Forms and distribution of phosphorus in a placic podzolic toposequence in a subtropical subalpine forest, Taiwan

Catena 140 (2016) 145–154

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Forms and distribution of phosphorus in a placic podzolic toposequence in a subtropical subalpine forest, Taiwan Shih-Hao Jien a, Ian Baillie b, Chi-Chieh Hu c, Tsai-Huei Chen d, Yoshiyuki Iizuka e, Chih-Yu Chiu c,⁎ a

Department of Soil and Water Conservation, National Pingtung University of Science and Technology, Pingtung 91201, Taiwan National Soil Resources Institute, Cranfield University, MK43 0BA, UK Biodiversity Research Center, Academia Sinica, Nankang, Taipei 11529, Taiwan d Taiwan Forestry Research Institute, Taipei 10066, Taiwan e Institute of Earth Sciences, Academia Sinica, Nankang, Taipei 11529, Taiwan b c

a r t i c l e

i n f o

Article history: Received 19 May 2015 Received in revised form 14 January 2016 Accepted 26 January 2016 Available online 4 February 2016 Keywords: P fractionation P distribution Placic podzolization EPMA

a b s t r a c t Phosphorous (P) is a limiting nutrient in subtropical and tropical mountain forests, where placic podzolization further restricts mobilization and the transformation of P fractionation. We examined how the distribution of different forms of phosphorus varied with increasing placic podzolization along a toposequence of seven pedons in subalpine forest soils in Taiwan. We used a variation of the Hedley procedure for the chemical sequential extraction of P and electron-probe micro-analysis (EPMA) to determine the micro-distribution and associations of P with other elements in selected illuvial horizons. The fractionation and P:Ti ratios indicated that P is not greatly depleted during the development of placic conditions. It is instead increasingly sequestered in sesquioxides and their organic complexes in the illuvial B horizons and in the accreting peaty surface layers. The EPMA micro-mapping in the Bsm horizons revealed that P was mainly associated with illuviated iron but not with aluminum. Forests on these soils must cope with a combination of an apparently adequate P supply and an increasingly stagnic moisture regime. When placic conditions develop in the soil, P appears to be increasingly sequestered in the sesquioxides and their complexes with illuvial humus in the B horizons and in the accreting organic epipedon. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Phosphorus (P) is a major limiting factor in the growth and function of many forests (Hou et al., 2014). Because P is a mineral nutrient derived from the weathering of rocks and drift, its deficiencies tend to become more acute as landscapes and soils age due to progressive leaching and sequestration (Walker and Syers, 1976). The combination of extensive old regoliths and intense leaching means that tropical soils are particularly prone to declining P availability. P eventually supplants N as the critically deficient macronutrient for many tropical forests in stable landscapes (Vitousek, 2004). As P totals decline, the proportion of the remainder that is sequestered in the subsoil tends to increase at the expense of more actively available P in the biomass and topsoil. General depletion, downward migration and increasing proportions of the remaining P sequestered by sorption on and occlusion in free sesquioxides of iron and aluminum are clear in mature kaolisols (Ferralsols, Nitisols and Acrisols; Oxisols and Ultisols) (Dabin, 1980). Subsoil P sorption and sequestration are particularly marked in tropical soils developed from mafic and ultramafic parent materials, as their weathering generates large quantities of sorbent sesquioxides (Dubus and Becquer, 2001). The intense fixation and low bio-availability often ⁎ Corresponding author. E-mail address: [email protected] (C.-Y. Chiu).

http://dx.doi.org/10.1016/j.catena.2016.01.024 0341-8162/© 2016 Elsevier B.V. All rights reserved.

make P a severely limiting macronutrient for plant growth on these soils, despite the relatively large quantities of P inherited from their parent materials (Proctor, 2003). In contrast, podzols mostly develop in siliceous parent materials, and they inherit relatively little Fe and P. Their low initial P contents are subject to similar trends of depletion and sequestration. However, they differ from the kaolisols in that the main loci for subsoil sequestration are the illuvial humus and its complexes with Fe and Al in the Bh and Bsm horizons. In coarse textured, well-drained podzols of the ortstein type, the humus in the Bh is mostly complexed and cemented with Al (Bockheim, 2010; Lapen and Wang, 1999). Thick ‘duff’ O horizons develop on the surfaces of these soils, the nutrients of which are only slowly mineralized because of the low quality of the litter, with low concentrations and availability of P and high C:P ratios. These horizons can therefore function as subordinate loci for P sequestration (Turner et al., 2012). Some podzols develop beyond the ortstein stage and become placic. The conventional model of placic podzolization postulates that the permeability of the illuvial B horizon(s) becomes sufficiently low that it impedes drainage in the overlying E, A and O horizons. The O horizons come wet and peaty and often increase in thickness. Patchy and fluctuating redoximorphic conditions mean that Fe in the A and E horizons is readily mobilized and leached downwards. Fe reduction and depletion accentuates the pallid colors of the E horizons. Much of the mobile Fe is oxidized and deposited as sesquioxides or ferric–organic complexes at the

