soil chemistry phosphate solubilization from poorly ...sciences, llc, personal communication, 2015),...

Soil Science Society of America Journal

Soil Sci. Soc. Am. J. 81:20–28 doi:10.2136/sssaj2016.08.0247 Received 1 Aug. 2016. Accepted 5 Nov. 2016. *Corresponding author ([email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA. All Rights reserved.

Phosphate Solubilization from Poorly Crystalline Iron and Aluminum Hydroxides by AVAIL Copolymer

Soil Chemistry

less than 40% of fertilizer phosphate applied to soils is generally taken up by crops because of strong retention of P by soil solids. our objective was to determine mechanisms by which AVAIl, a maleic-itaconic copolymer used as a fertilizer additive, potentially affects retention of applied phosphate, and consequently plant availability. We measured competitive sorption of AVAIl and orthophosphate in aqueous suspensions of ferrihydrite and poorly crys-talline Al hydroxide [pxl-Al(oh)3] at ph 6.2, and characterized phosphate bonding distribution between Fe(III) and Al(III) in 1:1 (w/w) mixtures of these solids using P K-edge X-ray absorption near edge structure (XANES) spectros-copy. With increasing co-additions of AVAIl and P at the levels evaluated, sorption results showed dissolved P increasing up to 0.45 and 1.25 mM for ferrihydrite and pxl-Al(oh)3, respectively, which represented 18 and 34% of added P. Negative relationships between sorbed P and sorbed AVAIl implied a competitive adsorption mechanism between these two ligands, and solubi-lization of Fe by AVAIl indicated complexometric dissolution of ferrihydrite. The XANES results showed that 72 to 86% of sorbed P was bonded with Al(III) in the ferrihydrite/pxl-Al(oh)3 mixtures, with only a minor (<15%) effect of AVAIl apparent when P was applied at the two levels tested in this study. our results suggest that optimized AVAIl application rates for enhanc-ing crop availability of P would depend on soil sorption characteristics and the soil content of residual P relative to its soil sorption capacity.

Abbreviations CEC, cation-exchange capacity; LCF, linear combination fitting; pxl-Al(OH)3, poorly crystalline Al hydroxide; XANES, X-ray absorption near edge structure.

Each year, 4 million metric tons of phosphate (PO4) fertilizers are used in US agriculture (USDA-ERS, 2011), making the United States the fourth largest consumer of PO4 fertilizers worldwide (FAO, 2013). Despite these

large fertilizer additions, <40% of applied PO4 is taken up by most crops in one growing season (Syers et al., 2008). This inefficiency in soil PO4 fertilizer use by crops is largely due to strong retention of PO4 in soil, mainly via sorption by iron and aluminum (hydr)oxides or by precipitation into PO4 minerals.

Additives that enhance soil PO4 availability to crops could diminish inputs of PO4 fertilizers, thereby improving the economics of crop production, con-serve PO4 resources, and diminish undesirable buildup of PO4 in soils. AVAIL (Verdesian Life Sciences) is one P-enhancing additive that consists of a polycar-boxylic copolymer made from maleic and itaconic acids (Verdesian Life Sciences, 2015). The copolymer is mixed directly with liquid or granular PO4 fertilizers. However, field trials have shown inconsistent effects of AVAIL on crop yields. For example, increased PO4 efficiency and yields have been reported in several stud-ies (Dunn and Stevens, 2008; Guertal and Howe, 2013; Hopkins, 2013; Gordon et al., 2014), while others showed either mixed or no positive crop responses (Dudenhoeffer et al., 2012; Chien et al., 2014). Although AVAIL is generally ap-

Sarah Doydora* Dean hesterberg

Dep. of Crop and Soil Sciences North Carolina State University 101 Derieux Pl. Raleigh, NC 27695

Wantana KlysubunSynchrotron Light Research Institute P.O. Box 93 Nakhon Ratchasima Thailand 30000

Core Ideas

•Dissolved P increased with increasing co-additions of AVAIl and P to metal oxides.

•AVAIl dissolved greater P with Al-hydroxide than with ferrihydrite.

•AVAIl had no effect on P bonding distribution between Al(III) and Fe(III) in mixed sorbents.

