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Speciation of Phosphorus in Phosphorus-Enriched Agricultural Soils Using X-RayAbsorption Near-Edge Structure Spectroscopy and Chemical Fractionation

Suzanne Beauchemin,* Dean Hesterberg, Jeff Chou, Mario Beauchemin, Regis R. Simard, and Dale E. Sayers

ABSTRACT give a relative measure of the ease of removing phospho-rus from soil solids. In contrast, deterministic ap-Knowledge of phosphorus (P) species in P-rich soils is useful forproaches for predicting the potential loss of phosphorusassessingP mobility andpotential transferto ground water andsurfacefrom agricultural soils with different properties and fer-waters. Soil P was studied using synchrotron X-ray absorption near-tilization histories could be based on knowledge ofedge structure (XANES) spectroscopy (a nondestructive chemical-

speciation technique) and sequential chemical fractionation. The chemical species (forms) of soil phosphorus and a moreobjective was to determine the chemical speciation of P in long- fundamental chemical understanding of P release fromterm-fertilized, P-rich soils differing in pH, clay, and organic matter these specific species.contents. Samples of three slightly acidic (pH 5.56.2) and two slightly The increase in soil P solubility that is often correlatedalkaline (pH 7.47.6) soils were collected from A or B horizons in to an increase in total soil P concentration may be ex-two distinct agrosystems in the province of Quebec, Canada. The soils

plained in part by changes in solid-phase speciation orcontained between 800 and 2100 mg total P kg1. Distinct XANES

by the affinity of orthophosphate (PO4) for sorbing soilfeatures for Ca-phosphate mineral standards and for standards ofcomponents. Adsorption appears to be the dominantadsorbed phosphate made it possible to differentiate these forms ofretention mechanism that regulates dissolved phosphateP in the soil samples. The XANES results indicated that phosphateat low concentrations, whereas phosphate mineral pre-adsorbed on Fe- or Al-oxide minerals was present in all soils, with a

higher proportion in acidic than in slightly alkaline samples. Calcium cipitation controls P solubility at high concentrationsphosphate also occurred in all soils, regardless of pH. In agreement (Lindsay et al., 1989). Consequently, distinguishing be-with chemical fractionation results, XANES data showed that Ca- tween adsorbed phases and precipitates is critical tophosphates were the dominant P forms in one acidic (pH 5.5) and in ascertain the long-term behavior of P in soils. Typically,the two slightly alkaline (pH 7.47.6) soil samples. X-ray absorption adsorption isotherms from laboratory experiments arenear-edge structure spectroscopy directly identified certain forms of

characterized by an L-curve (Sposito, 1984), which cansoil P, while chemical fractionation provided indirect supporting data

be fitted with a Langmuir or Freundlich isotherm model.and gave insights on additional forms of P such as organic pools thatThis type of isotherm predicts that as the PO4 loadingwere not accounted for by the XANES analyses.rate approaches the maximum adsorption capacity ofthe soil, additional phosphate cannot be retained bythe soil. Unlike surface-adsorbed chemical species, the

Buildup of phosphorusin excessively fertilized soils solubility of a solid-phase precipitate is essentially inde-is of environmental concern as P transfer from soilspendent of the amount of the solid phase present (Lind-to surface and subsurface waters is increased (Sharpleysay, 1979). Thus, precipitation of minerals such as Ca-,et al., 1993; Pote et al., 1996; Heckrath et al., 1995;Al-, or Fe-phosphates at higher soil P concentrations

Eghball et al., 1996). This situation has led many coun- may represent a sink for P that has a constant solubilitytries to define critical threshold levels of soil test P tounder given chemical conditions. However, the type oflimit the potential negative effects of P on water qualityphosphate mineral formed and soil conditions such as(Sharpley et al., 1996; Centre de Reference en Agricul-pH and presence of dissolved complexing species willture et Agroalimentaire du Quebec, 2003; Breeuwsmadetermine the phosphate activity in solution (Lindsay,et al., 1995). At the same time, new soil tests that are1979).designed to more directly predict the potential for P

In the past, thermodynamic models of mineral solubil-loss to waters have been proposed (e.g., ion exchangeity predicted that dissolved PO4would be controlled atmembranes, iron oxidecoated filter paper, easily de-equilibrium by Fe- and Al-phosphates in acidic soils andsorbable P; Sims et al., 2000). Such invasive methodsby Ca-phosphates in neutral and alkaline soils (Lindsay,1979). However, kinetics of P transformations were not

S. Beauchemin, Natural Resources Canada, CANMET, 555 Booth considered (Bohn et al., 1985), even though kinetic limi-Street, Office 332A, Ottawa, ON, Canada K1A 0G1. D. Hesterberg, tations often exert considerable influence on P specia-Department of Soil Science, North Carolina State University, Box tion in natural environments. Also, adsorbed P phases7619, 3235 Williams Hall, Raleigh, NC 27695-7619. J. Chou, National were poorly understood and difficult to include in suchInstitute of Environmental Health Sciences,P.O. Box 12233, Research

models. For example, the concentrations of ammoniumTriangle Park, NC 27709. M. Beauchemin, Canada Centre for RemoteSensing, 588 Booth Street, 4th floor, Ottawa, ON, Canada K1A 0Y7. oxalateextractable Al and Fe in soils have often beenR.R. Simard (deceased), SoilScience Department,University of Man-itoba, 362 Ellis Building, Winnipeg, MB, Canada R3T 2N2. D.E.

