–40.5 — — — 4.66 3.66 19.8 0.33

Bsm2 (lower) 40.5–41 — — — 4.71 3.43 16.6 0.61

Bt1 41–65 86.1 366 548 4.58 1.01 17.2 1.72

Bt2 65–100 134 360 507 4.52 0.84 15.2 1.79

TP-4 O 10–0 — — — 3.77 28.7 — —

A 0–22 245 348 407 3.65 26.3 74.8 1.37

E 22–30 38.2 554 408 4.09 0.91 20.3 0.70

Bsm (simple) 30–30.3 — — – 4.19 2.07 21.7 1.79

Bt1 30.3–46 24.0 388 588 4.44 1.69 22.4 1.19

BC >46 27.6 361 612 4.76 1.25 15.4 1.56

TP-5 O 30–0 — — — — 36.7 — —

A 0–10 44.8 564 392 4.23 9.09 32.9 2.68

E 10–22 153 437 410 4.25 2.34 22.9 1.12

EB 22–55 162 316 522 4.50 1.81 22.9 0.77

Bsm1 (upper) 55–55.5 — — — 4.91 3.28 21.7 0.30

Bsm2 (lower) 55.5–56.5 — — — 4.84 2.65 21.9 0.34

BC 57–100 351 304 345 4.82 1.12 13.5 1.38

— = not determined.

Micromorphological features and SEM, EDS and EPMAanalyses

SEM examination revealed similar micromorphological features

in all five of the Bsm samples studied. Therefore, only those

from TP-1 are discussed here. The upper and lower Bsm

sub-horizons are separated by abrupt, smooth boundaries. The

matrices of the upper and lower sub-horizons were both spotted

with skeleton grains and had a greater quantity of quartz grains

than the upper horizon (Figure 4b). Furthermore, the upper

and lower sub-horizons showed different coarse/fine (c/f)-related

distributions: a bimodal porphyric distribution in the upper sub-

horizon and a porphyric narrower ratio distribution in the lower,

with fewer quartz grains observed in the upper sub-horizon. The

micromorphological observations corresponded with those in Bsm

horizons elsewhere, which suggests that porphyric c/f distributions

are the norm (Hseu et al., 1999; Pinheiro et al., 2004; Wu &

Chen, 2005). The SEM findings suggested that the upper Bsm

sub-horizon mainly consisted of dense plasma and our other

results indicated that it was probably cemented by oxidized Fe

materials.

The EDS elemental mapping (Figure 4c,d) showed that the

matrix materials in the Bsm horizons consisted mainly of Fe

and O for both sub-horizons, but Al and Si contents were

greater in the lower than the upper sub-horizon. In addition, the

SEM and EDS suggested the presence of both Fe and clay in

the lower sub-horizon (Figure 5a,b). The EPMA results showed

banded distributions of Fe, Al and Si in the inter-quartz matrix

(Figure 5c,d).



Table 4 Fe, Al and Mn fractionation by dithionite-citrate-bicarbonate, oxalate, and pyrophosphate extractions

Profile Horizon Depth Fed Ald Mnd Feo Alo Fep Alp Fet Alt Feo/Fed Fep/Fed Fep/Feo Fed/Ald Feo/Alo Fet/Alt/ cm – – – – – – – – – – – – – – – –- / g kg−1 – – – – – – – – – – – – – – – –

TP-1 O 5–0 3.73 1.16 — 2.02 1.06 0.85 0.81 4.37 9.76 0.54 0.23 0.42 3.22 1.91 0.45A 0–7 5.13 2.11 0.06 2.86 1.92 2.52 1.65 5.94 24.5 0.56 0.49 0.88 2.43 1.49 0.24E 7–21 0.38 2.23 0.01 0.18 3.15 0.17 2.43 3.79 36.9 0.47 0.45 0.94 0.17 0.06 0.10Bsm1 (upper) 21–21.7 327 10.2 0.06 174 4.73 71.3 4.49 331 45.4 0.53 0.22 0.41 32.1 36.8 7.29Bsm2 (lower) 21.7–22.7 195 14.3 0.08 89.0 4.61 76.5 7.31 205 39.9 0.46 0.39 0.86 13.6 19.3 5.14Bt1 22.7–45 61.0 9.24 0.08 18.0 4.39 38.1 8.27 62.0 45.5 0.30 0.62 2.12 6.60 4.10 1.36