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upper margin of the B horizon. These deposits coalesce to become a thin and often convoluted iron pan, which further reduces the permeability of the B horizon and accentuates the epiaquic conditions (Crompton, 1956). The Fe sesquioxides and their organic complexes in the iron pan are able to immobilize and occlude P. The transition from ortstein to placic podzolization accentuates the tendency for increasing P immobilization in the O and Bsm horizons. The P in the O horizons will slowly be remobilized by microbial mineralization. The part of the P in the Bsm that is complexed with illuvial organic matter (Cade-Menun et al., 2000) may also be mineralized eventually, although the B horizon humus is recalcitrant, and microbial populations are small at these depths. The P sorbed onto and occluded within sesquioxidic accretions is more intractably immobilized (Kitayama et al., 2004; Xavier et al., 2011). Extended chronosequences show that even very coarse textured podzols on siliceous parent materials may eventually become placic (Turner et al., 2012). Podzolic soils are uncommon in medium or fine textured regoliths derived from feldspathic or micaceous source lithologies. However, where they do occur, their water retention and greater initial Fe content make them particularly prone to placic development. Fine- and medium-textured placic podzolic soils on argillaceous parent materials have been noted just below the treeline on high mountains across a wide range in subtropical Asia (Baillie et al., 2004; Jien et al., 2010, 2013; Podwojewski et al., 2011; Wu and Chen, 2005). Placic podzols with distinctively high silt contents have also been described on temperate mountains (Kabala et al., 2012) and at high altitudes on equatorial mountains (Tie et al., 1979), but these soils are thought to be inextensive. In this study, we aim to examine how P is diverted and stored as podzolic soils become more placic along a toposequence in a subalpine forest. Sequential chemical extractions are used to clarify the spatial distribution of P, and electron-probe microanalysis (EPMA) is used to identify its association with Fe and Al oxides in selected illuvial horizons. 2. Materials and methods 2.1. Study sites We described and sampled seven soil profiles along a toposequence from the summit to lakeshore (Table 1) at altitudes ranging from 1800 to 2100 m a.s.l. near Lake Tsuifeng (24°32′ N, 121°56′ E) in the Taiping Mountains of northeastern Taiwan (Fig. 1). The mountains were uplifted during the Miocene (Ho, 1988), and the dominant soil parent materials are shales and slates of the Lushan Formation, in which quartz, chlorite, epidote, muscovite, biotite and sphene are the primary minerals. The mean annual air temperature is approximately 10 °C and ranges from 24 °C in August to 4 °C in February. The total annual rainfall is approximately 3200 mm and is heavy throughout the year with peaks in July–November. Rapid response shallow return flows of the subsurface runoff appear to feed into the lake, which fluctuates in level by up to six meters and periodically inundates the shore. The vegetation on the slopes is naturally regenerating secondary forests of Taiwan red cypress (Chamaecyparis formosensis Matsum.), Taiwan Chinese fir (Taiwania cryptomerioides Hay.) and Japanese cedar (Cryptomeria japonica). Silver grass (Miscanthus transmorrisonensis Hay.), Schoenoplectus mucronatus and other aquatic plants predominate along the intermittently flooded lakeshore.