Abbreviations:Aloxand Feox, ammonium oxalateextractable alumi-Sayers, Department of Physics, North Carolina State University, Boxnum or iron; HCl-P, phosphorus extracted with 1MHCl;IHP, inositol8202, Raleigh, NC 27695-8202. Contribution no. 02-091(J). Receivedhexametaphosphate; LCF, linear combination fitting; M3P, Mehlich6 Nov. 2002. *Corresponding author ([email protected]).IIIextractable phosphorus; NaOH-P, phosphorus extracted with0.1MNaOH; PCA, principal component analysis; Pi, inorganic phos-Published in J. Environ. Qual. 32:18091819 (2003).

ASA, CSSA, SSSA phorus; Po, organic phosphorus; Pt, total soil phosphorus; XANES,X-ray absorption near-edge structure.677 S. Segoe Rd., Madison, WI 53711 USA

1809

Published September, 2003


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1810 J. ENVIRON. QUAL., VOL. 32, SEPTEMBEROCTOBER 2003

found to be the best variable to predict P sorption capac- MATERIALS AND METHODSities of acidic (Laverdiere and Karam, 1984; van der Soil Sample Selection and PreparationZee and van Riemsdijk, 1986; Freese et al., 1992; Simard

X-ray absorption near-edge structure spectroscopy andet al., 1994) and neutral to calcareous soils (Ryan et al.,chemical fractionation analyses were performed on five com-1984; Tran and Giroux, 1987). This correlation suggestsposite soil samples collected in the province of Quebec, Can-

that oxide mineral surfaces are significant P-sorbing ada. All soils are naturally poorly drained and were classifiedcomponents at all pH levels. Likewise, results of energy- as Humaquepts (Table 1). One sample (designated sb2.1) withdispersive X-ray analyses of excessively fertilized soils high total phosphorus content (Pt 2076 mg kg1; Table 1)

was collected from the Ap horizon of an acidic silt loam Leshowed that P-rich particles contained P predominantlyBras soil that had been intensively cropped with potato (Sola-associated with Al in amorphous solid phases, even fornum tuberosumL.). The P fertilization was mainly from inor-neutral to slightly alkaline soil samples (Pierzynski etganic sources. Four other soil samples were collected from A

al., 1990). Such observations illustrate the need for director B horizons within two distinct agroecosystems. For each

identification of soil P species, regardless of soil proper- type of horizon, we selected two samples of comparable Pt,ties, when trying to understand and quantitatively model but with contrasting properties such as pH, clay, and organic

matter contents, and source of P inputs (Table 1). Sampleslong-term changes in P solubility in P-enriched soils.designated Ma2 and Ma3 were from the loamy Mawcook soilIn the present study, X-ray absorption near-edgeseries in the Beaurivage River watershed, and are representa-structure (XANES) spectroscopy was used in conjunc-tive of acidic soils that were historically (25 yr) amendedtion with sequential chemical fractionation to character-with animal manure. The samples were taken from hay fields

ize the dominant solid-phase species of P in selected of farms having no surplus (Ma2) or a known surplus (Ma3)soils. Total-electron-yield XANES studies at the P K-edge of manure (Simard et al., 1995). The clayey Providence (PV2)

of several commercial phosphate powders have shown and loamy St-Aime(AI2) soils were sampled in the St. Law-rence lowlands (Beauchemin and Simard, 2000). Soils fromthat each compound had a unique spectrum that re-this area are mostly tile-drained and intensively cropped withflected the specific molecular environment of P (Frankecorn (Zea maysL.) and soybean [Glycine max(L.) Merr.], andand Hormes, 1995; Okude et al., 1999). Rose et al. (1997)the source of P is mainly inorganic. The PV2 soil developed ondetermined the local structure of P during hydrolysisa noncalcareous parent material and the AI2 soil developed

of FeCl3 in the presence of phosphate using P K-edge on calcareous parent material. The precise fertilization historyextended X-ray absorption fine structure (EXAFS) is not known for the soils sampled.spectroscopy. For soils, XANES spectroscopy has been Soil sampling strategy was discussed in Simard et al. (1995)

for the Beaurivage soils (Ma2 and Ma3) and in Beaucheminmainly applied to sulfur and metal speciation (Fendorfet al. (1998) for the lowland samples (PV2 and AI2). For alland Sparks, 1996), but Hesterberg et al. (1999) havesamples, five 7-cm-diameter cores were taken and mixed. Soilshown the feasibility of using this approach for moresamples were air-dried and subsequently ground to2 mm

direct identification of some soil P species. X-ray ab-before chemical analysis.

sorption near-edge structure spectroscopy has the main

advantages of being element specific and nondestructive Soil Characterization(no sample pretreatment required). It further providesParticle-size analysis was performed by the hydrometerinformation on the local molecular bonding environ-

method except for the use of the pipette method for PV soilment of the element (Fendorf and Sparks, 1996). Unlike very rich in clay (Sheldrick and Wang, 1993). Organic C con-X-ray diffraction, poorly ordered mineral phases can tent was determined by wet oxidation (Tiessen and Moir,also be characterized by XANES spectroscopy (Schulze 1993). Soil pH was measured in distilled water with a soil to

solution ratio of 1:2. Mehlich IIIextractable P and Ca (M3P,and Bertsch, 1995). The objective of this study was toM3Ca) contents were obtained as described by Tran and Si-determine chemical speciation of P in long-term-fertil-mard (1993). Ammonium oxalateextractable Fe and Al (Feox,ized, P-enriched soil samples using synchrotron XANESAlox) and dithionite citrateextractable Fe (Fedc) contentsspectroscopy and sequential chemical fractionation. Forwere determined on the soil samples according to Ross and

this purpose, soil samples were selected to represent a Wang (1993). A modified Hedley et al. (1982) chemical extrac-range of properties such as pH, texture, organic matter tion procedure, as described by Simard et al. (1995), was used

to fractionate soil phosphorus. Briefly, after grinding to100content, and P source.