TP-2 O 3–0 4.18 3.18 — 1.62 2.07 1.64 1.7 — — 0.39 0.39 1.01 1.31 0.78 —A 0–5 2.65 1.85 0.01 1.56 1.89 1.82 1.44 4.92 33.3 0.59 0.69 1.17 1.43 0.83 0.15E 5–22 5.20 1.86 0.01 1.14 2.20 1.17 1.88 7.66 31.2 0.22 0.23 1.03 2.80 0.52 0.25Bsm (simple) 22–23 104 10.3 0.06 34.0 5.30 41.0 7.54 110 36.1 0.36 0.44 1.21 9.13 6.42 3.05Bt1 23–42 67.0 16.5 0.19 23.4 6.39 40.1 9.85 63.5 37.0 0.35 0.60 1.71 4.06 3.66 1.72

TP-3 O/A 10–0 17.4 1.56 — 2.87 1.09 1.63 0.69 34.0 37.9 0.16 0.09 0.57 11.2 2.63 0.90E 0–10 3.87 1.67 0.01 0.79 1.52 0.68 1.16 7.1 29.0 0.20 0.18 0.86 2.32 0.52 0.24EB 10–30 36.8 4.36 0.01 3.81 2.02 10.7 2.76 38.7 43.7 0.10 0.29 2.81 8.44 1.89 0.89Bsm1 (upper) 30–30.5 241 8.54 0.12 103 4.05 47.1 3.54 262 27.8 0.43 0.20 0.46 28.2 25.4 9.42Bsm2 (lower) 30.5–31 166 10.4 0.18 55.3 4.41 44.5 6.14 140 47.3 0.33 0.27 0.80 16.0 12.5 2.96Bt1 31–55 58.2 11.6 0.41 13.1 4.02 27.5 7.05 59.5 51.8 0.23 0.47 2.10 5.02 3.26 1.15Bt2 55–90 54.0 10.3 0.57 12.7 4.30 20.2 6.13 57.0 44.5 0.24 0.37 1.59 5.24 2.95 1.28

TP-4 O 10–0 6.91 2.53 — 2.84 2.07 3.31 1.83 8.76 23.2 0.41 0.48 1.17 2.73 1.37 0.38A 0–22 1.31 5.02 0.01 0.82 4.17 0.85 4.71 2.26 24.5 0.63 0.65 1.04 0.26 0.20 0.09E 22–30 9.05 3.02 0.01 1.61 2.27 3.37 2.24 11.8 36.8 0.18 0.37 2.09 3.00 0.71 0.32Bsm (simple) 30–30.3 136 11.7 0.15 42.8 4.11 56.0 6.78 146 47.0 0.32 0.41 1.71 11.6 7.98 3.11Bt 30.3–46 75.0 10.5 0.16 20.4 4.12 47.2 8.01 76.1 48.2 0.27 0.63 2.31 7.14 4.95 1.58BC >46 59.7 12.3 0.26 15.3 4.39 34.2 8.68 58.2 54.7 0.26 0.57 2.24 4.85 3.49 1.06

TP-5 O 30–0 4.06 2.16 — 2.02 1.84 1.95 1.81 5.13 11.4 0.50 0.48 0.97 1.88 1.10 0.45A 0–10 6.12 2.14 0.01 4.50 2.04 4.34 1.66 9.03 28.6 0.74 0.71 0.96 2.86 2.21 0.32E 10–22 3.61 2.79 0.00 1.45 2.17 1.79 2.28 10.0 29.3 0.40 0.50 1.23 1.29 0.67 0.34EB 22–55 8.02 9.49 0.01 1.65 5.11 5.41 6.82 20.1 43.0 0.21 0.67 3.28 0.85 0.32 0.47Bsm1 (upper) 50–55.5 230 11.4 0.10 102 5.67 43.0 6.00 227 44.5 0.44 0.19 0.42 20.2 18.0 5.10Bsm2 (lower) 55.5–56.5 130 11.7 0.12 47.5 5.48 55.3 9.40 133 60.9 0.37 0.43 1.16 11.1 8.70 2.18BC 57–100 31.6 7.48 0.46 12.2 3.99 14.5 5.06 54.9 35.8 0.39 0.46 1.19 4.22 3.06 1.53

Subscriptsd,o and p denote dithionite-citrate-bicarbonate, oxalate and pyrophosphate extracts. Total contents of Fe and Al determined by X-ray fluorescence(XRF).— = not determined.

Major-element chemistry

Triangular diagrams showing XRF contents of bases, Al and Fe(Figure 6) indicated clear pedogenetic trends in the formationof Bsm horizons in our soils. The small contents of Ca,Na, K and Mg in the Bt horizons revealed intense leaching(Figure 6). The large Al contents of E horizons probably resultedfrom residual accumulation as the other main elements wereeluviated.