absorption spectrophotometry (FAAS) (Hitachi Z-8100, Japan). The total Fe (Fet) and total Ti were determined using an X-ray fluorescence (XRF) analyzer (Rigaku ZSX Mini II XRF Analyzer, Japan). The free Fe sesquioxides (Fed) were extracted with a dithionite–citrate–bicarbonate (DCB) extraction (Mehra and Jackson, 1960), and the amorphous Fe (Feo) was extracted with an ammonium oxalate extraction at pH 3.0 (McKeague and Day, 1966). The Fed and Feo extracts were assayed by inductively coupled plasma atomic emission spectroscopy (ICP–AES) (JY124, HORIBA Jobin–Yvon, France). Fed − Feo was taken as the crystalline fraction of the Fe sesquioxides, and (Fed − Feo)/Fet was used as the Fe weathering index (Nagatsuka, 1972; Maejima et al., 2002). We collected undisturbed blocks in Kubiena boxes from the Bsm horizons, and slides were set at room temperature overnight in coldmounting epoxy resin (EpoFix, Struers Co., Denmark) in a 1-in. diameter mold. The mounted samples were ground with silicon carbide and finely polished using alumina paste (b 0.3 μm). The elements were mapped with a field-emission gun-type EPMA (JEOL EPMA 73 JXA-8500F, Japan). The beam was focused at 15 keV and 1.8 nm with a diffracting thallium acid phthalate crystal for Na, Mg, Si and Al, a penta-erythritol crystal for P, K, Ca, Ti and Mn and a lithium fluoride crystal for Fe (Jien et al., 2013). 2.3. P fractionation In nutrient fractionation (Richter et al., 2006), a sample is successively treated with a series of increasingly aggressive extractants that aim to remove ever more recalcitrant forms of the target nutrient. The residue from each extraction forms the substrate for the next stage. The fractionation of P in this study (Table 2) was based on the procedure of Hedley et al. (1982). The main variations from the Hedley procedure (Tiessen and Moir, 1993) were mainly related to the extraction of the most labile forms and the differentiation of the inorganic and organic forms of P in some extracts. For this study, we extracted the labile P using a 0.1 M KCl rather than a resin (Elliot et al., 2002; Olila et al., 1997). To distinguish the inorganic and organic forms in the NaHCO3 and NaOH extracts, we determined the inorganic Pi in the initial extracts and then digested them at 200 °C with H2SO4 and H2O2 to solubilize the organic Po. The assayed P in the digest was taken as the total Pt, and the organic Po was calculated as Pt − Pi. We also determined the total organic P (TOP) using the difference of Pt and the sum of the Pi fractions. The P concentrations in all extracts and digests were assayed colorimetrically by the malachite green procedure (Lajtha et al., 1999). 2.4. P stocks, gains and losses The stocks of the different extracts of P in each horizon were estimated from the concentrations, horizon thickness and bulk density. To compare the pedons, the stocks were summed by horizons down to 75 cm. This depth was chosen to incorporate as much subsoil as possible without excessive extrapolation in shallow pedons. Pedon 6 was omitted from the stock estimates because it was too shallow. In three selected profiles, one on each of the main slope facets, the enrichment and depletion of P were estimated from changes in its concentration relative to Ti, which we took as a stable reference element (Nesbitt, 1979; Zhang et al., 2007), i.e., ΔPð%Þ ¼ ½ðPs =Tis Þ=ðPr =Tir Þ–1  100%;

ð1Þ

2.2. Soil analysis Soil samples from the main horizons were air-dried, ground and sieved to 2 mm and analyzed by conventional methods (Klute, 1986). The fine earth particle size was determined by the pipette method. The pH was determined in a 1:2.5 soil:water suspension. The bulk density (BD) was obtained by the core method. The total organic carbon content was determined using a Fisons NA1500 element analyzer. The cation exchange capacity (CEC) and exchangeable bases were estimated by leaching with neutral ammonium acetate and assay by flame atomic

where the subscripts ‘s’ and ‘r’ indicate the concentrations in the soil and parent rock, respectively. The net changes were correlated against the soil weathering index ((Fed − Feo)/Fet) to trace the enrichment or depletion of P during pedogenesis. 2.5. Statistical methods All of the statistical analyses used SPSS for Windows v10.0 (SPSS Inc., Chicago, IL, USA), with p b 0.05 as the significance criterion.

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Table 1 Study sites and soils in a subalpine forest, Taiwan. Pedon

Altitude

Slope Position

m

Horizon

%

1⁎

2021

Summit

8

2⁎

2078

Summit

5

3⁎

1960

Backslope

10

4

1870

Footslope

15

5⁎

1858

Footslope

8

6

1860

Lakeshore

7

1855

Depth

Color & field texture

pH

Gradient

Lakeshore

cm O A E Bsm Bt O A E Bsm Bt1 Bt2 O A E Bsm Bt BC O A E Bt O A E EB Bsm BC O A Bw OA AE BC

3–0 0–5 5–22 22–25 25–42 5–0 0–7 7–21 21–22 22–45 45–65 10–0 0–12 12–20 20–20.3 20.3–36 N36 5–0 0–10 10–20 20–45 30–0 0–10 10–22 22–55 55–57 57–100 2–0 0–3 3–12 0–11 11–25 25–50

Very dark brown humus Dark brown silty clay loam Light gray mottled silty clay loam Yellowish red iron pan Reddish yellow mottled clay Very dark brown humus Very dark brown silty clay Light gray mottled silty clay Black & dark red iron pan Reddish yellow clay Reddish yellow clay Very dark brown humus Very dark brown silty clay loam Light gray mottled silty clay Black & dark red iron pan Reddish yellow mottled clay Reddish yellow mottled clay Very dark gray humus Very dark gray silty loam Light gray silty clay Brownish yellow silt clay Very dark gray humus Very grayish dark brown loam Light yellowish gray mottled silty clay Light grayish yellow mottled clay Dark brownish & yellowish red iron pan Yellowish red mottled clay Very dark gray humus Dark grayish brown silty loam Light olive brown silty clay loam Very dark grayish brown silt loam Pale brown loam Brownish yellow silty clay loam