Table 1. Basic properties of the selected soil samples.

Soil Soil Source pHSample horizon classification of P Sand Clay OM (water) M3Ca M3P Pt Alox Feox Feox/Fedc

g kg1 mg kg1 mmol kg1

sb2.1 Ap TH mineral 280 192 82 5.8 1116 38 2076 673 116 0.81Ma2 Ap AH manure 800 50 66 6.2 1374 103 1189 282 117 0.81PV2 Ap AH mineral 77 750 40 7.4 3112 63 1223 76 122 0.72Ma3 Bg AH manure 620 30 28 5.5 316 10 814 182 92 0.61AI2 Bg TH mineral 438 254 3 7.6 1519 4 884 29 58 0.23

OM, organic matter content; M3Ca and M3P, Mehlich IIIextractable calcium and phosphorus; Pt, soil total phosphorus determined after digestion withconcentrated H2SO4 H2O2 as in the last step of the fractionation; Al oxand Feox, ammonium oxalateextractable aluminum and iron; Fe dc, dithionite citrateextractable iron.

TH, Typic Humaquepts; AH, Aeric Humaquepts.


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BEAUCHEMIN ET AL.: SPECIATION OF P IN P-ENRICHED AGRICULTURAL SOILS 1811

mesh, the soil samples were sequentially extracted for 16 h energy of the maximum of the first peak in the first derivativespectrum for a variscite standard. According to X-ray photo-(each treatment) using an anionic exchange resin (Dowex

1X8-50, HCO3 form; Dow, Indianapolis, IN), 0.5MNaHCO3 electron spectral data and other total-electron-yield XANESstudies, the binding energy of the P K-shell electron is, in fact,(pH 8.5), 0.1 MNaOH, 1 MHCl, and concentrated H2SO4

H2O2. In all extracts, inorganic phosphorus (Pi) was measured at a higher energy than the E0defined this way (Franke andHormes, 1995; Li et al., 1994; Okude et al., 1999).by the molybdenum blue method (Murphy and Riley, 1962).

The NaHCO3 and NaOH extracts were also digested with The XANES data were collected directly on air-dried soilsamples ground to pass through a 125-m sieve. Dried pow-H2SO4H2O2 to determine total phosphorus (Pt); organic phos-

phorus (Po) was then calculated as Pt Pi. The extractions ders of all mineral and organic P standards were diluted to800 mmol P kg1 in boron nitride (BN). All mineral powderswere designed to target the following forms of P (Hedley et

al., 1982): (i) resin P labile inorganic phosphorus directly and soil samples were pressed into a 1.3-cm-diameter sampleplexiglass holder well of 1 mm thickness. Standards of ad-exchangeable and soil solution phosphorus, (ii) NaHCO3P

labile inorganic and organic phosphorus sorbed to soil mineral sorbed PO4 containing 500 mmol P kg1 were prepared as

moist pastes, and mounted in the 1.3-cm-diameter well behindsurfaces plus some microbial phosphorus, (iii) NaOH-Pinorganic phosphorus chemisorbed to aluminum- and iron- a 3-m-thick film of Mylar X-ray film (Spex Industries, Met-

uchen, NJ) for data collection. Mylar is known to have detect-oxide minerals and organic phosphorus from humic com-pounds, (iv) HCl-P relatively insoluble apatite-type miner- able phosphorus XANES peak due to contamination, but this

peak was trivial compared with the fluorescence yield of ourals, and (v) H2SO4P residual insoluble inorganic phospho-rus and the most stable organic phosphorus forms. adsorbed PO4 standards at 15-fold higher concentration.

The XANES spectra were analyzed using principal compo-nent analysis (PCA) and nonlinear, least-squares fittinglinearPhosphorus Standards for X-Ray Absorptioncombination fitting (LCF). Both approaches were described

Near-Edge Structure Spectroscopy in detail in Beauchemin et al. (2002). Principal component

analysis wasfirst performedto definethe numberof significantThe following phosphate standards for XANES spectros- orthogonal components in our dataset composed of the nor-copy were either purchased from a chemical supply companymalized, interpolated spectra (background and baseline cor-or synthesized according to the references cited (see Hester-rected) of the five soils. Target transformation was then usedberg et al., 1999 for some details): noncrystalline Fe-phosphateto test which standards would be the most likely species inand strengite (FePO42H2O) treated hydrothermally for 3 orour samples based on two criteria: the SPOIL value and the30 d to vary crystallinity (Dalas, 1991); PO4 adsorbed on poorlyFtest. According to Malinowski (1991), tested standards withcrystalline Fe hydroxide (2-line ferrihydrite; Schwertmann andSPOIL values of3 are acceptable whereas SPOIL values ofCornell, 1991, p. 9094) or Al hydroxide; PO4 adsorbed on6 are considered unacceptable. SPOIL values between 3goethite (-FeOOH) or alumina (-Al2O3) (Oh et al., 1999);and 6 represent marginal standards. In the one-tailed F testnoncrystalline Al-phosphate and variscite (AlPO42H2O) (Hsuproposed by Malinowski (1991), thetested standard is retainedand Sikora, 1993); berlinite (AlPO4) (purchased); octacalciumas valid when the probability of the calculatedFis greater thanphosphate [Ca4H(PO4)32.5H2O] (Christoffersen et al., 1989);a given critical threshold value such as 0.05 (5% probability).and monetite (CaHPO4), brushite (CaHPO42H2O), hydroxy-

