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a Natural Resources Canada, CANMET, 555 Booth Street, Office 332A, Ottawa, ON, Canada K1A 0G1
b Department of Soil Science, North Carolina State University, Box 7619, 3235 Williams Hall, Raleigh, NC 27695-7619
c National Institute of Environmental Health Sciences, P.O. Box 12233, Research Triangle Park, NC 27709
d Canada Centre for Remote Sensing, 588 Booth Street, 4th floor, Ottawa, ON, Canada K1A 0Y7
e Soil Science Department, University of Manitoba, 362 Ellis Building, Winnipeg, MB, Canada R3T 2N2
f Department of Physics, North Carolina State University, Box 8202, Raleigh, NC 27695-8202
* Corresponding author (sbeauche{at}nrcan.gc.ca).
Received for publication November 6, 2002.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: Alox and Feox, ammonium oxalate–extractable aluminum or iron • HCl-P, phosphorus extracted with 1 M HCl • IHP, inositol hexametaphosphate • LCF, linear combination fitting • M3P, Mehlich III–extractable phosphorus • NaOH-P, phosphorus extracted with 0.1 M NaOH • PCA, principal component analysis • Pi, inorganic phosphorus • Po, organic phosphorus • Pt, total soil phosphorus • XANES, X-ray absorption near-edge structure
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The increase in soil P solubility that is often correlated to an increase in total soil P concentration may be explained in part by changes in solid-phase speciation or by the affinity of orthophosphate (PO4) for sorbing soil components. Adsorption appears to be the dominant retention mechanism that regulates dissolved phosphate at low concentrations, whereas phosphate mineral precipitation controls P solubility at high concentrations (Lindsay et al., 1989). Consequently, distinguishing between adsorbed phases and precipitates is critical to ascertain the long-term behavior of P in soils. Typically, adsorption isotherms from laboratory experiments are characterized by an L-curve (Sposito, 1984), which can be fitted with a Langmuir or Freundlich isotherm model. This type of isotherm predicts that as the PO4 loading rate approaches the maximum adsorption capacity of the soil, additional phosphate cannot be retained by the soil. Unlike surface-adsorbed chemical species, the solubility of a solid-phase precipitate is essentially independent of the amount of the solid phase present (Lindsay, 1979). Thus, precipitation of minerals such as Ca-, Al-, or Fe-phosphates at higher soil P concentrations may represent a sink for P that has a constant solubility under given chemical conditions. However, the type of phosphate mineral formed and soil conditions such as pH and presence of dissolved complexing species will determine the phosphate activity in solution (Lindsay, 1979).
In the past, thermodynamic models of mineral solubility predicted that dissolved PO4 would be controlled at equilibrium by Fe- and Al-phosphates in acidic soils and by Ca-phosphates in neutral and alkaline soils (Lindsay, 1979). However, kinetics of P transformations were not considered (Bohn et al., 1985), even though kinetic limitations often exert considerable influence on P speciation in natural environments. Also, adsorbed P phases were poorly understood and difficult to include in such models. For example, the concentrations of ammonium oxalate–extractable Al and Fe in soils have often been found to be the best variable to predict P sorption capacities of acidic (Laverdière and Karam, 1984; van der Zee and van Riemsdijk, 1986; Freese et al., 1992; Simard et al., 1994) and neutral to calcareous soils (Ryan et al., 1984; Tran and Giroux, 1987). This correlation suggests that oxide mineral surfaces are significant P-sorbing components at all pH levels. Likewise, results of energy-dispersive X-ray analyses of excessively fertilized soils showed that P-rich particles contained P predominantly associated with Al in amorphous solid phases, even for neutral to slightly alkaline soil samples (Pierzynski et al., 1990). Such observations illustrate the need for direct identification of soil P species, regardless of soil properties, when trying to understand and quantitatively model long-term changes in P solubility in P-enriched soils.