– 3.96 3.43 3.78 3.81 – 3.67 3.16 4.97 4.96 4.83 – 3.65 4.09 4.76 4.44 4.32 – 3.87 4.21 4.48 – 4.23 4.25 4.50 – 4.82 – 4.34 4.73 4.64 4.59 5.24

Organic carbon

Base saturation

International soil classification WRB

Soil Taxonomy

%

%

(FAO, 2014)

35.5 25.8 1.45 3.21 0.92 35.7 25.1 1.31 2.25 0.91 0.99 28.7 26.3 0.91 2.07 1.69 1.25 35.5 10.8 2.27 1.24 36.7 9.09 2.34 1.81 – 1.12 20.1 7.50 1.12 3.32 1.58 1.62

– 4.36 1.86 0.95 0.92 – 7.05 1.12 0.50 1.05 1.07 – 4.02 0.74 0.93 1.72 2.57 – 2.83 2.06 1.02 – 2.68 1.12 0.79 0.64 1.38 – 5.14 3.63 12.2 6.57 7.84

Albic Stagnic Alisol

(Soil Survey Staff, 2014) (Humic) Epiaquult

Albic Stagnic Alisol

(Humic) Epiaquult

Albic Stagnic Alisol

Typic Epiaquult

Albic Alisol

(Humic) Epiaquult

Haplic Humic Cambisol

Humic Epiaquept

Haplic Humic Cambisol Haplic Humic Cambisol

Humic Epiaquept Humic Epiaquept

⁎ Further details in Jien et al. (2010).

3. Results 3.1. Pedogenesis and soil classification These soils show podzolic features combined with fine textures (Jien et al., 2010). They have wet O and A horizons over a bleached eluvial E,

which lies over somewhat convoluted and slowly permeable placic podzolic horizons over argillic horizons (Jien et al., 2013). The placic horizons are thin but complex because the brightly colored upper part is a true Bsm iron pan, which has high contents of free Fe. The placic podzolic features and horizons are more developed in the upslope soils and weaken towards the lakeshore. However, even the

Fig. 1. Location of the study area and pedons along the toposequence.

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Table 2 P fractionation procedure in subalpine forest soils, Taiwan. Stage

Target P

Extracted by

Substrate

1

Pre-treatment

2

Labile

KCl

3

Moderately labile inorganic & organic

NaHCO3

4 5

Inorganic & organic on Fe & Al NaOH sesquioxides Apatite HCl

6

Occluded

H2SO4 & H2O2 Residue from Stage 5

Non-sequential

Total P

H2SO4 & H2O2 Pre-treated fine earth

Initial filtrate Digestion of filtrate with gives: H2SO4, & H2O2 at 200 °C gives:

Procedure

Fresh soil

Air-dry, grind in agate mortar, sieve to 2 mm, weigh 0.5 g Pre-treated fine earth Swirl sample in 30 ml 0.1 M KCl for 30 min at room temperature

–

–

KCl-Pi

Residue from Stage 2

Swirl sample in 30 ml 0.5 M NaHCO3-Pi NaHCO3 for 16 h at room temperature

NaHCO3-Pt

Residue from Stage 3

Swirl sample in 30 ml 0.1 M NaOH for 16 h at room temperature Digest sample in 15 ml 1 M HCl for 16 h room temperature Digest sample in 97% H2SO4, 30% H2O2 & MgCl2 at 300 °C Digest sample in 97% H2SO4, 30% H2O2 & MgCl2 at 300 °C

NaOH-Pi

NaOH-Pt

HCl-Pi

–

Occluded-Pi

–

Residue from Stage 4

TP

NaHCO3-Po = NaHCO3-Pt − NaHCO3-Pi; NaOH-Po = NaOH-Pt − NaOH-Pi Total organic P (TOP) TOP = TP − (KCl-Pi + NaHCO3-Pi + NaOH-Pi + HCl-Pi + Occluded-Pi) P occluded Pr = TP − Sum of extracts

Organic P (Po)

upslope pedons are only moderately placic, as evidenced by their relatively thin O and weak E horizons. Although they have bright ferruginous colors and coherent pans, the Bsm horizons are not so strongly

colored to qualify any of the soils as Podzols in the World Reference Base (WRB) (FAO, 2014) or as Spodosols in Soil Taxonomy (ST) (Soil Survey Staff, 2014). Rather, their argillic horizons and acidity qualify