Linear combination fitting of soil XANES spectra was alsoapatite [Ca5(PO4)3OH], adenosine triphosphate (ATP), andperformed on the current dataset using all possible binary andinositol hexametaphosphate (IHP) (all purchased). Resultsternary combinations of the 14 available standards according

from X-ray diffraction analysis showed that the various stan- to the Vairavamurthy et al. (1994) procedure (for n 2 ordards were mineralogically pure, except that thestrengitestan-3, possible combinations 91 or 364, respectively). Lineardards contained detectable levels of phosphosiderite (mono-combination fitting included energy offset parameters. Thisclinic FePO42H2O).fitting approach assumes that the standards chosen are repre-sentative of soil phosphorus species present in the soil samples.

X-Ray Absorption Near-Edge Structure Standards were not corrected for self-absorption, but self-Spectroscopy Analysis absorption would decrease the fluorescence signal at the white

line peak by less than 8% at a 800 mmol kg1 concentrationThe XANES data collection for standards and soil samplesfor mineral standards (Hesterberg et al., 1999). Linear combi-was done at the National Synchrotron Light Source at Brook-nation fitting was done using in-house programs running onhaven National Laboratory (Upton, New York) using theScilab 2.6 (Scilab Group, 2002). Normalized XANES spectraBeamline X-19A equipped with a Si(III) monochromator.were fit over the relative energy range of10 to 15 eV. LinearWith a Si(III) monochromator and collimating mirror, thecombination fitting computes the best-fit weighting factorsresolution at the P K-edge is 0.2 eV. The electron beam energyfor the selected standards using the LevenbergMarquardtwas 2.5 GeV, and the maximum beam current was 300 mA.method (Nielsen, 1999). The weighting factors correspond toThe XANES data were collected in fluorescence mode atthe proportion of each standard yielding the best fit to theambient temperature using a solid-state passivated implantedXANES spectrum for a given soil sample. Chi-squared valuesplanar silicon (PIPS) detector and a He flight path. Thewere adopted as a goodness-of-fit criterion. In addition, fitsXANES data were taken between 2129 and 2299 eV, with awere considered unacceptable when the energy offset parame-minimum step size of 0.2 eV from 2139 to 2174 eV. Multipleters were greater than 1 eV or when the weighting factorsscans (at least two for standards and four to eight for soilwere negative.samples) across the P K-edge were averaged. Data were back-

ground- and baseline-corrected, and normalized to the K-edgeaccording to procedures described in Sayers and Bunker

RESULTS(1988). A linear baseline correction was made between 20and 5 eV (relative energy), and a single-point background General Soil Characterizationnormalization was made at a flat part of the spectrum near

Despite their high total P contents (2076 and 1223 mg30 eV (relative energy). The energy scale was normalized toa reference energy (E0) of 2149 eV,which was calibrated as the kg1), sb2.1 and PV2A horizon samples had Mehlich


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Table 2. Sequential P fractionation of the soil samples standard deviations.

NaHCO3 NaOH

Sample Resin P Pi Po Pt Pi Po Pt HCl H2SO4

mg kg1

sb2.1-A 98 7 (5) 127 3 (6) 100 6 (5) 227 9 (11) 743 6 (34) 456 43 (21) 1199 46 (55) 434 18 (20) 200 39 (9)Ma2-A 77 4 (5) 107 6 (8) 76 12 (5) 183 18 (13) 427 28 (30) 194 29 (14) 621 50 (44) 374 32 (27) 147 23 (11)PV2-A 86 1 (7) 34 3 (3) 48 5 (4) 82 6 (7) 84 5 (7) 154 10 (13) 238 13 (20) 476 4 (40) 304 43 (26)

Ma3-B 18 10 (2) 15 1 (2) 47 6 (6) 62 7 (8) 82 3 (10) 133 13 (17) 215 16 (27) 401 30 (50) 102 12 (13)AI2-B 24 4 (3) 5 1 (1) 19 2 (2) 23 2 (3) 10 0 (1) 19 4 (3) 29 4 (4) 577 6 (71) 155 17 (19)

P i, inorganic phosphorus; Po, organic phosphorus; Pt, total phosphorus; resin P, the most available inorganic phosphorus; NaHCO 3P, labile phosphorussorbed on the soil surface; NaOH-P, phosphorus chemisorbed to aluminum or iron; HCl-P, apatite-type minerals; H2SO4P, chemically stable organicphosphorus and relatively insoluble inorganic phosphorus.

The numbers in parentheses are the percentage of each fraction relative to the sum of all fractions.