In the present study, X-ray absorption near-edge structure (XANES) spectroscopy was used in conjunction with sequential chemical fractionation to characterize the dominant solid-phase species of P in selected soils. Total-electron-yield XANES studies at the P K-edge of several commercial phosphate powders have shown that each compound had a unique spectrum that reflected the specific molecular environment of P (Franke and Hormes, 1995; Okude et al., 1999). Rose et al. (1997) determined the local structure of P during hydrolysis of FeCl3 in the presence of phosphate using P K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. For soils, XANES spectroscopy has been mainly applied to sulfur and metal speciation (Fendorf and Sparks, 1996), but Hesterberg et al. (1999) have shown the feasibility of using this approach for more direct identification of some soil P species. X-ray absorption near-edge structure spectroscopy has the main advantages of being element specific and nondestructive (no sample pretreatment required). It further provides information on the local molecular bonding environment of the element (Fendorf and Sparks, 1996). Unlike X-ray diffraction, poorly ordered mineral phases can also be characterized by XANES spectroscopy (Schulze and Bertsch, 1995). The objective of this study was to determine chemical speciation of P in long-term-fertilized, P-enriched soil samples using synchrotron XANES spectroscopy and sequential chemical fractionation. For this purpose, soil samples were selected to represent a range of properties such as pH, texture, organic matter content, and P source.
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MATERIALS AND METHODS |
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Soil Characterization
Particle-size analysis was performed by the hydrometer method except for the use of the pipette method for PV soil very rich in clay (Sheldrick and Wang, 1993). Organic C content was determined by wet oxidation (Tiessen and Moir, 1993). Soil pH was measured in distilled water with a soil to solution ratio of 1:2. Mehlich III–extractable P and Ca (M3P, M3Ca) contents were obtained as described by Tran and Simard (1993). Ammonium oxalate–extractable Fe and Al (Feox, Alox) and dithionite + citrate–extractable Fe (Fedc) contents were determined on the soil samples according to Ross and Wang (1993). A modified Hedley et al. (1982) chemical extraction procedure, as described by Simard et al. (1995), was used to fractionate soil phosphorus. Briefly, after grinding to <100 mesh, the soil samples were sequentially extracted for 16 h (each treatment) using an anionic exchange resin (Dowex 1X8-50, HCO-3 form; Dow, Indianapolis, IN), 0.5 M NaHCO3 (pH 8.5), 0.1 M NaOH, 1 M HCl, and concentrated H2SO4–H2O2. In all extracts, inorganic phosphorus (Pi) was measured by the molybdenum blue method (Murphy and Riley, 1962). The NaHCO3 and NaOH extracts were also digested with H2SO4–H2O2 to determine total phosphorus (Pt); organic phosphorus (Po) was then calculated as Pt - Pi. The extractions were designed to target the following forms of P (Hedley et al., 1982): (i) resin P = labile inorganic phosphorus directly exchangeable and soil solution phosphorus, (ii) NaHCO3–P = labile inorganic and organic phosphorus sorbed to soil mineral surfaces plus some microbial phosphorus, (iii) NaOH-P = inorganic phosphorus chemisorbed to aluminum- and iron-oxide minerals and organic phosphorus from humic compounds, (iv) HCl-P = relatively insoluble apatite-type minerals, and (v) H2SO4–P = residual insoluble inorganic phosphorus and the most stable organic phosphorus forms.
Phosphorus Standards for X-Ray Absorption Near-Edge Structure Spectroscopy
The following phosphate standards for XANES spectroscopy were either purchased from a chemical supply company or synthesized according to the references cited (see Hesterberg et al., 1999 for some details): noncrystalline Fe-phosphate and strengite (FePO4·2H2O) treated hydrothermally for 3 or 30 d to vary crystallinity (Dalas, 1991); PO4 adsorbed on poorly crystalline Fe hydroxide (2-line ferrihydrite; Schwertmann and Cornell, 1991, p. 90–94) or Al hydroxide; PO4 adsorbed on goethite (
-FeOOH) or alumina (
-Al2O3) (Oh et al., 1999); noncrystalline Al-phosphate and variscite (AlPO4·2H2O) (Hsu and Sikora, 1993); berlinite (AlPO4) (purchased); octacalcium phosphate [Ca4H(PO4)3·2.5H2O] (Christoffersen et al., 1989); and monetite (CaHPO4), brushite (CaHPO4·2H2O), hydroxyapatite [Ca5(PO4)3OH], adenosine triphosphate (ATP), and inositol hexametaphosphate (IHP) (all purchased). Results from X-ray diffraction analysis showed that the various standards were mineralogically pure, except that the strengite standards contained detectable levels of phosphosiderite (monoclinic FePO4·2H2O).