Table 3 Granulometric and Fe profiles in subalpine forest soils, Taiwan. Pedon

Horizon

Particle size distribution Sand

Silt

Fe forms Clay

Clay ratio (Maximum /A horizon)

2

3

4

5

6

7

Fed

Fet

(Fed − Feo)/Fet

g kg−1

% 1

Feo

O A E Bsm Bt O A E Bsm Bt1 Bt2 A E Bsm Bt BC

– 5.70 16.4 15.6 16.1 – 2.10 2.90 – 3.70 6.10 24.5 3.90 – 2.50 2.70

– 59.2 53.3 26.4 27.6 – 50.9 47.5 – 37.9 39.7 34.8 55.3 – 38.8 36.1

– 35.1 30.3 58.0 56.3 – 47.0 49.6 – 58.4 54.2 40.7 40.8 – 58.7 61.2

1 0.86 1.65 1.60 – 1 1.06 – 1.24 1.15 1 1.00 – 1.43 1.50

1.62 1.56 1.14 34.0 23.4 2.02 2.86 0.18 174 18.0 – 2.87 0.79 121 13.1 12.7

4.18 2.65 5.20 104 97.0 3.73 5.13 0.38 327 64.0 – 17.4 3.87 251 68.2 54.0

4.35 4.92 7.66 97.6 63.5 4.37 5.94 3.79 330 62.0 – 34.0 7.07 262 59.5 56.9

0.58 0.22 0.53 0.72 1.16 0.39 0.38 0.05 0.46 0.74 – 0.33 0.42 0.50 0.93 0.73

O A E Bt O A E EB Bsm BC

– 9.50 18.7 7.60 – 4.50 15.2 16.2 – 35.1

– 52.5 43.1 39.5 – 56.3 43.7 31.6 – 30.4

– 38.0 38.2 52.9 – 39.2 41.1 52.2 – 34.5

– 1 1.01 1.34 – 1 1.05 1.33 – 0.88

2.18 3.07 1.28 10.6 2.02 4.50 1.45 1.71 136 12.2

4.93 3.98 4.77 53.4 4.06 6.12 3.61 12.8 201 31.6

6.02 6.49 8.18 58.4 5.13 9.03 10.0 21.0 200 54.9

0.33 0.14 0.43 0.73 0.40 0.18 0.22 0.52 0.32 0.35

O A Bw O/A AE BC

– 4.90 6.5 – 21.4 17.8

– 52.5 63.5 – 38.7 42.9

– 42.6 30.0 – 39.9 39.3

– 1 0.70 – 1 0.98

7.70 4.92 8.91 4.24 3.64 8.74

17.2 17.5 36.9 10.8 10.9 24.8

25.1 37.1 59.0 31.1 29.2 44.4

0.38 0.34 0.47 0.90 0.25 0.36

The non-italicized columns are assay data, and the italicized columns are derived.

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them as Stagnic Alisols (WRB) or Haplaquults (ST). The less podzolic soils on the lower slope are classified as Alisols and Cambisols in WRB or Hapludults and Dystrudepts in ST (Table 1). Some of the O horizons are quite thick in the lower slope soils, possibly because the concave sites are kept wet for long periods by resurfacing throughflow. The particle size distributions confirm the fine field textures, and all of the profiles have at least one horizon with N 40% clay (Table 3). The sand contents are all b20%, and silt is the predominant non-clay fraction. All of the profiles except those along the lakeshore have at least one subsoil horizon in which the clay content was 1.3 higher than in the topsoils. The visible argillans in the subsoils and structural indications of argillic (Bt) horizons confirm that these soils are Alisols/Ultisols. The upslope soils have the highest clay contents, widest clay ratios and most pronounced argillic horizons. The contents of Feo, Fed and Fet in the B horizons were considerably higher than in the topsoils (Table 3), and the highest contents of all forms were in the Bsm horizons. In the B horizons, the upslope soils had virtually all of their Fe in free forms, and the Fed contents were almost equal to those of Fet, which suggests that their mineral weathering and soil development are well advanced. In contrast, Fed accounted for only approximately half of the Fet in the Bw and BC horizons of the lakeshore soils (Table 3). This is attributed to Fe retention in incompletely weathered minerals. There are indications that free Fe tends to be more crystalline in the B horizons, with Feo/Fed, values of approximately 0.3, compared with N0.5 in some topsoils (Table 3). Despite the bright ferruginous colors, the Fe sesquioxides in the Bsm appear to be no more crystalline than in other B horizons.