IIIextractable P (M3P) contents of less than 70 mg dards. As shown in Fig. 1A, spectra for Fe-phosphateminerals and PO4 adsorbed on Fe-oxides had a pre-edgekg1 (Table 1). According to previous Quebec fertilizerfeature between 5 and 2 eV [relative to the P(V)recommendations (Conseil des Productions VegetalesK-edge], which increased in intensity with increasingdu Quebec, 1996), the P fertility levels of sb2.1, Ma2,mineral crystallinity. The standards of PO4adsorbed onand PV2 soils would be considered respectively as low,ferrihydrite or goethite were further characterized byhigh, and adequate. The lower available P content ofan intense white line peak. Their very similar XANESthe sb2.1 sample may be related to its high P sorptionspectra indicate that these two adsorbed species maycapacity as estimated by the Alox Feoxcontent (Table

not be distinguishable from each other when fitting spec-1). For a given horizon, soils from the Beaurivage Rivertra of soils. Calcium-phosphate mineral standards allwatershed have greater P sorption capacities, as indi-exhibited a shoulder on the high-energy side of thecated by greater levels of Alox Feox, than soils fromabsorption edge, between 2 and 6 eV (relative energy;the lowland area (Table 1; Ma2 vs. PV2, or Ma3 vs. AI2).Fig. 1B). The structure of the shoulder varied amongthe different Ca-phosphate minerals. For example, hy-Sequential Phosphorus Fractionationdroxyapatite and octacalcium phosphate had well-de-

Chemical fractionation provided information on op- fined shoulders compared with monetite (Fig. 1B) anderationally defined P pools of varying solubilities brushite (CaHPO42H2O; not shown). The XANES(Table 2). For all samples, the largest P fraction was spectra for Al-phosphate minerals showed a weak pre-found in moderately labile (NaOH-P) or nonlabile frac- edge inflection at about1 eV (relative energy). Thistions (HCl-P and H2SO4P). Fractionation data for the feature was better defined on the first-derivative spectraA horizon samples suggested that P was mainly associ- (data not shown) and was more distinct for crystallineated with Al or Fe oxides in the acidic sb2.1 and Ma2 (variscite, berlinite) than noncrystalline Al-phosphate

soils (up to 55% of Pt as NaOH-P), whereas apatite- (see Fig. 1C for variscite). Similar to what was observedtype minerals represented the major forms of P in the with PO4adsorbed on Fe-oxides, PO4species adsorbedslightly alkaline PV2 sample (40% of Pt as HCl-P). In on Al hydroxide or alumina had an intense white line

peak but no pre-edge feature (Fig. 1C). The XANESboth B horizon samples, HCl-P was the main pool, sug-spectra for ATP showed an inflection at 1 eV, similargesting that Ca-bound P minerals accounted for 71%to that of Al-phosphate minerals (data not shown). Theof P tin the calcareous AI2 sample and for 50% of Ptinspectrum for IHP was mostly featureless and had athe acidic Ma3 sample. Labile P (resin PNaHCO3Pt)broad white line peak (Fig. 1C).accounted for 14 to 18% of total P in A horizon soils,

whereas this pool was less than 10% for the B horizonsamples. On average, 30% of the labile P in A horizons Soil Sampleswas organic (NaHCO3Po). For B horizons, 40% of

Principal component analysis performed on the nor-labile P was organic P. Moderately labile organic Pmalized K-XANES spectra of the five soil samples(NaOH-Po), a fraction considered to be associated withshowed two significant orthogonal components at humic compounds, represented between 13 and 21% of10%. Target transformation retained most of the stan-total P in most soils, except for the calcareous AI2 Bdards as likely species except for strengite, variscite,horizon, which had only 3% of total P as NaOH-Po.and amorphous iron phosphate that had unacceptableSPOIL values of8 and probabilities of F values of0.05 (data not shown). When using three orthogonalX-Ray Absorption Near-Edge Structurecomponents, all standards came out as likely species.Spectroscopy ResultsThese results indicated that the first PCA step lacked

Phosphate Standardssensitivity and that target transformation could not dis-

Details of P K-XANES spectra for a number of phos- criminate well the most likely species among our set ofphate standards were discussed by Hesterberg et al. standards. Because target transformation is an oblique(1999). A subset of representative spectra are presented rotation, the selected targets may be correlated (Beau-

chemin et al. 2002), and the fact that most standardshere to illustrate the range of spectral features for stan-


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Fig. 1. Stacked P K-XANES (X-ray absorption near-edge structure) spectra for selected phosphate standards: (A ) P related to Fe, (B ) Caphosphates, and (C) others. Data are background- and baseline-corrected and normalized to the P K-edge at 2149 eV.

were potential targets suggested correlation among our The goodness of fit indicated by 2 was typically 0.2(Table 3). The sum of fractions before normalizationstandards. For this reason and because of lack of sensi-

tivity observed with PCA results, LCF was used to can also indicate, to some extent, the goodness of fit,as the individual component should ideally sum to 1achieve the best characterization of our soil samples

using all standards. The least-squares fitting procedure within the experimental error. Good fits can still beobtained with a sum as low as 0.6 to 0.7, as it is the casewas not restricted to two standards only (based on the

number of orthogonal significant components identified for AI2B (Table 3), but then the origin of the deviationshould ideally be investigated (Manceau et al., 2000).in the first step via PCA), and a maximum of three

standards was allowed in the fitting. For the data in Table 3, the greatest deviation (30%)from the ideal sum of 1 was obtained for samples sb2.1The XANES spectra and least-squares fits for each

soil sample are illustrated in Fig. 2. Table 3 reports and AI2B.The P K-XANES fitting results for the PV2-A andthe relative normalized proportions of each phosphate

species in the soil as determined by fitting each soil AI2-B samples indicatedthat morethan one best combi-nation could be fitted for these samples (Table 3). Asspectrum as a linear combination of standard spectra.