X-Ray Absorption Near-Edge Structure Spectroscopy Analysis
The XANES data collection for standards and soil samples was done at the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, New York) using the Beamline X-19A equipped with a Si(III) monochromator. With a Si(III) monochromator and collimating mirror, the resolution at the P K-edge is 0.2 eV. The electron beam energy was 2.5 GeV, and the maximum beam current was 300 mA. The XANES data were collected in fluorescence mode at ambient temperature using a solid-state passivated implanted planar silicon (PIPS) detector and a He flight path. The XANES data were taken between 2129 and 2299 eV, with a minimum step size of 0.2 eV from 2139 to 2174 eV. Multiple scans (at least two for standards and four to eight for soil samples) across the P K-edge were averaged. Data were background- and baseline-corrected, and normalized to the K-edge according to procedures described in Sayers and Bunker (1988). A linear baseline correction was made between -20 and -5 eV (relative energy), and a single-point background normalization was made at a flat part of the spectrum near 30 eV (relative energy). The energy scale was normalized to a reference energy (E0) of 2149 eV, which was calibrated as the energy of the maximum of the first peak in the first derivative spectrum for a variscite standard. According to X-ray photoelectron spectral data and other total-electron-yield XANES studies, the binding energy of the P K-shell electron is, in fact, at a higher energy than the E0 defined this way (Franke and Hormes, 1995; Li et al., 1994; Okude et al., 1999).
The XANES data were collected directly on air-dried soil samples ground to pass through a 125-µm sieve. Dried powders of all mineral and organic P standards were diluted to 800 mmol P kg-1 in boron nitride (BN). All mineral powders and soil samples were pressed into a 1.3-cm-diameter sample plexiglass holder well of 1 mm thickness. Standards of adsorbed PO4 containing 500 mmol P kg-1 were prepared as moist pastes, and mounted in the 1.3-cm-diameter well behind a 3-µm-thick film of Mylar X-ray film (Spex Industries, Metuchen, NJ) for data collection. Mylar is known to have detectable phosphorus XANES peak due to contamination, but this peak was trivial compared with the fluorescence yield of our adsorbed PO4 standards at >15-fold higher concentration.
The XANES spectra were analyzed using principal component analysis (PCA) and nonlinear, least-squares fitting–linear combination fitting (LCF). Both approaches were described in detail in Beauchemin et al. (2002). Principal component analysis was first performed to define the number of significant orthogonal components in our dataset composed of the normalized, interpolated spectra (background and baseline corrected) of the five soils. Target transformation was then used to test which standards would be the most likely species in our samples based on two criteria: the SPOIL value and the F test. According to Malinowski (1991), tested standards with SPOIL values of <3 are acceptable whereas SPOIL values of >6 are considered unacceptable. SPOIL values between 3 and 6 represent marginal standards. In the one-tailed F test proposed by Malinowski (1991), the tested standard is retained as valid when the probability of the calculated F is greater than a given critical threshold value such as 0.05 (5% probability).
Linear combination fitting of soil XANES spectra was also performed on the current dataset using all possible binary and ternary combinations of the 14 available standards according to the Vairavamurthy et al. (1994) procedure (for n = 2 or 3, possible combinations = 91 or 364, respectively). Linear combination fitting included energy offset parameters. This fitting approach assumes that the standards chosen are representative of soil phosphorus species present in the soil samples. Standards were not corrected for self-absorption, but self-absorption would decrease the fluorescence signal at the white line peak by less than 8% at a 800 mmol kg-1 concentration for mineral standards (Hesterberg et al., 1999). Linear combination fitting was done using in-house programs running on Scilab 2.6 (Scilab Group, 2002). Normalized XANES spectra were fit over the relative energy range of -10 to 15 eV. Linear combination fitting computes the best-fit weighting factors for the selected standards using the Levenberg–Marquardt method (Nielsen, 1999). The weighting factors correspond to the proportion of each standard yielding the best fit to the XANES spectrum for a given soil sample. Chi-squared values were adopted as a goodness-of-fit criterion. In addition, fits were considered unacceptable when the energy offset parameters were greater than ±1 eV or when the weighting factors were negative.