3.2. Total P stocks and balances The estimates for the total P stocks in the top 75 cm are variable but show no consistent depletion or accumulation (Fig. 2a). This accords with the changes in TP relative to Ti with depth (Table 4), which indicate net gains in all of the surface soils and net losses in all of the E horizons, except in the lakeshore soil (Fig. 3). There were small net losses in the B horizons, except in the soil lakeshore pedon. There is a suggestion that the Bsm has slightly lower net losses than the other B horizons in the slope soil, but the differences are small.

3.3. Distributions of stocks of P fractions with depth and along the toposequence TP concentrations were higher in the surface soils (O and A horizons) and decreased with depth in all profiles, often with slight minima in the E horizons (Table 4). However, the total P stocks were greater in the subsoils than topsoils because of the thicker horizons and higher bulk densities. This pattern of higher topsoil concentrations but lower stocks is repeated for the separate fractions (Table 4, Fig. 2b). A feature of these soils is the high proportion of TP that is organic at all depths. The total organic P (TOP), the difference between the TP and the summation of the Pi, accounts for approximately half of the TP in the O and A horizons and still approximately 30% in the E horizons and 25% in B horizons. The stocks in the P fractions from the main horizons of the contrasting Pedons 1 and 7 confirm that the bulk of the P is held in the subsoils (Fig. 2b). Unfortunately, the Bsm horizon of Pedon 1 was too thin and its P stocks were too small to be shown in Fig. 2b, despite the high inorganic P concentrations (Table 4). The general pattern is somewhat similar in the two pedons, with substantial stocks of labile P and less labile organic P (NaOH-Po) in the surface horizons and higher proportions of the subsoil stocks in the mineral fractions (NaOH-Pi and occluded P). The pattern was more pronounced in the more developed soil upslope (Fig. 4).

Fig. 2. Stocks (g.m−2) of P fractions with increasing placic podzolization. (a) Stocks in the top 75 cm. (b) Stocks by horizons in Pedons 1 and 7. Labile Pi = KCl Pi + NaHCO3 Pt.

3.4. Concentrations of the P fractions The concentration data complement those for the stocks, as they indicate the spatial aggregations and likely loci of the P dynamics. The concentrations of the labile P fractions were highest in the O horizons, high in the A horizons and decreased to low levels in the subsoils, with no dip in the E horizons (Table 4). They were not higher in the placic Bsm than in the E and other B horizons. The concentrations of the less labile inorganic P (NaOH-Pi) were moderate and were generally highest in the topsoils and only moderate in the Bsm horizons, despite their high free Fe concentrations, which were up to an order of higher than the topsoils (Table 3). It therefore appears that this fraction cannot be wholly designated as sesquioxide-sorbed P. The highest concentrations of the relatively labile organic P (NaHCO3-Po) were in the O and A horizons. There were pronounced decreases with depth in the upslope soils, and the subsoil values were an order of magnitude less than in the topsoils. The decrease with depth weakened downslope, and the labile P was more or less uniform throughout the lakeshore soil (Pedon 7). The concentrations of the less labile organic P (NaOH-Po) were approximately twice as high as those of NaHCO3-Po. Overall, they declined with depth, often with slight minima in the E horizons and increases in the Bsm horizons, but the depth pattern was not as marked as for NaHCO3-Po. As with the more labile form, the depth pattern was most pronounced in the upslope soils, and the concentrations were fairly constant with depth in the lakeshore soil. Although the concentrations of the combined NaOH-P fractions decreased below the topsoil, they accounted for up to 60% of the total non-residual P

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Table 4 Concentrations of P fractions (mg kg−1) in subalpine forest soils, Taiwan. Pedon

1

2

3

4

5

6

7

Horizon

O A E Bsm Bt O A E Bt1 Bt2 O A E Bsm Bt BC O A E Bt O A E EB BC O A BW OA AE BC

Inorganic P

P occluded

Organic P (Po) in extracts (from Pt − Pi)

Total organic P

Total P

KCl-Pi

NaHCO3-Pi

NaOH-Pi

HCl-Pi

Pr

NaHCO3-Po

NaOH-Po

Sum

TOP

TP

56.8 27.3 1.10 1.10 0.90 99.3 71.0 1.40 1.30 1.60 46.6 12.8 1.10 1.10 1.20 1.20 37.0 6.30 1.40 1.20 71.8 3.90 1.20 1.00 0.90 2.70 1.40 0.90 1.20 1.10 0.90

103 74.3 6.90 2.80 3.10 60.4 125 4.50 2.90 3.60 91.9 56.9 3.80 3.50 3.20 3.40 50.0 51.4 8.20 4.00 58.5 63.0 10.6 11.7 5.20 52.8 18.3 5.10 24.5 7.10 3.90

81.0 70.9 21.4 33.9 42.1 77.2 94.6 20.9 37.3 38.4 117 139 17.4 40.1 41.2 40.1 84.3 76.6 22.2 37.8 82.8 92.4 39.5 86.9 97.5 113 62.9 34.8 106 47.8 50.8