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Fig. 2. Least-squares fits of the P K-XANES (X-ray absorption near-edge structure) spectra of the five soil samples: (A ) sb2.1-A, (B ) Ma2-A,(C) PV2-A, (D) Ma3-B, and (E) AI2-B (P/ferrihydrite, P/goethite, and P/alumina PO4 adsorbed on ferrihydrite, goethite, or alumina;octaCaPO4 octacalcium phosphate; hydroxyap. hydroxyapatite; NC FePO4 noncrystalline FePO4). For PV2-A and AI2-B, the firstbest fit among the best solutions reported in Table 3 is illustrated.


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was discussed earlier in reference to Fig. 1A, the spec-trum of PO4 adsorbed on ferrihydrite could not be easilydistinguished from that of PO4 adsorbed on goethite.However, standards of PO4 adsorbed on Fe-oxidesshould be distinguishable from species adsorbed on Al-oxides, based on the weak but typical pre-edge featurein Fe-containing species (Fig. 1A vs. 1C). The fitting

results for PV2A and AI2B demonstrated that PO4ad-sorbed on Fe-oxides could not be reliably distinguishedfrom PO4adsorbed on Al-oxides for the soil data. How-ever, adsorbed species could be distinguished fromother P minerals due to the characteristic intense whiteline peak in their spectra. Hence, we regrouped inTable 3 all PO4 adsorbed species under the generalterm adsorbed on Fe- or Al-oxides and averaged theproportions obtained from the best reported combi-nations.

The K-XANES fitting results indicated that phos-phate adsorbed on Fe- or Al-oxide minerals was presentin all soil samples, but in greater proportion for thethree acidic soil samples (44%) than for the slightly

alkaline PV2-A and AI2-B soils (25%; Table 3). Thegreatest proportion of adsorbed P was found in samplesb2.1-A (88% of total P). This can be seen by an intensewhite line peak near 1 eV in the spectrum of this sampleand a weak pre-edge feature near 3 eV that wouldreflect the presence of PO4 adsorbed on Fe-oxides(Fig. 2A). The XANES data suggested that the acidicMa2 soil sample contained 22%of P as poorly crystallineiron phosphate (Table 3). Overall, the proportions ofall P species associated with Fe or Al (adsorbed PO4on either Fe- or Al-oxides Fe-phosphate) determinedfrom the XANES spectral fitting were significantly cor-related with proportions of NaOH-extractable Pi (r0.99, p 0.001, n 5; Fig. 3B), although proportions

determined by XANES fitting tended to be greater thanthose obtained by chemical fractionation. This overesti-mation could be due to the fact that we restricted theXANES fitting to three standards, or to a lack of speci-ficity in the chemical fractionation.

The XANES spectra of all soil samples exhibited ashoulder on the high-energy side of the white-line peak,indicating that some form of calcium phosphate waspresent in all samples (Fig. 2AE). Fitting results sug-gested that hydroxyapatite occurred in all soils but inlower proportions in acidic (18%) than in neutral toslightly alkaline soil samples (25%; Table 3). Thistrend is consistent with the increased solubility of hy-droxyapatite under acidic conditions (Lindsay, 1979).

The calcareous AI2-B sample contained the greatestproportion of hydroxyapatite (59%), as can be seenfrom the pronounced shoulder on the high-energy sideof the white line in its spectrum (Fig. 2E). For the twoslightly alkaline soils (PV2-A and AI2-B) and for theacidic soil sample for which the Ca-phosphate appearedas a major sink for P (Ma3-B; Tables 2 and 3), a signifi-cant proportion of octacalcium phosphate was alsodetected. The correlation between the proportion ofHCl-extractable P (apatite-like P) and the summedproportions of all Ca-phosphate species determined by

Table3.PhosphorusK-XANES(

X-rayabsorptionnear-edgestructure)fit

tingresultsshowingtherelativeproportion(percentage,normalizedtosum

100)ofeachphosphate

standardthatyieldedthebestfittothesoilXANESdatainlinearcomb

inationfitting.

Sumoffractions

Goodness

before

PO4

on

PO4

on

P

O4

onAl

PO4

on

Noncrystalline

Octacalcium

To

talPO

4

on

Fe-

To

talCa

Sample

offit(2)

norm

alization

ferrihydrite

goethite

hydroxide

alumina

FePO4

H

ydroxyapatite

phosphate

or

Al-ox

ide

phosp

ha

te

%

oftotalP

standarderror

%

oftotalP

sb2.1-A

0.15

1.32

54

2

34

3

12

1

88

12

Ma2-A

0.06

0.95

60

1

22

2

18

1

60

18

PV2-A(1)

0.16

0.91

23

1

24

2

53

2

23

77

PV2-A(2)

0.17

0.91

27

1

27

2

46

2

27

73

PV2

-Amean

25

75

Ma3-B

0.12

0.88

44

1

11

1

45

2

44

56

AI2-B(1)

0.05

0.66

17

1

59

3

24

3

17

83

AI2-B(2)

0.05

0.68

15

1

59

2

26

3

15

85

AI2-B(3)

0.05

0.68

18

1

58

2

24

3

18

82

AI2-B(4)

0.05

0.71

16

1

59

2

25

3

16

84

AI2

-Bmean

17

83

TotalPO4

onFe-orAl-oxideisthesumofPO4

adsorbedonferrihydrite,goethite,Alhydroxide,oralumina.TotalCaphosphateisthesumofhydroxyapatiteandoctacalcium

phosphate.