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RESULTS |
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Sequential Phosphorus Fractionation
Chemical fractionation provided information on operationally defined P pools of varying solubilities (Table 2). For all samples, the largest P fraction was found in moderately labile (NaOH-P) or nonlabile fractions (HCl-P and H2SO4–P). Fractionation data for the A horizon samples suggested that P was mainly associated with Al or Fe oxides in the acidic sb2.1 and Ma2 soils (up to 55% of Pt as NaOH-P), whereas apatite-type minerals represented the major forms of P in the slightly alkaline PV2 sample (40% of Pt as HCl-P). In both B horizon samples, HCl-P was the main pool, suggesting that Ca-bound P minerals accounted for 71% of Pt in the calcareous AI2 sample and for 50% of Pt in the acidic Ma3 sample. Labile P (resin P + NaHCO3–Pt) accounted for 14 to 18% of total P in A horizon soils, whereas this pool was less than 10% for the B horizon samples. On average, 30% of the labile P in A horizons was organic (NaHCO3–Po). For B horizons, 
40% of labile P was organic P. Moderately labile organic P (NaOH-Po), a fraction considered to be associated with humic compounds, represented between 13 and 21% of total P in most soils, except for the calcareous AI2 B horizon, which had only 3% of total P as NaOH-Po.
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= 10%. Target transformation retained most of the standards as likely species except for strengite, variscite, and amorphous iron phosphate that had unacceptable SPOIL values of >8 and probabilities of F values of <0.05 (data not shown). When using three orthogonal components, all standards came out as likely species. These results indicated that the first PCA step lacked sensitivity and that target transformation could not discriminate well the most likely species among our set of standards. Because target transformation is an oblique rotation, the selected targets may be correlated (Beauchemin et al. 2002), and the fact that most standards were potential targets suggested correlation among our standards. For this reason and because of lack of sensitivity observed with PCA results, LCF was used to achieve the best characterization of our soil samples using all standards. The least-squares fitting procedure was not restricted to two standards only (based on the number of orthogonal significant components identified in the first step via PCA), and a maximum of three standards was allowed in the fitting.
The XANES spectra and least-squares fits for each soil sample are illustrated in Fig. 2
. Table 3 reports the relative normalized proportions of each phosphate species in the soil as determined by fitting each soil spectrum as a linear combination of standard spectra. The goodness of fit indicated by 
2 was typically <0.2 (Table 3). The sum of fractions before normalization can also indicate, to some extent, the goodness of fit, as the individual component should ideally sum to 1 within the experimental error. Good fits can still be obtained with a sum as low as 0.6 to 0.7, as it is the case for AI2B (Table 3), but then the origin of the deviation should ideally be investigated (Manceau et al., 2000). For the data in Table 3, the greatest deviation (
30%) from the ideal sum of 1 was obtained for samples sb2.1 and AI2B.