9.80 8.10 1.60 2.10 2.20 10.8 10.2 1.50 1.70 1.90 9.80 3.40 1.80 2.40 2.20 2.40 6.10 7.20 1.80 1.80 7.10 8.20 2.60 2.10 5.10 8.70 2.60 1.90 8.70 2.70 2.30

175 175 92.1 129 147 215 193 92.1 134 153 228 188 152 160 154 157 283 201 110 130 234 153 117 181 196 192 133 93.6 149 118 144

137 150 21.7 11.3 10.9 119 101 1.20 1.10 1.20 169 116 6.70 13.3 8.30 11.1 33.6 40.6 13.7 12.9 155 133 30.4 19.9 17.5 144 54.3 9.10 27.3 31.9 27.4

236 241 51.3 69.7 92.9 214 243 10.5 12.8 11.2 159 108 49.7 67.3 92.6 106 220 228 78.2 91.7 271 332 99.8 101 119 332 204 52.3 171 172 153

373 397 73 81 104 333 344 12 14 12 328 224 57 70 101 117 254 269 92 105 426 465 130 121 137 476 258 61 198 204 180

407 403 74 55 79 501 444 37 3 3 340 238 57 46 69 69 321 278 83 63 466 447 121 46 85 455 269 57 212 196 144

833 758 197 224 274 963 938 157 180 200 834 638 233 253 270 272 781 643 226 238 920 767 293 329 390 825 487 194 501 373 346

TP/Ti

– 0.24 0.03 – 0.04 – 0.20 0.03 0.05 0.04 – 0.14 0.02 – 0.04 0.03 – 0.17 0.03 0.04 – 0.12 0.05 0.07 0.08 – 0.12 0.02 – 0.10 0.05

The non-italicized columns are assay data, and the italicized columns are derived.

in the Bt horizons. The inorganic NaOH-Pi increased in importance with depth and may have matched or exceeded NaOH-P o in some of the B horizons. The concentrations of HCl-Pi were negligible, b 1% TP throughout (Table 4). They were slightly higher in the topsoils and decreased with depth in all of the profiles, which suggests that some of the P was mobilized from the recalcitrant organic matter as well as from the main phosphatic primary mineral, apatite. The low values in the subsoils suggest that apatite does not persist long once weathered free from the rock matrix. The subsoil values were slightly higher in the downslope soils, but the differences were minor. The residual P was the largest fraction in all of the horizons and accounted for half or more of TP in the Bt horizons. The proportion increased from 40–50% in the lakeshore soils to N 70% upslope. It tended to be slightly higher in the topsoil. 3.5. Micro-mapping P in the placic horizon The element micro-mapping of the illuvial domains in the placic (Bsm) horizon in Pedon 1 (Fig. 5a) shows the micro-distribution of the Al, Fe and P (Fig. 5b–d). In a planar domain of illuvial clay, the distribution of P coincided closely with that of Fe but not Al, which suggests that the P is mainly absorbed onto ferric sesquioxides. 4. Discussion 4.1. Methodological limitations and data quality The fractionation procedure used has the same limitations that are common to all sequential extractions, irrespective of the target element. As the extractants become successively more stringent, they modify other soil components and affect general soil conditions and processes.

This means that the results of the later extractions do not reflect entirely natural conditions. Variations in soil pre-treatments, particularly the intensity of grinding, can affect physical accessibility and extraction efficiency (Levy and Schlesinger, 1999) and lead to variations in the estimates of different forms. The original Hedley procedure and its later variations have been widely applied in pedology, edaphology, agronomy and ecology. The assignation of the contents extracted at each stage to specific soil stores has aided in the understanding of a wide range of phenomena and processes. However, the distinction between stores may be somewhat arbitrary, and there are instances where an extractant appears to be mobilizing P from sources other than the target store. In our results, the higher values of HCl-Pi in the organic topsoils than in the subsoils suggest that this extractant is mobilizing P from organic matter as well as from apatite. Similarly, NaOH-Pi may not be wholly derived from sesquioxide-sorbed P. In addition to these general considerations, our particular choice of methods may affect the results. Thus, the use of KCl instead of a resin can influence estimates of labile P. However, this fraction is small and of marginal relevance to our main emphasis on changes in the less available storage forms. The omission of HF from the extractant for total P, on safety grounds, is now commonplace. Despite these caveats, the fractionation results in Table 4 appear to be generally satisfactory. The sums of the concentrations of the fractions are within 10% of the independent one-off measurement of total P (obtained by acid digestion), both for the pedons and single horizons. The sums of the Po estimates for the NaHCO3 and NaOH fractions mostly agree well with the independently determined TOP (TP − sum of Pi fractionations), although they are noticeably larger in the subsoils of the downslope soils. EPMA can be affected by carbonaceous materials (Chenery et al., 1996), which could be problematic in podzolic B horizons with illuvial