Published February 28, 2017

www.soils.org/publications/sssaj 21

Soil Chemistry

plied at a constant rate relative to P fertilizer inputs, we hypoth-esize that the effectiveness could vary across soils with different P sorption capacities.

To optimize the effectiveness of AVAIL (and other simi-lar polymer systems) as a fertilizer additive for improving PO4 availability to crops, it is important to understand the mode(s) of action in soils. Presently, no study has evaluated the specific reaction mechanisms of AVAIL for maintaining co-added PO4 in dissolved form. Because of the carboxylic functionalities of AVAIL, several possible mechanisms could be hypothesized for soils, including: competitive adsorption of the polymer with PO4 on mineral surfaces when added simultaneously, ligand ex-change of the polymer with previously adsorbed PO4, competi-tive binding with PO4 to Fe(III) or Al(III) in soil organo-metal complexes, and complexometric or ligand-enhanced dissolution of PO4 sorbents or minerals. Simple polycarboxylic acids such as citric acid are known to enhance phosphate solubilization from soils, presumably by competitive sorption between carboxylate groups and PO4 on oxide surfaces, or complexometric dissolu-tion of surface metal ions (Guppy et al., 2005; Geelhoed et al., 1998; Johnson and Loeppert, 2006; Shi et al., 2010). However, the natural abundance of these simple organic acids tends to be low in soil solutions because of decomposition by microorgan-isms (Guppy et al., 2005; Fischer et al., 2010). Understanding the dominant PO4 solubilization mechanisms, that is, the modes of action, could improve management practices for optimizing the effectiveness and predictability of AVAIL and other such polymer systems when applied with PO4 fertilizer to a given soil and crop.

The specific objectives of this research were to (i) measure solubilization of PO4 in equilibrium with synthesized, poorly crystalline (pxl) Fe and Al hydroxides as a function of PO4 and co-added AVAIL concentrations; (ii) determine potential mechanisms of PO4 solubilization from these hydroxides; and (iii) evaluate the distribution of sorbed PO4 between Fe and Al hydroxides in physical mixtures of these solids for different concentrations of P and co-added AVAIL. Poorly crystalline Fe and Al (hydr)oxides are considered dominant sorbents for PO4 in soils. Similar to simple polycarboxylic acids, we hypothesized that competitive sorption between the AVAIL copolymer and PO4 on pxl-Fe- and Al-hydroxide surfaces would block PO4 up-take and maintain greater dissolved PO4 concentrations. We also hypothesized that quantifying the bonding distribution of PO4 between Fe and Al in binary mixtures of Fe and Al hydroxides would indicate whether PO4 or the polymer preferentially bind with one of these solids. To test these hypotheses, we measured dissolved PO4 in batch sorption experiments as a function of si-multaneously added PO4 and AVAIL concentrations in aqueous suspensions of synthesized ferrihydrite or pxl-Al(OH)3. We also performed P K-edge XANES spectroscopic analysis in mixed ferrihydrite/pxl-Al(OH)3 systems to determine preferential bonding of PO4 between these solids in the presence of different levels of co-added AVAIL.

MATErIAlS AND METhoDSSynthesis of Iron and Aluminum (hydr)oxides

Ferrihydrite and pxl-Al(OH)3 were synthesized in the labo-ratory following the procedures of Schwertmann and Cornell (1991) and Prodromou and Pavlatou (1995). For ferrihydrite preparation, 40 g of Fe(NO3)3 × 9H2O were dissolved in 500 mL of CO2–free water, and approximately 330 mL of 1 M NaOH were added while vigorous stirring to bring the suspension pH between 7 and 8. The precipitated ferrihydrite was then washed (shaken for 30 min and centrifuged) three times with 1 M NaCl and another three times with 0.01 M NaCl. The Na-saturated ferrihydrite suspension in a 0.01 M NaCl background solution was adjusted to pH 6.2 using 0.01 M NaOH or 0.01 M HCl, then stored at ~4°C for <3 wk until used. For pxl-Al(OH)3 preparation, 0.25 M Al2(SO4)3 solution was titrated with 1.5 M KOH. The pxl-Al(OH)3 was washed three times with 1 M KCl and four times with CO2–free water. The Al(OH)3 suspension in 0.001 M KCl was adjusted to pH 6.2 using 0.001 M KOH or 0.001 M HCl, and it was kept in the refrigerator until used. Potassium hydroxide was used instead of NaOH in synthesiz-ing the pxl-Al(OH)3 because reaction between NaOH with Al2(SO4)3 produces a mixture of gibbsite, bayerite and pseudo-boehmite (Prodromou and Pavlatou, 1995).