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The K-XANES fitting results indicated that phosphate adsorbed on Fe- or Al-oxide minerals was present in all soil samples, but in greater proportion for the three acidic soil samples (>44%) than for the slightly alkaline PV2-A and AI2-B soils (<25%; Table 3). The greatest proportion of adsorbed P was found in sample sb2.1-A (88% of total P). This can be seen by an intense white line peak near 1 eV in the spectrum of this sample and a weak pre-edge feature near -3 eV that would reflect the presence of PO4 adsorbed on Fe-oxides (Fig. 2A). The XANES data suggested that the acidic Ma2 soil sample contained 22% of P as poorly crystalline iron phosphate (Table 3). Overall, the proportions of all P species associated with Fe or Al (adsorbed PO4 on either Fe- or Al-oxides + Fe-phosphate) determined from the XANES spectral fitting were significantly correlated with proportions of NaOH-extractable Pi (r = 0.99, p = 0.001, n = 5; Fig. 3B) , although proportions determined by XANES fitting tended to be greater than those obtained by chemical fractionation. This overestimation could be due to the fact that we restricted the XANES fitting to three standards, or to a lack of specificity in the chemical fractionation.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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X-ray absorption near-edge structure spectroscopy complemented chemical fractionation results by more directly identifying probable P species within the NaOH (chemisorbed P) or HCl-P (apatite-like) pools. The correlation between chemical fractionation results and XANES results with respect to the sum of P species associated with Fe- and Al- or Ca-phosphates indicates a consistency between those two sets of results. However, chemical fractions are macroscopic and operationally defined fractions that cannot be verified as being specific to particular chemical species. Because XANES analysis is a direct, nondestructive physical method, the chemical species determined by this technique are expected to be more chemically similar to the standards used in the fitting. For example, XANES fitting indicated the presence of hydroxyapatite in all soils, while octacalcium phosphate would occur in the two slightly alkaline PV2-A and AI2-B soils but not in the acidic sb2.1 and Ma2-A samples. These results are in line with results from solubility diagrams for a range of representative surface soils in Québec (Laverdière and Karam, 1984). The latter study reported that the soil solution compositions were consistent with hydroxyapatite formation in most soils, whereas formation of brushite (CaHPO4·2H2O) or octacalcium phosphate would only be favored in soils with pH > 6 and high P concentrations (>90 mg M3P kg-1). The XANES data further provided spectroscopic evidence for the occurrence of a significant proportion of PO4 as adsorbed species on Fe- or Al-oxide surfaces for all soil samples, including the AI2 soil developed on calcareous material. This result is also in agreement with previous observations regarding the apparent contribution of Fe- and Al-oxide mineral surfaces in the P sorption capacity of neutral and calcareous soils from Québec (Tran and Giroux, 1987; Beauchemin and Simard, 1999). Although the proportion of adsorbed PO4 species on Fe- or Al-oxides was relatively low for our calcareous AI2-B sample, Fe-oxides in calcareous soils were suggested to have high-energy phosphate adsorbing surfaces compared with calcium carbonate (Holford and Mattingly, 1975). Therefore, the contribution of Fe-oxides to P sorption capacity in calcareous soils can be significant.
Direct identification of P species is useful for predicting the probability of increased P concentrations in solution under given conditions. For example, the soil sb2.1 illustrates well the dilemma of meeting both agronomic P needs and environmental standards to protect water quality in some cases. For this soil, which contains the highest level of total P, the adoption of a new critical threshold of soil P saturation degree (M3P/M3Al x 100 = 15%; Centre de Référence en Agriculture et Agroalimentaire du Québec, 2003) to restrict P inputs to P exports is not likely to prevent additional accumulation of P. Given the low soil M3P content and its M3P/M3Al saturation degree of <2%, 225 kg P2O5 ha-1 are recommended to obtain optimal potato yields for a mean P export of 30 kg P2O5 ha-1 (Centre de Référence en Agriculture et Agroalimentaire du Québec, 2003). Even though its high Alox + Feox content (Table 1) should reduce the risk of P desorption into surface runoff waters, eroded particles may still reach surface waters. Once the eroded particles enter a water body, PO4 associated with Fe-oxide minerals may be solubilized under more reducing conditions (Pierzynski et al., 1994). Given its large proportion of phosphate sorbed to Fe- or Al-oxide minerals (Table 3) (with a detectable level associated to Fe-oxides as indicated by the pre-edge feature in Fig. 2A), the sb2.1 soil may be more vulnerable to reductive dissolution of Fe and associated P than the PV2-A soil, for example, which has less PO4 adsorbed on Fe- or Al-oxide surfaces and a dominant fraction of Ca-phosphates.
