HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (20) Citing Articles via Google Scholar Google Scholar Articles by Zysset, M. Articles by Gehring, A.U. Search for Related Content PubMed Articles by Zysset, M. Articles by Gehring, A.U. GeoRef GeoRef Citation Agricola Articles by Zysset, M. Articles by Gehring, A.U.

DIVISION S-2-SOIL CHEMISTRY

Aluminum Solubility Control in Different Horizons of a Podzol

^a Dep. of Soil Sci., Swedish Univ. of Agric. Sci. (SLU), 750 07 Uppsala, Sweden [present address: Jägerweg 6, 3014 Bern, Switzerland]
^b Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), 8903 Birmensdorf, Switzerland
^c Institute of Terrestrial Ecology, ETH Zurich, Grabenstrasse 3, 8952 Schlieren, Switzerland

bmzysset{at}dplanet.ch

	ABSTRACT

TOP ABSTRACT INTRODUCTION Samples and methods Results Discussion Conclusions REFERENCES

 
Aluminum extractability and solubility were investigated in detail in six horizons of a Typic Haplohumod (FAO: Haplic Podzol) from southern Switzerland. Pyrophosphate and oxalate extractions as well as successive acid leaching indicated that in the Ah, (AE), and Bh horizons reactive Al is mainly bound to soil organic matter, whereas in the Bs, BC1, and BC2 horizons it is of inorganic nature. In the latter three horizons, infrared (IR) spectroscopy and transmission electron microscopy (TEM) revealed the presence of imogolite. Batch equilibrium experiments at 20°C in the pH range of approximately 3.5 to 5.5 showed that the podzol profile can be divided into two parts of different Al solubility control. In the Ah and (AE) horizons, Al solubility was found to be controlled by complexation reactions to soil organic matter. Kinetic studies with samples of the Bh, Bs, BC1, and BC2 horizons showed that ion activity products with respect to both Al(OH)₃ and imogolite, (HO)₃Al₂O₃SiOH, reached a constant value after reaction times of 16 d. For pH >4.1, the compilation of all data revealed and . These data could be shown to be consistent with either Al solubility control by imogolite-type material (ITM) with a log , which dissolves incongruently, or a simultaneous equilibrium with ITM and hydroxy-Al interlayers of clay minerals. For pH <4.1, data indicated solubility control by a 1:1 aluminosilicate, e.g., poorly crystalline kaolinite.

Abbreviations: AAS, atomic absorption spectroscopy • BS, base saturation • CEC, cation-exchange capacity • FIA, flow injection analysis • IAP, ion-activity products • ICP-AES, inductively coupled plasma atomic emission spectroscopy • ICP-MS, inductively coupled plasma mass spectroscopy • IR, infared • ITM, imogolite-type material • TEM, transmission electron microscopy

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Samples and methods Results Discussion Conclusions REFERENCES

 
IN THE LAST TWO DECADES, the anthropogenically induced acceleration of forest soil acidification has been a topic of environmental concern. In many industrialized areas, the atmospheric deposition of sulfur and nitrogen compounds is a major source of proton input to soils (see Matzner, 1992; Paces, 1985; van Breemen et al., 1984). In acidic soils at pH values 4.5, a major part of the acid input is buffered by dissolution of reactive Al phases. The released Al in soil solution can be phytotoxic and, thus, can pose a major ecological risk (see Sumner et al., 1991; Ulrich, 1989; Foy, 1984). Despite this toxicity, Al solubility control in soils is not understood completely. In acidification models such as SAFE (Sverdrup et al., 1995), it is assumed that Al activity in the soil solution is regulated by equilibrium with an Al(OH)₃ phase.

On the basis of both field measurements and laboratory experiments several different solid phases have been proposed to control Al solubility in soils. Walker et al. (1990) attributed Al solubility control in organic soil horizons to exchange reactions between H and Al at soil organic matter. They found that equilibrium solubility was attained within hours and increased with decreasing pH and increasing degree of saturation of soil organic matter with Al. Their results agreed with earlier studies by Cronan et al. (1986) and Bloom et al (1979). Mulder et al. (1989) concluded from results of a leaching experiment that even in acid mineral soil horizons Al solubility can be controlled by complexation with soil organic matter. This finding was confirmed by soil solution data obtained from acid brown forest soils in the Netherlands (Mulder and Stein, 1994). Berggren and Mulder (1995) concluded in their study that, depending on pH range, different phases can control Al solubility. They attributed Al solubility in acid mineral soil horizons from southern Sweden at pH <4.1 to complexation reactions with soil organic matter, and, at higher pH values, to an Al(OH)₃ phase with a higher solubility than gibbsite. David and Driscoll (1984) found that soil solutions from two Spodosol B horizons were often close to equilibrium with synthetic gibbsite. In a laboratory study, Dahlgren et al. (1989) observed fast equilibrium of Spodosol Bs horizons with respect to Al(OH)₃ from both conditions of undersaturation and oversaturation. They attributed control of Al solubility to hydroxy-Al interlayers of expansible 2:1 layer silicates, since (i) no or only trace quantities of gibbsite could be detected in the soil samples, and (ii) Al activities were oversaturated with respect to gibbsite. Similar results were reported by Dahlgren and Walker (1993). Hydroxy-Al interlayers are less stable than gibbsite as was shown for smectites (Turner and Brydon, 1965; 1967).

Another phase that was proposed to control Al solubility in acid soils, is imogolite, a short range ordered aluminosilicate with an Al:Si ratio of 2:1. A less ordered material with the same chemical composition is often referred to as proto-imogolite allophane (Farmer et al., 1980), and the sum of imogolite and proto-imogolite allophane as ITM (imogolite type material; Gustafsson et al., 1995). Imogolite was detected in podzols from different countries (see Tait et al., 1978; Farmer et al., 1980; Ross and Kodama, 1979; Childs et al., 1983; Gustafsson et al., 1995). However, not all podzols contain imogolite (see Wang et al., 1986). According to Farmer (1987) the formation of imogolite and allophane requires a pH value >4.6 to 4.9. Farmer (1987) suggested that Al solubility in soils containing imogolite may be controlled by a simultaneous equilibrium between an Al(OH)₃ phase and imogolite. Dahlgren and Ugolini (1989) found evidence for this mechanism in soil solutions obtained from Bs and C horizons in a Tephritic Spodosol. Data from batch equilibrium experiments using spodic B horizons also suggested this mechanism (Su et al., 1995).