The solids concentrations of both clay suspensions were determined by oven-drying subsamples of the suspensions over-night at 110°C, and correcting dry weights for the background salts. Suspensions were shaken for at least 1 h prior to use in ex-periments. X-ray diffraction patterns of dialyzed subsamples of the final clay samples were characteristic of two-line ferrihydrite and a pxl-Al(OH)3 showing several broad peaks, with minor diffraction peaks at 4.71, 4.36, 3.20, 2.20, and 1.72 nm in the batch used for sorption isotherms indicating contamination with bayerite (data not shown).

Polymer Source and PropertiesAn aqueous formulation of AVAIL was received from

Verdesian Life Sciences, LLC on 9 Sept. 2015. The material was reported to have a specific gravity of 1.287, and contain 40.62% w/w of the AVAIL polymer and 0.63% w/w monomer residues. The estimated cation exchange capacity (CEC) at pH 6.2 was equivalent to 1000 cmolc kg-1 of the polymer (Verdesian Life Sciences, LLC, personal communication, 2015), which we at-tributed to carboxyl charge for our experiment.

Batch Sorption ExperimentBatch sorption experiments were performed at a target pH

of 6.2 using the synthesized ferrihydrite or pxl-Al(OH)3 as sor-bents and an added background electrolyte of 20 mM NaCl. Residual K+ in the pxl-Al(OH)3 suspensions was <2 mM. Well-mixed subsamples of each stock clay suspension were weighed into 50-mL polycarbonate centrifuge tubes to achieve a final solids concentration of 1.5 g kg-1 in a 30.0-g suspension (Khare et al., 2004). Each sample was first diluted in 20 mM NaCl and vigorously stirred on a magnetic stirrer while slowly adding (vol-

22 Soil Science Society of America Journal

umetrically) treatment solutions comprising various proportions of PO4 and AVAIL. For isotherms, varying amounts of PO4 or AVAIL were added alone. For competitive sorption experiments, treatment solutions comprising PO4 and AVAIL mixtures in varying proportions were prepared before reacting with the clay suspensions. Assuming that competitive adsorption between PO4 and AVAIL was a dominant mode of interaction, AVAIL treatments were based on the maximum PO4 sorption capacity of each sorbent and assumed that 1 mol of polymer carboxyl-group charge could compete with 1 mol of PO4 anions—mainly H2PO4

-—for sorption sites. Phosphate treatments in the mix-tures corresponded with 27 (P30), 53 (P50), and 80% (P80) of maxima observed PO4 sorption capacities [2200 mmol kg-1 ferrihydrite and 3100 mmol kg-1 pxl-Al(OH)3 discussed be-low], with AVAIL co-added at 0 to ~140% of maximum PO4 sorption capacities on a carboxylic charge basis. Following pH adjustment to ~6 using 20 mM HCl, samples were shaken for 60 h at ambient temperature (23°C). pH was (re-)adjusted to pH 6.2 periodically during the equilibration period (Khare et al., 2004). Equilibrated samples were centrifuged at 27,000 × g for 15 min and the pH of supernatant solutions were measured before vacuum filtering through 0.2-mm polycarbonate mem-brane filters (Isopore Membrane Filters, Merck Millipore, Ltd., Ireland). Filtrates were analyzed for dissolved P, C (from poly-mer), Fe, and Al.