Limits of X-Ray Absorption Near-Edge Structure Speciation for Phosphorus
For sulfur and metal XANES data, PCA combined with target transformation was powerful in demonstrating how closely selected standards fitted the experimental spectra (Wasserman, 1997; Ressler et al., 2000; Alcacio et al., 2001; Beauchemin et al., 2002) and complemented well the LCF analysis. In the current study, the PCA approach showed a lack of sensitivity for the P K-XANES data. Although it rejected strengite, variscite, and noncrystalline Fe-phosphate using two orthogonal components ( = 10%), these standards were retained with the use of three orthogonal components. In spite of these mixed results, some consistency was found with LCF as neither strengite nor variscite were included in best-fit results from LCF analysis. The K-XANES data for phosphorus are characterized by one main white-line peak with subtle features around that single peak, which reduces the power of target transformation to discriminate among the available standards. For example, most Ca-phosphate standards in our dataset came out as equally good targets (SPOIL values ranging from 0.7–1.7). Consequently, P K-XANES speciation was mainly achieved through a least-squares fitting procedure. The XANES spectra represent the weighted average of all forms of phosphorus in the soil samples, and results of fitting analysis indicate the dominant forms present. Various minor components would not be distinguishable, mainly due to limitations on the number of variables that can be included in the linear combination fitting without "overfitting."
Chemical fractionation results indicated that some soil samples contained up to 26% of total P as organic P (NaHCO3–Po + NaOH-Po; Table 2). Neither of the two organic P standards included in the fitting of XANES spectra came out as a significant component to explain the variation in our spectra. This result may be partly explained by the absence of strong and unique spectral features in the spectrum of IHP (Fig. 1C), which is considered the most important fraction of organic P in soil (Harrison, 1987). Also, it is likely that the IHP concentration in soils was a limiting factor for XANES analysis. The greatest Po pool as determined by sequential fractionation was found in sample sb2.1, 556 mg kg-1 (Table 2). If we consider that inositol phosphate may account for up to 20% of Po (Tisdale et al., 1984; Harrison, 1987), the highest expected amount of inositol P in the sb2.1 sample would be around 110 mg kg-1 (<5% of total P), which may not be detectable by XANES analysis. Similarly, ATP represents an even lower fraction of Po in soils than IHP (nucleic acids 
2% of Po; Tisdale et al., 1984). The ATP was probably present at concentrations below detection, despite its unique XANES spectral features. The detection limit of the technique was not tested using carefully controlled standard mixtures. We expect to be able to detect a species if it represents 10 to 15% of total P and has a spectrum that is unique from other standards. For this reason, other complementary techniques such as NMR spectroscopy (in iron depleted samples) might prove better suited for direct soil organic P speciation than XANES.
X-ray absorption near-edge structure data speciation based on fitting techniques is inherently restricted by (i) the data quality and (ii) how well the chosen set of standards actually represents real species in the samples of unknown composition (Beauchemin et al., 2002). Phosphorus K-XANES data collected on the soil samples were noisy due to the relatively low P concentration in soils (26–67 mmol kg-1) compared with spectra acquired on the standards (500–800 mmol kg-1). Therefore, soil data quality might be increased by averaging a large number of scans, which is not always feasible due to beamtime constraints. In the current study, four to eight scans were taken for each soil sample. Alternatively, XANES analysis could be preferentially performed on the clay fraction only, where P is typically more concentrated (Leinweber et al., 1997), and tends to accumulate in long-term-fertilized soils (Beauchemin and Simard, 2000). This alternative approach, however, requires a pretreatment of particle-size fractionation of the soil sample, with possible P loss and changes in chemical forms. Another possible method to improve sensitivity that would minimally alter the sample composition would be to analyze the silt and clay fraction obtained by sieving to <50 µm the dry samples (Lookman et al., 1996). In this study, we wanted to assess the feasibility of using XANES spectroscopy on whole soil samples for a normal range of P concentrations observed in agricultural fields. In addition to the limit related to low P concentration in soils, P K-XANES data collected with a Si(III) monochromator typically have a short baseline, which makes the data processing (baseline correction and normalization) more difficult. Since the present study was completed, a Ge(III) monochromator has been commissioned at the Beamline X-19A and significant improvements were achieved in the stability of the baseline for P K-XANES spectra with a consequent increase in the overall consistency of normalized data obtained.
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CONCLUSIONS |
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ACKNOWLEDGMENTS |
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NOTES |
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of the selected soil samples.