PercentoftotalPafternormalizationtosum

100%

computedstandarderrorsforthelinearcoefficients.

XANES fitting was significant (r 0.87,p 0.05,n 5;


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fertilized with inorganic P (Condron and Goh, 1989).Also, a large fraction of P in liquid manure is presentas relatively soluble Ca-phosphates and Mg-NH4phos-phates (de Haan and van Riemsdijk, 1986; Bril andSalomons, 1990). Lookman et al. (1996) suggested thatthe Ca-phosphate phase observed in excessively fertil-ized, acidic surface soils with a history of large manure

inputs was stable because of saturation of the Al- andFe-phosphate pool (Pox/(AlFe)ox 0.3) andthe conse-quent high P concentration in solution. Similarly, deHaan and van Riemsdijk (1986) reported that, in fieldssaturated with P by long-term applications of liquidmanure, soil solution was more or less in equilibriumwith CaHPO42H2O(s) (brushite). In relation to theMa3-B sample, previous leaching experiments on Maw-cook soil samples showed that long-term manure inputshad considerably lowered the P sorption capacity of Ahorizons and that the risk of P leaching was highest inthe agricultural soil samples associated with high animaldensity (Beauchemin et al., 1996). Therefore, leachingof Ca and P from the A horizon with subsequent precipi-

tation of P in the B horizon might partly explain ourresults for the B horizon of the acidic Ma3 sample.

X-ray absorption near-edge structure spectroscopycomplemented chemical fractionation results by moredirectly identifying probable P species within the NaOH(chemisorbed P) or HCl-P (apatite-like) pools. The cor-relation between chemical fractionation results andXANES results with respect to the sum of P speciesassociated with Fe- and Al- or Ca-phosphates indicates aconsistency between those two sets of results. However,chemical fractions are macroscopic and operationallydefined fractionsthat cannot be verified as being specific

Fig. 3. Relationships between the proportions of different P species to particular chemical species. Because XANES analysisdetermined by X-ray absorption near-edge structure (XANES) is a direct, nondestructive physical method, the chemicalfitting versus those obtained via sequential chemical fractionation

species determined by this technique are expected tofor corresponding pools: (A ) P associated with Ca; (B ) P relatedbe more chemically similar to the standards used into Fe or Al phase.the fitting. For example, XANES fitting indicated the

Fig. 3A), and showed nearly a 1:1 relationship between presence of hydroxyapatite in all soils, while octacalciumthese two measurements. phosphate would occur in the two slightly alkaline

PV2-A and AI2-B soils but not in the acidic sb2.1 andMa2-A samples. These results are in line with resultsDISCUSSIONfrom solubility diagrams for a range of representative

Insight about Phosphorus Speciation Gained surface soils in Quebec (Laverdie` re and Karam, 1984).by X-Ray Absorption Near-Edge The latter study reported that the soil solution composi-

Structure Spectroscopy tions were consistent with hydroxyapatite formation inmost soils, whereas formation of brushite (CaHPO4Both XANES spectroscopy and chemical fraction-2H2O) or octacalcium phosphate would only be favoredation results indicated that Ca-phosphates were presentin soils with pH 6 and high P concentrations (90 mgin all of the soil samples analyzed, even those of acidic

M3P kg1). The XANES data further provided spectro-pH. Of the three acidic soils, both techniques indicatedscopic evidence for the occurrence of a significant pro-that sample Ma3-B (the one of lowest pH) containedportion of PO4as adsorbed species on Fe- or Al-oxidethe greatest proportion of total P as Ca-phosphate. Ma-surfaces for all soil samples, including the AI2soil devel-hapatra and Patrick (1969) have also observed (fromoped on calcareous material. This result is also in agree-chemical fractionation) a dominance of Ca-phosphatement with previous observations regarding the apparentforms in an acidic soil. For agricultural soils from thecontribution of Fe- and Al-oxide mineral surfaces in theBeaurivage watershed, Simard et al. (1995) reportedP sorption capacity of neutral and calcareous soils fromthat Ca-phosphate and amorphous Al and Fe pools wereQuebec (Tran and Giroux, 1987; Beauchemin and Si-important sinks of P, despite their acidic pH. They re-mard, 1999). Although the proportion of adsorbed PO4lated this observation to long-term applications of ma-species on Fe- or Al-oxides was relatively low for ournure and lime as sources of Ca. Lime addition has been

shown to increase Ca-bound P forms in acidic topsoils calcareous AI2-B sample, Fe-oxides in calcareous soils


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were suggested to have high-energy phosphate ad- cedure. The XANES spectra represent the weightedaverage of all forms of phosphorus in the soil samples,sorbing surfaces compared with calcium carbonate

(Holford and Mattingly, 1975). Therefore, the contribu- and results of fitting analysis indicate the dominantforms present. Various minor components would nottion of Fe-oxides to P sorption capacity in calcareous

soils can be significant. be distinguishable, mainly due to limitations on thenumber of variables that can be included in the linearDirect identification of P species is useful for pre-

dicting the probability of increased P concentrations in combination fitting without overfitting.