Most previous studies on Al solubility in soils investigated specific horizons, mainly A or spodic B horizons. Many dealt only with a narrow pH range, and this can lead to difficulties in assessing the solubility controlling phase. The aim of this research was to investigate the Al-solubility behavior in a complete sequence of horizons of an acidic soil profile in the approximate pH range between 3.5 and 5.5, which is characteristic for many podzolized soils.

	Samples and methods

TOP ABSTRACT INTRODUCTION Samples and methods Results Discussion Conclusions REFERENCES

 
Soil Profile
Samples were collected from a soil profile at Copera in southern Switzerland (46°8' N, 8°59' E). The pedon was classified as a Typic Haplohumod (Soil Survey Staff, 1996), Haplic Podzol (FAO, 1988), or as cryptopodzolic soil (Blaser et al., 1997). The soil developed on mica-rich gneissic bedrock under chestnut forest and is characterized by uniform incorporation of organic matter deep into the profile leading to only a weakly developed, barely visible eluvial horizon.

Soil Analysis
Field moist soil samples from each of the six horizons were passed through a 2-mm sieve and stored at -20°C. Preliminary tests showed that freezing and melting did not affect the results of the experiments. All chemical analyses were performed in three replicates. The mass of added soil samples to extraction solutions were corrected for soil moisture content to obtain equal soil:solution ratios (mass:mass) for all horizons. The pH value was measured potentiometrically in deionized water (soil:solution ratio = 1:2; 30 minutes equilibration time). Exchangeable cations were determined in 1 M NH₄Cl extracts (soil:solution ratio = 1:10; 1-h extraction time) by inductively coupled plasma atomic emission spectrometry (ICP-AES; Optima 3000, Perkin Elmer Corp., Norwalk, CT). Exchangeable protons were calculated by the difference of total and Al-induced acidity, which were determined according to Yuan (1959). Charge equivalents of exchangeable H, Al, Na, K, Ca, Mg, Mn, and Fe were used to estimate effective cation-exchange capacity (CEC_eff) and base saturation (BS). Reactive forms of Al and Si were estimated by pyrophosphate (Al_p) and oxalate extractions (Al_ox and Si_ox) using the methods of Bascomb (1968) and Schwertmann (1964), respectively. The concentrations of these elements in the extracts were determined by flame atomic absorption spectrometry (AAS, Pye Unicam PU 9200, Unicam Ltd., Cambridge, UK).

A portion of the <2-mm fraction was dried at 60°C for 48 h and used for the following analyses. After removing organic matter by H₂O₂, particle size distribution was determined by the pipet method described by Gee and Bauder (1986). Total Al concentrations (Al_tot) in the samples were determined by analyzing Na-tetraborate digests dissolved in dilute nitric acid with inductively coupled plasma mass spectrometry (ICP-MS, VG PlasmaQuad PQ 2 Plus, VG Elemental, Winsford, UK). The carbon content was measured with a CN analyzer (Carlo Erba Instruments NA 1500 Series 2, Milan, Italy).

Leaching Experiment
To test whether added hydrogen ions are mainly buffered by the release of organically or inorganically bound Al, samples were successively leached with a solution of the chemical composition of 0.01 M HNO₃ and 0.03 M NaNO₃ and subsequently extracted with oxalate or pyrophosphate as described above. Since it was intended to leach 40–50% of reactive Al, the number of leachings were varied for the different horizons depending on their contents of Al_ox: Ah: 5 leachings; (AE): 6; Bh: 8; Bs: 6; BC1: 5; BC2: 4. The leachings were performed in duplicates in batch reactors with a soil: solution ratio of 1:10. After a 15-h extraction on an end-over-end shaker at a constant temperature of 20 ± 1°C, the suspension was centrifuged for 15 min at 7000 g and the supernatant filtered (cellulose nitrate 0.45 µm; Sartorius Corp., Edgewood, NY). The removed solution was replaced by fresh reaction solution and the extraction procedure repeated. In all filtrates concentrations of Al were determined by AAS (Pye Unicam PU 9200). After the last leaching step, pyrophosphate or oxalate extractable Al was determined in the solid residue as described above and referred to as Al^R_p or Al^R_ox. For all horizons total leached Al (Al_L) differed less than 3% within two replicates.

Infrared Spectroscopy and Transmission Electron Microscopy
Infrared spectroscopy was performed on acid dispersible clay fractions of Bs, BC1, and BC2 material. For the preparation of the <0.5-µm fraction, the method described by Farmer et al. (1980) was used. The IR spectra were obtained in transmission mode on pellets containing 1.5 mg of freeze-dried clay in 150 mg of a matrix of KBr. The pellets were dried at 150°C in an oven for 12 h. Infrared spectra were recorded between 4000 and 400 cm^-1 (Perkin Elmer 1760 X) and between 500 and 200 cm^-1 (Perkin Elmer 1700 X). In a second run, the same pellets were heated at 400°C for 12 h and then another set of IR spectra was measured. The heating step was performed to distinguish between mineral phases in terms of their thermal stability. For TEM, one drop of diluted suspension containing the acid dispersable <0.5-µm fraction was dried on a carbon-coated Cu grid. Analyses were performed with a JEM-2000 EX TEM (JEOL Ltd., Akishima, Japan).

Batch Experiments
Al solubility behavior was investigated by batch experiments on an end-over-end shaker at 20 ± 1°C. All reaction solutions contained a constant ionic background of 0.03 M NaNO₃ to reduce variation in ionic strength. To determine the time required to reach equilibrium with inorganic Al phases, kinetic series were performed at selected HNO₃ concentrations [(AE): 18.0 mM; Bh: 5.5 and 27.5 mM; Bs: 5.7 and 16.7 mM; BC1: 2.9 and 9.7 mM; BC2: 2.9 and 9.8 mM) at soil:solution ratios of 1:10 and reaction times varying between 3 h and 35 d. At the end of the reaction time the suspensions were centrifuged for 15 min at 7000 g. To avoid any impact of filters on H⁺-activity, pH values were measured in a small portion of supernatant with a combination glass electrode. The remaining supernatant was filtered (Sartorius cellulose nitrate 0.45 µm) and used for the following analyses. Labile aluminum (Al_labile), was operationally defined as Al quickly reacting with 8-hydroxyquinoline, and was determined by the manual method described by Luster et al. (1993). Concentrations of monomeric Si (Si_m), were measured colorimetrically (Koch and Koch, 1964). Total concentrations of Al, Si, and S were determined by ICP-AES (Perkin Elmer Optima 3000).