The XRF totals show an enrichment of FeO in the Bsm horizon,especially for the upper Bsm sub-horizon, which is attributed tothe illuvial accumulation of Fe. The accumulation modes of Fewere clearly different between Bsm and Bt horizons. Our upperBsm sub-horizons matched those of Fe nodules in India (Tripathi& Rajamani, 2007) and placic horizons in Denmark (Breuning-Madsen et al., 2000) (Figure 6), suggesting similar, predominantlyinorganic redoximorphic-formation processes. The incipient Bsm

horizon of the TP-2 and TP-4 and lower Bsm sub-horizons inTP-1, TP-3 and TP-5 are intermediate between the Bt horizonsand the upper Bsm sub-horizons of the TP-1, TP-3 and TP-5profiles.

Pedogenic processes in placic horizons

Previous studies suggest that the formation of the Bsm horizon isthrough the redoximorphic processes of Fe and/or Mn mobilizationand translocation (Crompton, 1956; Crampton, 1963; Clayden elal., 1990; Conry et al., 1996; Hseu et al., 1999; Lapen & Wang,1999; Wu & Chen, 2005). In our profiles, the large Fet/Alt, Fed/Aldand Feo/Alo ratios in the Bsm horizons indicate preferentialaccumulation of Fe over Al (Table 4). The translocation of Feis attributed mainly to redox processes (Conry et al., 1996;Breuning-Madsen et al., 2000; Kaczorek et al., 2004; Pinheiroet al., 2004). The relatively large Fed content and (Fed + Alo)/OC



Figure 4 Scanning electron microscopic (SEM) backscattered image, analytical results from energy-dispersive spectrometry (EDS) and electron probemicro-analysis (EPMA) of the placic horizon of the profile TP-1. (a) Location of the sample in the placic horizon. The SEM backscattered image wastaken in the white square frame. (b) The white point A is the EDS beam location for the upper placic sub-horizon, and B for the lower sub-horizon. (c)EDS spectra. (d) Elemental mapping. Light colours denote large quantities, and dark colours denote small quantities.

ratio (over the range 5–10) suggested that our Bsm horizonswere composed mainly of inorganic Fe. McKeague et al. (1967)suggested that organically complexed Fe/Al is insoluble when the(Fed + Alo)/OC ratio is larger than 3.0. The large value of thisratio in our Bsm horizons therefore implied that it was not, inthe main, organically complexed Fe that accumulated in the Bsmhorizon, especially the upper sub-horizon.

Although chemical analyses revealed that both the lower, aswell as the upper, Bsm sub-horizon contained much inorganicFe, large contents of Fep and Alp and large Fep/Fed ratios in the

lower Bsm sub-horizon were taken to indicate the presence ofsome organically complexed Fe and/or Al. We therefore assumedthat organically complexed Fe and/or Al, clay and reducedFe were leached synchronously from the A and E horizons,and then oxidized and precipitated at the textural interface toform an incipient Bsm horizon. In addition, the Feo/Fed andFep/Fed ratios of the single-layered incipient Bsm horizons inthe TP-2 and TP-4 profiles were similar to those of the lowerBsm sub-horizon in the other profiles and in the Bt horizons.Therefore, the formation process of the single-layered incipient



Figure 5 Scanning electron microscopic (SEM) backscattered image, analytical results from EDS and EPMA for the sample of the lower placic sub-horizonof profile TP-1. (a) Photo of the sample of the placic horizon (only the lower sub-horizon). White frame in the SEM backscattered image is the micro-probearea. (b) SEM backscattered image of the lower placic sub-horizon. Point A is EDS beam location for illuvial Fe, and point B for illuviated clay. (c) EDSspectra. (d) Elemental mapping. Light colours denote large quantities, and dark colours denote small quantities.

TP-2 and TP-4 Bsm horizons is similar to that of the lower

Bsm sub-horizons in the more developed TP-1, TP-3 and TP-5

profiles. Once the incipient Bsm horizon was established, it

further retarded drainage. This accentuated reducing conditions

in the horizons above, and the Fe was increasingly reduced and

leached from the A and E horizons. The reduced Fe content

was oxidized and precipitated above the incipient Bsm horizon.

Consequently, the upper Bsm sub-horizon was mainly composed

of Fe oxides.

Micromorphological observation and elemental mapping sup-

port this hypothesis of the formation of Bsm horizons as a two-

stage process. The EPMA results showed that the upper Bsm

sub-horizon is mainly composed of Fe oxides (Figures 4 and 7),

with mixed Fe oxides and aluminosilicates in the lower Bsm sub-

horizon (Figures 5 and 8). This agrees with Conry et al. (1996),

who found the Bsm horizon formed by crystalline Fe oxides as

well as other colloids, in peat lands in Ireland.