Chemical fractionation results indicated that somesolution under given conditions. For example, the soilsb2.1 illustrates well the dilemma of meeting both agro- soil samples contained up to 26% of total P as organic

P (NaHCO3Po NaOH-Po; Table 2). Neither of thenomic P needs and environmental standards to protectwater quality in some cases. For this soil, which contains two organic P standards included in the fitting of

XANES spectra came out as a significant componentthe highest level of total P, the adoption of a new criticalthreshold of soil P saturation degree (M3P/M3Al to explain the variation in our spectra. This result may

be partly explained by the absence of strong and unique100 15%; Centre de Reference en Agriculture etAgroalimentaire du Quebec, 2003) to restrict P inputs spectral features in the spectrum of IHP (Fig. 1C), which

is considered the most important fraction of organic Pto P exports is not likely to prevent additional accumula-tion of P. Given the low soil M3P content and its M3P/ in soil (Harrison, 1987). Also, it is likely that the IHP

concentration in soils was a limiting factor for XANESM3Al saturation degree of2%, 225 kg P2O5ha1 are

recommended to obtain optimal potato yields for a analysis. The greatest Popool as determined by sequen-tial fractionation was found in sample sb2.1, 556 mgmean P export of 30 kg P2O5ha

1 (Centre de Referenceen Agriculture et Agroalimentaire du Quebec, 2003). kg1 (Table 2). If we consider that inositol phosphate

may account for up to 20% of Po (Tisdale et al., 1984;Even though its high Alox Feox content (Table 1)Harrison, 1987), the highest expected amount of inositolshould reduce the risk of P desorption into surface run-P in the sb2.1 sample would be around 110 mg kg1off waters, eroded particles may still reach surface wa-(5% of total P), which may not be detectable byters. Once the eroded particles enter a water body, PO4XANES analysis. Similarly, ATP represents an evenassociated with Fe-oxide minerals may be solubilizedlower fraction of Po in soils than IHP (nucleic acids under more reducing conditions (Pierzynski et al., 1994).2% of Po; Tisdale et al., 1984). The ATP was probablyGiven its large proportion of phosphate sorbed to Fe-present at concentrations below detection, despite itsor Al-oxide minerals (Table 3) (with a detectable levelunique XANES spectral features. The detection limit ofassociated to Fe-oxides as indicated by the pre-edgethe technique was not tested using carefully controlledfeature in Fig. 2A), the sb2.1 soil may be more vulnera-standard mixtures. We expect to be able to detect able to reductive dissolution of Fe and associated P thanspecies if it represents 10 to 15% of total P and has athe PV2-A soil, for example, which has less PO4 ad-spectrum that is unique from other standards. For thissorbed on Fe- or Al-oxide surfaces and a dominantreason, other complementary techniques such as NMRfraction of Ca-phosphates.

spectroscopy (in iron depleted samples) might provebetter suited for direct soil organic P speciation thanLimits of X-Ray Absorption Near-EdgeXANES.Structure Speciation for Phosphorus

X-ray absorption near-edge structure data speciationFor sulfur and metal XANES data, PCA combined based on fitting techniques is inherently restricted by

with target transformation was powerful in demon- (i) the data quality and (ii) how well the chosen set ofstrating how closely selected standards fitted the experi- standards actually represents real species in the samplesmental spectra (Wasserman, 1997; Ressler et al., 2000; of unknown composition (Beauchemin et al., 2002).Alcacio et al., 2001; Beauchemin et al., 2002) and com- Phosphorus K-XANES data collected on the soil sam-plemented well the LCF analysis. In the current study, ples were noisy due to the relatively low P concentrationthe PCA approach showed a lack of sensitivity for the in soils (2667 mmol kg1) compared with spectra ac-P K-XANES data. Although it rejected strengite, varis- quired on the standards (500800 mmol kg1). There-cite, and noncrystalline Fe-phosphate using two orthog- fore, soil data quality might be increased by averagingonal components ( 10%), these standards were re- a large number of scans, which is not always feasible

tained with the use of three orthogonal components. In due to beamtime constraints. In the current study, fourto eight scans were taken for each soil sample. Alterna-spite of these mixed results, some consistency was found

with LCF as neither strengitenor variscitewereincluded tively, XANES analysis could be preferentially per-formed on the clay fraction only, where P is typicallyin best-fit results from LCF analysis. The K-XANES

data for phosphorus are characterized by one main more concentrated (Leinweber et al., 1997), and tendsto accumulate in long-term-fertilized soils (Beaucheminwhite-line peak with subtle features around that single

peak, which reduces the power of target transformation and Simard, 2000). This alternative approach, however,requires a pretreatment of particle-size fractionation ofto discriminate among the available standards. For ex-

ample, most Ca-phosphate standards in our dataset the soil sample, with possible P loss and changes inchemical forms. Another possible method to improvecame out as equally good targets (SPOIL values ranging

from 0.71.7). Consequently, P K-XANES speciation sensitivity that would minimally alter the sample compo-sition would be to analyze the silt and clay fractionwas mainly achieved through a least-squares fitting pro-


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Dr. Regis Simard (19562002) could not see the final productobtained by sieving to 50 m the dry samples (Look-of this study; his valuable support is fully appreciated.man et al., 1996). In this study, we wanted to assess the

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Comparison of different models for phosphate sorption as a func-research. We are grateful to Dr. Young-Mi Oh for supplying tionof theiron andaluminium oxides of soils. J. SoilSci. 43:729738.oxide minerals and to Dr. April Leytem for supplying organic Harrison, A.F. 1987. Soil organic phosphorus. A review of worldphosphorus standards. Thanks are extended to Dr. Henri Di- literature. CABI Publ., Wallingford, UK.nel, Dr. Thi Sen Tran, and Dr. Eugene J. Kamprath for re- Heckrath, G., P.C.Brookes, P.R.Poulton,and K.W.T. Goulding. 1995.

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