On the basis of the results of the kinetic series, a reaction time of 30 d was chosen for the equilibration experiments. The reaction solutions contained HNO₃ (0–55 mM) or NaOH (0–8.8 mM). In most experiments, a soil:solution ratio of 1:10 was used. To evaluate a possible effect of this ratio, a few additional experiments were performed with a soil:solution ratio of 1:2.5. For all equilibrium experiments, concentrations of Al_labile were determined by the flow injection analysis (FIA) method of Clarke et al. (1992) as modified by Berggren and Sparén (1996) and concentrations of Si_m by the FIA method of Thomsen et al. (1983) as modified by Gustafsson et al. (1998). Results obtained with the corresponding manual and FIA methods differed less than 10% for Al_labile and less than 15% for Si_m.

Chemical Speciation and Evaluation of Aluminum Solubility Controlling Phases
To evaluate Al-solubility, free Al³⁺ concentrations were calculated from measured Al_labile by the computer program ALCHEMI 4.0 (Schecher and Driscoll, 1987). It was assumed that Al_labile includes monomeric Al-hydroxo-complexes as well as AlSO⁺₄ and AlH₃SiO²⁺₄ (Berggren and Mulder, 1995). Al-organic and polynuclear Al-complexes are not included to any significant degree (Clarke et al., 1992). The influence of considering AlSO⁺₄ on calculated Al³⁺-concentrations was tested in cases in which total S-concentrations were measured, assuming that all S was bound to SO^2-₄. Free Al³⁺-concentrations differed <1.3% when calculated with or without the consideration of SO^2-₄. Therefore, AlSO⁺₄ was neglected in the speciation calculations.

Stability constants log ß⁰_n, and log *K⁰_s, as well as heats of reaction H⁰_r used in chemical equilibrium calculations are listed in Table 1 . Concentrations of Al³⁺ were converted to activities by the Davies equation. Since dissolved Si_m occurs as uncharged H₄SiO₄ in the investigated pH-range, its activity coefficient was assumed to be 1. Logarithms of ion-activity products (log IAP) were calculated for Al(OH)₃ and imogolite, (HO)₃Al₂O₃SiOH, according to Eq. [1a] and [1b], where pH, pAl, and pSi stand for the negative logarithms of H⁺-, Al³⁺- and H₄SiO₄-activities:

(1a)

(1b)

Neal et al. (1987) showed that, due to autocorrelation, diagrams as used here can produce apparent order in data, even when the data consists of random numbers, and this can lead to false interpretation of experimental results. Xu and Harsh (1995), however, concluded in their study that autocorrelation does not interfere with the interpretation of solid phase control for natural gibbsite at pH <5.5.

In addition to Al solubility control by Al(OH)₃ or ITM, two special cases were tested: (i) simultaneous equilibrium with ITM and an Al-hydroxide, and, (ii) congruent dissolution of ITM. The dissolution of an Al-hydroxide can be described by Eq. [2a] and, in equilibrium, pAl vs. pH is given by Eq. [2b], where log *K_A is the stability constant.

(2a)

(2b)

The dissolution of an aluminosilicate can generally be described by Eq. [3a], and pAl vs. pH in equilibrium by Eq. [3b], with log *K_I being the stability constant.

(3a)

(3b)

If Al is in equilibrium with both phases, the right hand sides of Eq. [2b] and [3b] become equal, and, therefore, pSi can be calculated according to Eq. [4].

(4)

For the derivation of the pAl vs. pH relationship assuming congruent dissolution of ITM, stoichiometric stability constants ^c*K⁰_s (solubility of ITM, see discussion section), ^cK₁ (formation of AlH₃SiO²⁺₄; Table 1), and ^cß_n (mononuclear 1:1, 1:2, and 1:4 Al-hydroxo-complexes; Table 1) were used. Equations [5a] and [5b] must be fullfilled, where [] denote concentrations and [Al]_total and [Si]_total stand for total concentrations of dissolved Al and Si, respectively.

(5a)

(5b)

It is assumed that [Al]_total consists of the species [Al³⁺], monomeric 1:1, 1:2, and 1:4 Al-hydroxo-complexes and , and [Si]_total is the sum of [H₄SiO₄] and . Thus, [Al]_total and [Si]_total can be calculated according to Eq. [6a] and [6b].

(6a)

(6b)

Equations [6a] and [6b] are put into Eq. [5b], which is solved for [H₄SiO₄] to replace this species in Eq. [5a]. After rearranging, Eq. [7] is obtained.

(7)

For a certain H⁺-concentration, Eq. [7] can be solved for Al³⁺ by using, e.g., the Newton-Raphson method. This was done for several pH values between 3.5 and 5.0, assuming an ionic strength of , which is similar to the mean I determined in the equilibrium experiments, and a temperature of 20°C. The corresponding plot pAl vs. pH revealed a slope of 2.0.

	Results

TOP ABSTRACT INTRODUCTION Samples and methods Results Discussion Conclusions REFERENCES

 
Physical and Chemical Properties of the Soil Samples
The particle size distribution was dominated by the sand fraction with relative contents between 51 and 62% in the different horizons. The contribution of the silt fraction was in the range 28 to 37%. The clay content showed a decrease with depth from 17 to 6% (data not shown). The pH determined in H₂O increased with depth from 4.27 to 5.49 (Table 2) . Organic C exhibited a decrease within the profile by almost two orders of magnitude. The effective cation exchange capacity decreased from 145 mmol_c/kg soil in the Ah horizon to 4.4 mmol_c/kg in the BC2 horizon. The base saturation varied between 3.7 and 19.2% with a minimum in the Bh horizon. Selective extractions showed for both Al_p and Al_ox a maximum in the Bh horizon (Table 3) . In the horizons in which organic matter represented a major component, Al_p was similar to Al_ox. In the lower horizons, the Al_p decrease was more pronounced than the one of Al_ox. The ratio Al_ox/Al_tot varied between 0.06 and 0.16. The (Al_ox - Al_p)/Si_ox ratio as a measure to estimate the Al/Si ratio in amorphous Al silicates (see Parfitt and Kimble, 1989) varied between 0.68 and 3.32 within the four lower horizons. The low value observed for the Bh horizon is uncertain as indicated by a high standard deviation. The contribution of exchangeable Al, (Al_exc), to Al_tot was 1% in the Ah and even smaller in the other horizons.