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151

Fig. 3. Vertical distribution of gains or losses for total P (%) relative to Ti in selected pedons.

organic matter. However, this problem is most acute at low voltages and is diminished at the voltage (15 kev) used here (Merlet and Llovet, 2012). 4.2. Soil development The morphology and classification characterize the more fully developed study soils as moderately placic. It appears that pedogenesis proceeds from Cambisol through Alisol almost to Placic Podzol, more or less without intermediate well-drained ortstein podzolics. The brevity and virtual elision of the ortstein stage are attributed to the combination of argillaceous parent material and a warm perhumid climate (Baillie et al., 2004). The moderately advanced pedogenesis is reflected in the distribution patterns of P. The relatively constant pedon TP stocks are corroborated by the P:Ti data. It appears that P is redistributed and accumulated, rather than depleted, during the development of these soils. The retention of P is partly achieved by protection through sequestration. The main loci of sequestration are the residual P fractions, the stocks of which are mostly located in the subsoils. Some of this P may be sorbed or embedded in recalcitrant illuvial humus but much can be attributed to occlusion by sesquioxides (Guo et al., 2000; Xavier et al., 2011). The movement of P into this store is likely to be by simple vertical leaching and in organic complexes with illuviating humus. However, there also appear to be lesser but substantial stocks of residual and less labile P in the organic surface horizons. This sequestration is due to the long residence times in the slowly decomposing peaty organic matter. It was

initially expected that this store would be more important in the more placic upslope soils. However, this is confounded by the thick organic epipedons near the lake. Placic podzolic soils present an unusual combination of growing conditions for forests and other vegetation (Ciampitti et al., 2011; Egli et al., 2012). Firstly, P appears to be less of a critical nutrient than in many tropical and subtropical forests on well drained and freely leaching soils (Gao et al., 2011; Vitousek, 2004; Wang et al., 2012; Zhang et al., 2011). There may be a considerable depletion in the early stages of soil development, but this appears to stop once the placic horizon forms and P is sequestered in the subsoil sesquioxides and peaty O horizons. By definition, sequestered P is not available to plants. However, there are degrees of sequestration, and it is more helpful and realistic to consider P in such stores in terms of ease of detachment and residence times. The P sequestered in placic podzolic soils is likely to be more available in the medium and long term than, for instance, P occluded by the plentiful sesquioxides of mafic and ultramafic Ferralsols and Nitisols. On balance, it seems unlikely that P is a critical limitation for the forests on our study site (Egli et al., 2012; Tyler, 2004). The development of placic conditions also affects the physical conditions of roots. The thin iron pan can become a hard and coherent mechanical barrier and may preclude deep rooting. Root mats are seen above strongly developed iron pans in some temperate placic podzols, and the forest must subsist on the resources of the O, A and E horizons. In our study area, there appears to be enough available P, and the perhumid climate is unlikely to impose prolonged or severe moisture stress. There may be stability constraints because trees with shallow

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Fig. 4. Correlations of 0–25 cm stocks (g.m−2) of P fractions with (Fed − Feo)/Fet: (a) total P; (b) NaHCO3-Pi; (c) labile Pi (KCl-Pi + NaHCO3-Pi); (d) NaOH-Po; and (e) organic P (Po) (NaOHPo + NaHCO3-Po).

root systems may prematurely topple in soils that are wet for long periods. If roots do penetrate the iron pan, they break into a betterdrained and aerated environment. Such rooting in distinctly different conditions is not unique to placic podzols and is encountered in other soils with stagnic morphologies (Adzmi et al., 2009).

epipedon. The stocks of Po, labile P and TP in topsoils increase with soil weathering because the formation of placic horizon retards the leaching of P and other elements. This and the intensifying epistagnic conditions that are caused by the combination of the perhumid climate and development of an impermeable Bsm iron pan create a particular set of edaphic conditions for the forest.

5. Conclusions The development of placic conditions in these soils is associated with an unusual diversion from the normal trend of gradual P depletion. P appears to be increasingly sequestered in the sesquioxides and their complexes with illuvial humus in the B horizons and in the accreting organic

Acknowledgments This work was supported by the Ministry of Science and Technology, Taiwan (MOST-103-2313-B-020-007-MY2).

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Fig. 5. EPMA element mapping in an illuvial planar domain in the placic (Bsm) horizon in Pedon 1; (a) Back electron image; (b–d) Spatial distributions of Al, Fe and P, showing the association of P with Fe but not Al.

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