Our hypothesis suggests that the surface soil needs to besaturated and reduced before the Bsm horizon is formed. In ourstudy, saturating and reducing conditions and gleying in the uppersoil profiles have been caused by heavy rainfall and markedtextural difference between surface and sub-surface soils. Thisexplanation agrees with Jackson (1984) and Tokin & Basher(2000), who studied Typic Placorthods with loamy texture in NewZealand. Additionally, episaturation could be inferred from theslightly larger Mnd contents in all of the Bsm horizons than inthe horizons above. Because Mn is more easily reduced thanFe in soils, we surmised that saturated and reduced conditionsoccurred before the formation of the full Bsm horizon, and thusMn was consequently preferentially accumulated in the lower Bsmsub-horizons in profiles TP-1, TP-3 and TP-5 and incipient Bsmhorizons in profiles TP-2 and TP-4 (Table 4).

Cool and humid climates appear to be essential for thedevelopment of placic horizons. In subtropical regions placichorizons develop only in subalpine soils (Hseu et al., 1999,



Figure 6 Triangular diagram of X-ray fluores-cence (XRF) totals Al2O3 – (CaO + Na2O +K2O + MgO) – (FeO), showing distributions andtrends (bold arrows) of accumulation.

2004; Pinheiro et al., 2004; Wu & Chen, 2005), with temperatureand moisture regimes similar to those in temperate regions(Crampton, 1963; McKeague et al., 1968; Brewer et al., 1973;Lapen & Wang, 1999; Tonkin & Basher, 2001). These conditionsretard the decomposition of plant litter, and this results in theaccumulation of soil organic matter. Additionally, heavy rainfallcauses episaturation and oxygen depletion for the decompositionof organic matter and contributes to the reduction of Fe.

To date, a major difference between placic horizons in temperateand subtropical regions seems to be associated with soil texture.Temperate placic horizons are largely found in sandy soils(Crampton, 1963; McKeague et al., 1968; Brewer et al., 1973;Fitzpatrick, 1983; Lapen & Wang, 1999; Phillips & Fitzpatrick,1999), whereas in subtropical subalpine forests, the placic horizonsare commonly found in loams and fine loams (Hseu et al., 1999,2004; Wu & Chen, 2005). These fine-textured soils are moresusceptible to episaturation, and this is a prerequisite for thedevelopment of placic horizons.

Conclusions

Results from SEM/EDS, EPMA and chemical analyses indicatedthat the Bsm horizons in subalpine soils in northeastern Taiwanwere largely composed of inorganic Fe oxides in both layered

and simple Bsm horizons. Illuvial clay and organically complexedFe/Al were more abundant in lower Bsm sub-horizons andin simple Bsm horizons. Our hypothesis for the formationmechanism and development sequence of subtropical subalpineBsm horizons agreed with micromorphological observation andelemental distribution. The process that we propose has four steps(Figure 9). First, warm temperature and heavy rainfall causedmoderate weathering and strong leaching, which led to formationof the Bt horizon. Second, with persistent clay illuviation,podzolization occurred, and then organically complexed Fe/Alwith illuvial clay migrated down into the upper parts of theBt horizon. Third, the textural differentiation thus generated,with coarser-textured topsoil over finer-textured subsoil, causedepisaturation. Fe was therefore reduced and mobilized, andmigrated downwards with illuvial clay and organically complexedFe and Al. This then oxidized and precipitated at the texturalinterface and with pH gradient to initiate the development ofthe simple Bsm horizon, which is equivalent to the lower sub-horizon of layered Bsm horizons. Finally, the simple Bsm horizonaccentuated the sharp permeability barrier and enhanced thereducing conditions in the surface soils. More topsoil Fe wasreduced and migrated downwards, and repeated oxidation andprecipitation occurred above the simple Bsm horizon to form theupper sub-horizon of the mature layered Bsm horizon.



Figure 7 Electron probe micro-analysis(EPMA) of cementing materials in the upperBsm sub-horizon of profile TP-1: (a) SEMbackscattered image, (b) distribution of Fe,(c) distribution of Al, and (d) distribution ofSi. Warm colours denote large contents, andcold colours denote small contents.

Figure 8 Electron probe micro-analysis(EPMA) of cementing materials in the lowerBsm sub-horizon of profile TP-1: (a) SEMbackscattered image, (b) distribution of Fe, (c)distribution of Al, and (d) distribution of Si.Warm colours denote large contents, and coldcolours denote small contents.



Figure 9 Hypothetical pedogenetic processes forthe formation of placic horizons in the soils studied.

Acknowledgements

We acknowledge our colleagues in the Biodiversity ResearchCenter, Academia Sinica, for the soil analyses. We also thankProfessor Zueng-Sang Chen from the National Taiwan Universityfor advice. We are grateful to the journal’s reviewers and editorsfor many detailed and constructive suggestions.

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