View larger version (28K): [in this window] [in a new window]   Fig. 1 Results of acid leaching experiment for samples from different depths in the profile (Table 2); a) oxalate extraction; b) pyrophosphate extraction; Alox and Alp:oxalate and pyrophosphate extractable Al in untreated samples; AlRox and AlRp:oxalate and pyrophosphate extractable Al in solid residues after successive acid leaching; AlL:total Al removed by acid leaching; AlL + AlRox and AlL + AlRp:sum of total leached Al and extractable Al in solid residues

View larger version (26K): [in this window] [in a new window]   Fig. 2 IR-spectra in the range 600 to 300 cm-1 of the acid dispersible clay fraction <0.5 µm from the horizons Bs, BC1, and BC2 heated at 150°C and at 400°C, respectively

View larger version (113K): [in this window] [in a new window]   Fig. 3 Transmission Electron Microscopy of the <0.5-µm acid dispersible clay fraction of the BC1 horizon

View larger version (22K): [in this window] [in a new window]   Fig. 4 Values of log IAP vs. time for selected kinetic series. a) log IAP for Al(OH)3 (Eq. [1a]); b) log IAP for imogolite (Eq. [1b]). Initial H+-concentrations were 18.0 mM for (AE), 27.5 mM for Bh, 5.7 mM for Bs, 9.7 mM for BC1, and 9.8 mM for BC2

View larger version (20K): [in this window] [in a new window]   Fig. 5 Results of batch equilibrium experiment. a) pAl vs. pH; the imogolite line assuming congruent dissolution obeys Eq. [7]; b) pSi vs. pH; c) (pAl + 0.5 pSi) vs. pH. Closed symbols, soil:solution = 1:10; open symbols, soil:solution = 1:2.5. The linear fittings in a) and c) were performed by structural analyses (see Webster, 1997) including all data at pH >4.1 of the Bh, Bs, BC1, and BC2 horizons. Structural analyses in b) was performed separately for both pH <4.1 and pH >4.1 for each of the four lower horizons. Solubilities of gibbsite (Palmer and Wesolowski, 1992), imogolite (Farmer and Fraser, 1982), proto-imogolite sol (Lumsdon and Farmer, 1995), quartz, and amorphous SiO2 (Stumm and Morgan, 1981) were calculated for 20°C by the van't Hoff equation

(8)

In many filtrates dissolved Si_m was oversaturated with respect to quartz, and in two filtrates at low pH values a slight oversaturation with respect to amorphous SiO₂ was observed (Fig. 5b; thermodynamic data from Stumm and Morgan, 1981). In the Ah and (AE) horizons, a slight increase of pSi vs. pH was found over the investigated pH range. In the four lower horizons, pSi increased strongly up to pH = 4.1 with slopes varying between 1.23 and 1.76. At higher pH markedly lower slopes between 0.25 and 0.40 were observed. In contrast to this qualitative similarity, the pSi vs. pH curves exhibited quantitative differences with respect to pSi decreasing in the order Bh > Bs > BC1 > BC2. The soil: solution ratio had only little effect on pSi.

The (pAl + 0.5 pSi) vs. pH data for the Ah and (AE) horizons differed markedly from the behavior of ITM (Fig. 5c). By contrast, (pAl + 0.5 pSi) for the four lower horizons increased with pH exhibiting slopes between 2.87 and 3.17 at pH >4.1 (r² 0.99) (Table 4). The compilation of all data yielded Eq. [9].

(9)

Lower and upper confidence limits of the slope at the 95% level were 2.91 and 3.20, respectively. Mean values of log IAP⁰ (imogolite) (Eq. [1b]) varied between 6.43 and 6.66 for the different horizons and the compilation of all data yielded log (Table 4).

	Discussion

TOP ABSTRACT INTRODUCTION Samples and methods Results Discussion Conclusions REFERENCES

 
First the extractability of Al in the different horizons of the Typic Haplohumod is discussed. Since Al_exc is much lower than Al_ox or Al_p, the reactive part of total Al is mainly associated with amorphous and/or poorly crystalline inorganic phases or bound to organic matter. Pyrophosphate is known to extract preferentially Al that is coordinated with soil organic matter (see Parfitt and Childs, 1988), whereas oxalate dissolves in addition Al in amorphous and poorly crystalline inorganic phases as well as Al from chloritized vermiculite (Fordham and Norrish, 1983). On the basis of this, the podzol profile can be subdivided into two parts of different extraction behavior. In the three upper horizons with high contents of C_org, Al_p Al_ox indicates that most of the reactive Al is associated with organic matter. In the three lower horizons which have low contents of C_org, the Al_p/Al_ox ratios in the range 0.18 to 0.34 provide evidence that the major part of reactive Al is bound to amorphous or poorly crystalline inorganic compounds.

In the four lower horizons, Si_ox is indicative of the occurrence of allophane or ITM (see Farmer et al., 1983; Dahlgren, 1994; Gustafsson et al. 1995). Furthermore, ratios of (Al_ox - Al_p)/Si_ox between 2.6 and 3.3 in the Bs, BC1, and BC2 horizons indicate that these phases are rich in Al.

A comparison of with Al_ox in the leaching experiment shows that added acid mainly dissolves oxalate extractable Al. The slightly higher values of compared with Al_ox suggest that minor amounts of crystalline Al bound to primary or secondary minerals are also dissolved. These findings are in good agreement with a study by Dahlgren and Walker (1993), who showed in kinetic experiments using spodic Bs horizons that acid mainly dissolves oxalate extractable Al. In the Ah, (AE), and Bh horizons similar values of compared with Al_p as well as with Al_ox provide evidence that added acid mainly dissolves Al bound to organic matter. In the three lowermost horizons, Al^R_p is similar to Al_p indicating that Al associated with inorganic amorphous or poorly crystalline compounds is the major source of acid leachable Al. These findings are consistent with an investigation by LaZerte and Findeis (1995) who stated that Al_ox is most affected by acidic leaching when the ratio (Al_ox - Al_p)/Al_p is above 0.3 to 0.7 and Al_p becomes most important at lower ratios.

The IR spectra reveal kaolinite in Bs, BC1, and BC2 horizons. The occurrence of this mineral is proven by the absorption bands at 3698, 3627, 536, and 471 cm^-1. Imogolite or proto-imogolite allophane have characteristic bands at 428 and 348 cm^-1 where they overlap with bands of kaolinite. The absorption bands which can unambiguously be assigned to kaolinite show no significant change in intensity after heating to 400°C. By contrast to kaolinite, imogolite as well as proto-imogolite allophane are unstable at this temperature. Thus, the decrease in intensity of the two bands at 428 and 348 cm^-1 is indicative of these phases. The detection of a thread-like morphology by TEM analysis provides further evidence for the occurrence of imogolite in the Bs, BC1, and BC2 horizons.

With respect to Al solubility as determined in batch experiments, the profile can be subdivided into two parts consisting of the (i) Ah and (AE) and (ii) Bh, Bs, BC1, and BC2 horizons. In neither the Ah nor the (AE) horizons, was equilibrium attained with respect to Al(OH)₃ or ITM within 30 d. The observed behavior rather suggests that Al solubility is controlled by complexation reactions to soil organic matter. In such a case, Al solubility is dependent on the amount of reactive Al in the system (see Walker et al., 1990; Berggren and Mulder, 1995). Cronan et al. (1986) and Walker et al. (1990) found in batch experiments with organic horizons that solutions were undersaturated with respect to gibbsite at pH values lower than 4.4 to 4.8, the exact value depending on the amount of reactive Al, whereas an oversaturation was observed at higher pH. Our results from the Ah and (AE) horizons are consistent with these findings, and, therefore, it is concluded that Al solubility is controlled by complexation to soil organic matter. This mechanism is consistent with the results of the selective extractions and acid leaching experiments with Ah and (AE) material.

In the Bh, Bs, BC1, and BC2 horizons dissolved Al attained equilibrium after 16 d in all kinetic series based on stable values of log IAP for Al(OH)₃ and imogolite, suggesting that Al solubility is controlled by an inorganic phase. The simultaneous rapid initial increases in pH, Al_labile, and Si_m in all kinetic series provide evidence that added acid is mainly buffered by release of Al and that a reactive aluminosilicate is involved in proton buffering processes.

As shown in Fig. 5a, the Bh, Bs, BC1, and BC2 horizons exhibit very similar pAl-pH reationships which suggest Al solubility control by the same solid phase. Considering pAl vs. pH, the mean log IAP⁰ of 8.35 for Al(OH)₃ (Eq. [1a]) at pH >4.1 is similar to values reported in the literature. Gustafsson et al. (1998) found log for spodic B horizons containing imogolite from central Sweden and Finland. Dahlgren et al. (1989) reported a lower value of 8.1 for Spodosol Bs horizons in the northeastern USA containing no or only small amounts of imogolite. Su et al. (1995) determined log for Bs and BC horizons of a Tephritic Spodosol which contained imogolite as a major component in the clay fraction. In this study, statistical analysis revealed a slope of pAl vs. pH <3 at the 95% confidence level, suggesting that Al solubility may not be adequately described by assuming equilibrium with Al(OH)₃. Dahlgren and Walker (1993) found in batch experiments with Spodosol Bs horizons a slope of 2.7 between pH 3.1 and 5.2. On the basis of a study by Bloom et al. (1977), these authors attributed Al-solubility control to hydroxy-Al interlayered 2:1 layer silicates arguing that if the hydroxy-Al interlayer phase exhibits a positive charge to compensate the permanent negative charge of the clay mineral, a slope of less than 3 is obtained. In our study, Al-solubility control by chlorite or hydroxy-interlayered vermiculite, which are major mineral compounds in the investigated soil profile (Eggenberger, 1995), could lead to a slope of less than 3 in the pAl vs. pH diagram.

Structural analysis of (pAl + 0.5 pSi) vs. pH for the four lower horizons at pH >4.1 yields a slope of 3.05 which, at the 95% level, does not significantly differ from the theoretical value of 3.00 for ITM. Therefore, the mean log IAP⁰ of 6.53 ± 0.09 can be considered to represent log *K⁰_s of ITM in the four lower horizons. This value is within the range of reported stability constants for synthetic imogolite (log , Farmer and Fraser, 1982; log , Su and Harsh, 1994) and for proto-imogolite sol (log , Lumsdon and Farmer, 1995). Thus, the consideration of (pAl + 0.5 pSi) vs. pH suggests Al-solubility control by ITM. The uniform increase of pSi from pH = 4.1 up to pH values significantly higher than 5 may indicate that Al-solubility control by ITM is also valid in the pH range 5 to 6, where Al_labile was below detection limit. There are few studies on selected podzol horizons in which indications for Al-solubility control by ITM were found. For a spodic Bs horizon, Gustafsson et al. (1998) reported log , and for Bs and BC horizons of a Tephritic Spodosol log was found (Su et al., 1995). For the Bs, BC1, and BC2 horizons in this study, Al-solubility control by ITM is consistent with the results of selective extractions, leaching experiments, IR spectroscopy, and TEM. While Si_ox provides evidence for the presence of low amounts of ITM also in the Bh horizon, selective extraction and acid leaching data indicate that reactive Al in this horizon is mainly bound to Al-organic phases. Thus, at pH <4.16, where undersaturation with respect to ITM occurs, Al solubility may be controlled by complexation to soil organic matter.

As shown above, the independent considerations of (pAl + 0.5 pSi) vs. pH and pAl vs. pH lead to different implications on the Al solubility control in the four lower horizons. However, there are two possibilities for a combined interpretation. As a first possibility, both relationships can be explained by solubility control by ITM, which dissolves incongruently. Solubility control by congruent dissolution of ITM is very unlikely, since this would lead to a pAl vs. pH relationship with a slope of 2.0 as shown in Fig. 5a. Additional Al release by Al-organic phases might increase this slope. Considering the contribution of Al_p to Al_L in the acid leaching experiments, a significant effect of such a mechanism cannot be excluded for the Bh horizon. In the Bs, BC1, and BC2 horizons, however, reactive Al bound to organic matter very likely has only a minor effect on pAl vs. pH. The assumption of solubility control by incongruently dissolving ITM is supported by data from the following studies. Su and Harsh (1994) reported that solutions in equilibrium with a synthetic imogolite were oversaturated with respect to gibbsite. Lumsdon and Farmer (1995) investigated the solubility characteristics of a synthetic proto-imogolite sol. Stuctural analysis of their data (runs D1–D4 and F1–F5) reveals slopes of 2.92, 2.36, and 1.14 for (pAl + 0.5 pSi) vs. pH, pAl vs. pH, and pSi vs. pH, respectively. The findings of these studies suggest that in presence of a short-range ordered aluminosilicate (i) Al activities can be oversaturated with respect to gibbsite and (ii) a slope of pAl vs. pH between 2 and 3 can occur.

As a second possibility, Al³⁺ is assumed to be in a simultaneous equilibrium with ITM and hydroxy-Al interlayer. The latter phase has to be of the form Al⁺_n with n 0.13 as indicated by the slope of pAl vs. pH. According to Eq. [4], this would lead to a slope of pSi vs. pH of 0.26, which is within the observed range of 0.25 to 0.40. Thus, our data are consistent with respect to this mechanism which has been proposed earlier in the literature. Farmer (1987) proposed an equilibrium of this type for podzol Bs horizons. Dahlgren and Ugolini (1989) found evidence for such an equilibrium in soil solutions from a Tephritic Spodosol, and this finding was confirmed in a laboratory study conducted by Su et al. (1995).

Finally, the pH range below 4.1, where a break in pSi vs. pH occurs, is discussed for the three lower horizons. In all these horizons, the equilibrium solutions were distinctly undersaturated with respect to ITM at pH <3.8. This behavior can be explained by either kinetically impeded dissolution of Al, or control of Al activities by a solid phase which is more stable than ITM. In the kinetic series with Bs material and a final pH value of 3.70, log IAP with respect to both Al(OH)₃ and imogolite did not significantly change after 16 d, indicating that Al activities were in equilibrium with an inorganic solid phase. Thus, it is likely that solutions attained equilibrium at pH <4.1. The significant higher slopes of pSi vs. pH in this pH range when compared with the pH range above 4.1 may indicate Al solubility control by another aluminosilicate than ITM. To evaluate this possibility it was assumed that filtrates in the pH range 3.58 to 4.07 (n = 14) were in equilibrium with an aluminosilicate of the general formula AlSi_y(OH)_wO_((3+4y-w)/2), the solubility constant log *K_s of which can be calculated according to Eq. [10].

(10)

The value of y was varied until the slope of log *K_s vs. pH was close to zero. A value of 1.05 was determined for y and log *K_s was calculated to be 5.04 ± 0.14. These results suggest that an aluminosilicate with an Al:Si ratio close to 1:1 controls Al solubility at pH values <4.1. A possible candidate is kaolinite, which was detected by IR spectroscopy. Kittrick (1970) pointed out, that the stability of kaolinite in soils might vary between the crystalline form and the one of halloysite. Thus, log might vary between 3.1 and 5.4 at 20°C (thermodynamic data reported by Manley et al., 1987; Stumm and Morgan, 1981; Kittrick, 1969). The value estimated here is within this range, and, therefore, it is possible that Al solubility at pH <4.1 is controlled by poorly crystalline kaolinite. It must be stated, however, that no direct evidence for its presence could be found.

	Conclusions

TOP ABSTRACT INTRODUCTION Samples and methods Results Discussion Conclusions REFERENCES

 
The first study on the Al solubility control for a complete set of horizons of a podzol over the entire ecologically significant pH range led to the following conclusions:

1. Specific extraction and acid leaching experiments suggest that reactive Al is mainly bound to organic phases in the Ah, (AE), and Bh horizons and to inorganic solid phases in the Bs, BC1, and BC2 horizons. In the latter three horizons, oxalate extraction, IR, and TEM data indicate ITM as reactive inorganic Al phase.
   
2. Batch equilibrium experiments provide evidence that in the Ah and (AE) horizons Al solubility is controlled by complexation reactions to soil organic matter over the entire investigated pH range. This behavior is in agreement with specific extraction and leaching data. In the Bh, Bs, BC1, and BC2 horizons Al solubility at pH > 4.1 can be explained either by an equilibrium with incongruently dissolving ITM or by a simultaneous equilibrium with both ITM and hydroxy-Al interlayers of clay minerals. The data below pH 4.1 for the three lower horizons suggest solubility control by a 1:1 aluminosilicate, e.g., poorly crystalline kaolinite.
   

	ACKNOWLEDGMENTS

 
The authors thank the following persons at WSL: A. Diarra, U. Beutler, R. Luescher, Dr. S. Zimmermann and K. Siegrist for technical support in the laboratory and at the field site, V. Michellod for performing selective extractions as well as leaching and kinetic experiments, and the staff of the central laboratory for performing ICP-AES analyses. Dr. M. Brechbühl (ETHZ) supplied total chemical analyses of the soil samples. The first author is grateful to Dr. D. Berggren, Dr. J.P. Gustafsson and M. Simonsson at SLU for many valuable discussions as well as for introduction to FIA-, IR-, and TEM-analyses. Dr. V.C. Farmer provided valuable comments on an earlier manuscript. Gratitude is also expressed to three anonymous reviewers for their constructive criticism. This study was supported by grants from the Swiss Federal Office of Environment, Forest and Landscape, the Swiss National Science Foundation (grant no. 8220-046488) and the Swedish University of Agricultural Sciences, Uppsala.

Received for publication February 23, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Samples and methods Results Discussion Conclusions REFERENCES

 

• Bascomb C.L. Distribution of pyrophosphate-extractable iron and organic carbon in soils of various groups. J. Soil Sci. 1968;19:251-268.
• Berggren D., Mulder J. The role of organic matter in controlling aluminum solubility in acidic mineral soil horizons. Geochim. Cosmochim. Acta 1995;59:4167-4180.[Web of Science]
• Berggren D., Sparén A. A modified FIA system for the determination of high levels of quickly reacting aluminium in aqueous solutions. Inter. J. Environ. Anal. Chem. 1996;62:115-128.
• Blaser P., Kernebeek P., Tebbens L., van Breemen N., Luster J. Cryptopodzolic soils in Switzerland. Europ. J. Soil Sci. 1997;48:411-423.
• Bloom P.R., McBride M.B., Chadbourne B. Adsorption of aluminum by a smectite: I. Surface hydrolysis during Ca²⁺–Al³⁺ exchange. Soil Sci. Soc. Am. J. 1977;41:1068-1073.[Abstract/Free Full Text]
• Bloom P.R., McBride M.B., Weaver R.M. Aluminum organic matter in acid soils: Buffering and solution aluminum activity. Soil Sci. Soc. Am. J. 1979;43:488-493.[Abstract/Free Full Text]
• Childs C.W., Parfitt R.L., Lee R. Movement of aluminum and an inorganic complex in some podzolized soils, New Zealand. Geoderma 1983;29:139-155.
• Clarke N., Danielsson L.-G., Sparén A. The determination of quickly reacting aluminium in natural waters by kinetic discrimination in a flow system. Intern. J. Environ. Anal. Chem. 1992;48:77-100.
• Cronan C.S., Walker W.J., Bloom P.R. Predicting aqueous aluminium concentrations in natural waters. Nature 1986;324:240-243.
• Dahlgren R.A. Quantification of allophane and imogolite. In: Amonette J.E., Zelazny L.W., eds. Quantitative methods in soil mineralogy. Madison, WI: SSSA, 1994:430-451.
• Dahlgren R.A., Ugolini F.C. Formation and stability of imogolite in a tephritic Spodosol, Cascade Range, Washington, U.S.A. Geochim. Cosmochim. Acta 1989;53:1897-1904.
• Dahlgren R.A., Walker W.J. Aluminum release rates from selected Spodosol Bs horizons: Effect of pH and solid-phase aluminum pools. Geochim. Cosmochim. Acta 1993;57:57-66.
• Dahlgren R.A., Driscoll C.T., McAvoy D.C. Aluminum precipitation and dissolution rates in Spodosol Bs horizons in the northeastern U.S.A. Soil Sci. Soc. Am. J. 1989;53:1045-1052.[Abstract/Free Full Text]
• David M.B., Driscoll C.T. Aluminum speciation and equilibria in soil solutions of a haplorthod in the adirondack mountains (New York, USA). Geoderma 1984;33:297-318.
• Eggenberger, U. 1995. Mineral weathering in soils: experiments, field studies, and modelling. Ph.D. dissertation, Univ. Berne, Switzerland.
• FAO. 1988. Soil map of the world. Revised legend. Food and Agriculture Organization of the United Nations, Rome.
• Farmer V.C. The Infrared Spectra of Minerals. London: Mineralogical Society, 1974.
• Farmer V.C. The role of inorganic species in the transport of aluminium in podzols. In: Righi D., Chauvel A., eds. Podzols et podzolization. Paris: AFES et INRA, 1987:187-194.
• Farmer V.C., Fraser A.R. Chemical and colloidal stability of soils in the Al₂O₃-Fe₂O₃-SiO₂-H₂O system: their role in podzolization. J. Soil Sci. 1982;33:737-742.
• Farmer V.C., Fraser A.R., Tait J.M. Characterization of the chemical structures of natural and synthetic aluminosilicate gels and sols by infrared spectroscopy. Geochim. Cosmochim. Acta 1979;43:1417-1420.
• Farmer V.C., Russel J.D., Berrow M.L. Imogolite and proto-imogolite allophane in spodic horizons: Evidence for a mobile aluminum silicate complex in podzol formation. J. Soil Sci. 1980;31:673-684.
• Farmer V.C., Russell J.B., Smith B.F.L. Extraction of inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon. J. Soil Sci. 1983;34:571-576.
• Fordham A.W., Norrish K. The nature of soil particles particularly those reacting with arsenate in a series of chemically treated samples. Aust. J. Soil Res. 1983;21:455-477.
• Foy C.D. Physiological effects of hydrogen, aluminum and manganese toxicities in acid soil. In: Adams F., ed. Soil acidity and liming, 2nd ed Madison, WI: ASA, 1984:57-97.
• Gee G.W., Bauder J.W. Particle-size analysis. In: Page A.L., et al. , ed. Methods of Soil Analysis. Part 1. Physical and Mineralogical Methods, 2nd ed Madison, WI: ASA, 1986:383-411.
• Gustafsson J.P., Bhattacharya P., Bain D.C., Fraser A.R., McHardy W.J. Podzolisation mechanisms and the synthesis of imogolite in northern Scandinavia. Geoderma 1995;66:167-184.
• Gustafsson J.P., Lumsdon D.G., Simonsson M. Aluminium solubility characteristics of spodic B horizons containing imogolite. Clay Minerals 1998;33:77-86.[Abstract]
• Kendall M.G., Stuart A. The advanced theory of statistics. Volume 2. Inference and relationship, 2nd edition London: Griffin, 1967.
• Kittrick J.A. Soil minerals in the Al₂O₃-SiO₂-H₂O system and a theory of their formation. Clays Clay Miner. 1969;17:157-167.
• Kittrick J.A. Precipitation of kaolinite at 25°C and 1 atm. Clays and Clay Miner. 1970;18:261-267.
• Koch O.G., Koch G.A. Handbuch der Spurenanalyse. Heidelberg, Germany: Springer-Verlag, 1964.
• LaZerte B.D., Findeis J. The relative importance of oxalate and pyrophosphate extractable aluminum to the acidic leaching of aluminum in Podzol B horizons from the Precambrian Shield, Ontario, Canada. Can. J. Soil. Sci. 1995;75:43-54.
• Lumsdon D.G., Farmer V.C. Solubility characteristics of proto-imogolite sols: how silicic acid can de-toxify aluminium solutions. Eur. J. Soil Sci. 1995;46:179-186.
• Luster J., Yang A., Sposito G. On the interpretation of labile aluminum as determined by reaction with 8-hydroxyquinoline. Soil Sci Soc. Am. J. 1993;57:976-980.[Abstract/Free Full Text]
• Manley E.P., Chesworth W., Evans L.J. The solution chemistry of podzolic soils from the eastern Canadian shield: a thermodynamic interpretation of the mineral phases controlling soluble Al³+ and H₄SiO₄. J. Soil Sci. 1987;38:39-51.
• Matzner E. Acidification of forests and forest soils: Current status. Studies in environmental science 1992;50:77-86.
• Mulder J., Stein A. The solubility of aluminum in acidic forest soils: Long-term changes due to acid deposition. Geochim. Cosmochim. Acta 1994;58:85-94.
• Mulder J., van Breemen N., Eijck H.C. Depletion of soil aluminium by acid deposition and implications for acid neutralization. Nature 1989;337:247-249.
• Neal C., Skeffington R.A., Williams R., Roberts D.J. Aluminium solubility controls in acid waters: the need for a reappraisal. Earth Planet. Sci. Lett. 1987;86:105-112.
• Nordstrom D.K., May H.M. Aqueous equilibrium data for mononuclear Al species. In: Sposito G., ed. The environmental chemistry of aluminum. 2nd. ed. Boca Raton, FL: Lewis Publ, 1996:39-80.
• Paces T. Sources of acidification in Central Europe estimated from elemental budgets in small basins. Nature 1985;315:31-36.
• Palmer D., Wesolowski D.J. Aluminum speciation and equilibria in aqueous solution: II. The solubility of gibbsite in acidic sodium chloride solutions from 30 to 70°C. Geochim. Cosmochim. Acta 1992;56:1093-1111.
• Parfitt R.L., Childs C.W. Estimation of forms of Fe and Al: A review, and analyses of contrasting soils by dissolution and Moessbauer methods. Aust. J. Soil Res. 1988;26:121-144.
• Parfitt R.L., Kimble J.M. Conditions for formations of allophane in soils. Soil Sci. Soc. Am. J. 1989;53:971-977.[Abstract/Free Full Text]
• Pokrovski G.S., Schott J., Harrichoury J.-C., Sergeyev A.S. The stability of aluminum silicate complexes in acidic solutions from 25 to 150°C. Geochim. Cosmochim. Acta 1996;60:2495-2501.
• Ross G.J., Kodama H. Evidence for imogolite in Canadian soils. Clays Clay Miner. 1979;27:297-300.[Abstract]
• Russel, J.D., and A.R. Fraser. 1994. Infrared methods. p. 11–67. In M.J. Wilson (ed.) Clay mineralogy: spectroscopic and chemical determinative methods. Chapman and Hall.
• Schecher W.D., Driscoll C.T. An evaluation of uncertainty associated with aluminum equilibrium calculations. Water Resour. Res. 1987;23:525-534.
• Schwertmann U. Differenzierung der Eisenoxide des Bodens durch Extraktion mit Ammoniumoxalat-Lösung. Z. Pflanzenern., Döng., Bodenk. 1964;105:194-202.
• Soil Survey Staff. 1996. Keys to soil taxonomy. 7th ed. U.S. Print. Office. Washington, DC.
• Stumm W., Morgan J.J. Aquatic chemistry. New York: Wiley Interscience, 1981.
• Su C., Harsh J. Gibbs free energies of formation at 298K for imogolite and gibbsite from solubility measurements. Geochim. Cosmochim. Acta 1994;58:1667-1677.[Web of Science]
• Su C., Harsh J.B., Boyle J.S. Solubility of hydroxy-aluminum interlayers and imogolite in a spodosol. Soil Sci. Soc. Am. J. 1995;59:373-379.[Abstract/Free Full Text]
• Sumner M.E., Fey M.V., Noble A.D. Nutrient status and toxicity problems in acid soils. In: Ulrich B., Sumner M.E., eds. Soil acidity. Berlin: Springer, 1991:149-182.
• Sverdrup H., Warfvinge P., Blake L., Goulding K. Modelling recent and historic soil data from the Rothamstead experimental station, England, using SAFE. Agriculture Ecosystems and Environment 1995;53:161-177.
• Tait J.M., Yoshinaga N., Mitchell B.D. The occurrence of imogolite in some Scottish soils. Soil Sci. Plant Nutr. (Tokyo) 1978;24:145-151.
• Thomsen J., Johnson K., Petty R. Determination of reactive silicate in seawater by flow injection analyses. Anal. Chem. 1983;55:2378-2382.
• Turner R.C., Brydon J.E. Factors affecting the solubility of Al(OH)₃ precipitated in the presence of montmorillonite. Soil Sci. 1965;100:176-181.
• Turner R.C., Brydon J.E. Effect of length of time on some properties of suspensions of Arizona bentonite, illite, and kaolinite in which aluminum hydroxide is precipitated. Soil Sci. 1967;103:111-117.
• Ulrich, B. 1989. Effects of acidic precipitation on forest ecosystems in Europe. In D.C. Adriano and A.H. Johnson (ed.) Acidic precipitation. Volume 2: Biological and ecological effects. Springer, New York.
• Van Breemen N., Driscoll C.T., Mulder J. Acid deposition and internal proton sources in acidification of soils and waters. Nature 1984;307:599-604.
• Walker W.J., Cronan C.S., Bloom P.R. Aluminum solubility in organic soil horizons from northern and southern forested watersheds. Soil Sci. Soc. Am. J. 1990;54:369-374.[Abstract/Free Full Text]
• Wang C., McKeague J.A., Kodama H. Pedogenic imogolite and soil environments: case study of spodosols in Quebec, Canada. Soil Sci. Soc. Am. J. 1986;50:711-718.[Abstract/Free Full Text]
• Webster R. Regression and functional relations. Europ. J. Soil Sci. 1997;48:557-566.
• Xu S., Harsh J.B. Influence of autocorrelation and measurement errors on interpretation of solubility diagrams. Soil Sci. Soc. Am. J. 1995;59:1549-1557.[Abstract/Free Full Text]
• Yuan T.L. Determination of exchangeable hydrogen in soils by titration method. Soil Sci. 1959;88:164-167.

This article has been cited by other articles:

				R. L. Parfitt Allophane and imogolite: role in soil biogeochemical processes Clay Minerals, March 1, 2009; 44(1): 135 - 155. [Abstract] [Full Text] [PDF]

				T. T. Brown, R. T. Koenig, D. R. Huggins, J. B. Harsh, and R. E. Rossi Lime Effects on Soil Acidity, Crop Yield, and Aluminum Chemistry in Direct-Seeded Cropping Systems Soil Sci. Soc. Am. J., May 1, 2008; 72(3): 634 - 640. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (20) Citing Articles via Google Scholar Google Scholar Articles by Zysset, M. Articles by Gehring, A.U. Search for Related Content PubMed Articles by Zysset, M. Articles by Gehring, A.U. GeoRef GeoRef Citation Agricola Articles by Zysset, M. Articles by Gehring, A.U.