XANES ExperimentA scaled-up batch sorption experiment similar to that de-

scribed above was performed to determine the effect of AVAIL on proportions of PO4 bound with Fe(III) vs. Al(III) in ferrihy-drite/pxl-Al(OH)3 mixtures. The clay solids concentration was 1.5 g clay kg-1 in 240 g suspensions prepared in 250-mL poly-carbonate centrifuge bottles. Ferrihydrite and pxl-Al(OH)3 were mixed at a 1:1 ratio by weight and reacted with various mixtures of PO4 and AVAIL (as described above) for PO4 treatments of 450 and 1350 mmol kg-1 mixed solids and AVAIL treatments between 0 and 1800 mmolc kg-1 solids based on the CEC (car-boxyl charge) reported for our batch of AVAIL. Samples with the same treatments of PO4 and AVAIL added to suspensions of individual clays were prepared as fitting standards in the XANES analysis. We did not measure the PO4 or AVAIL sorp-tion capacities of the mixed solids. Our PO4 and AVAIL inputs were calculated to be <100% of the maximum sorption capacity of our mixtures (2650 mmol kg-1) estimated as a linear combi-nation of sorption capacities for our individual clays. All other experimental conditions were the same as those described above. After equilibration for 60 h at pH 6.2, the suspensions were cen-trifuged at 1500 × g for 20 min., and the supernatant solutions were collected for final pH measurement. The sedimented solids were stored wet at 4°C until prepared for XANES spectroscopy within 1 wk.

To prepare moist pastes of samples for XANES analysis, 1 to 3 mL of filtered supernatant solution were added to the respective sediment in each centrifuge bottle and thoroughly

mixed with a spatula to create a thick slurry. The slurries were partially dewatered under vacuum on polycarbonate filter mem-branes (0.2 mm) to form a waxy paste. The pastes collected on the filters were dried for 2 d in a glovebox under N2 gas, and col-lected into 2-mL sample vials. For XANES data collection 3 wk later, each powder sample was crushed and homogenized using an agate mortar and pestle.

Phosphorus K-edge XANES data were collected at beamline BL8 (Klysubun et al., 2012) at the Synchrotron Light Research Institute (SLRI) in Nakhon Ratchasima, Thailand. The elec-tron beam energy was 1.2 GeV, with a stored beam current of 100 to 150 mA. The beamline was equipped with an InSb(111) double crystal monochromator, which was energy calibrated every 12 h (after storage ring injection) to the absorption edge energy, 2145.5 eV, of elemental P powder measured in transmis-sion mode. The XANES spectra for samples were collected in fluorescence yield mode at energies between 2110 and 2446 eV, with a minimum step size of 0.2 eV between 2145 and 2211 eV. All XANES scans were normalized using the Athena software (Ravel and Newville, 2005) as follows: -40 to -10 eV relative to E0 (maximum in first derivative spectra) for linear baseline sub-traction and 30 to 45 eV relative energy for normalization using a quadratic fit. Any additional quadrature was removed using the “flatten” feature in Athena. Linear combination fitting (Kelly et al., 2008) was used to model the pre-edge region (between −7 and -1 eV or -7 and -2 eV relative energy—Khare et al., 2007) of normalized XANES spectra for clay mixtures, using as fitting standards the spectra from single-sorbent ferrihydrite and pxl-Al(OH)3 suspensions with equivalent concentrations of added PO4 and/or AVAIL.

rESulTS AND DISCuSSIoNSorption Isotherms

Sorption isotherms for PO4 or AVAIL on ferrihydrite and pxl-Al(OH)3 were characterized as L-curves (Fig. 1). Sorption isotherms of PO4 were fit with Freundlich models (Essington, 2004), and the observed maxima sorption capacities used as refer-ence for our PO4 and AVAIL treatments were 2200 mmol P kg-1 ferrihydrite and 3100 mmol P kg-1 Al(OH)3 (Fig. 1a). These maxima are within 15% of those reported by Khare et al. (2005) for ferrihydrite and non-crystalline Al-hydroxide (1860 and 3400 mmol kg-1, respectively). Sorption isotherms for AVAIL were fit using the Langmuir model (Essington, 2004), with b pa-rameters corresponding to sorption maxima of 2960 mmolc kg-1 ferrihydrite and 3413 mmolc kg-1 pxl-Al(OH)3 (Fig. 1b). While the Langmuir isotherm model describes saturation of sorption sites with AVAIL on the surfaces of both solids, the Freundlich model indicates that phosphate sorption will continue to in-crease with further additions of aqueous phosphate. However, neither of these empirical, macroscale models provide molecular sorption mechanisms of the polymer or PO4.

Spectroscopic studies showed that phosphate sorption to Fe and Al oxides occurs largely through bidentate and/or mono-dentate inner-sphere surface complexation, which could either


coexist with or later evolve to surface precipitation (Arai and Sparks, 2007; Li et al., 2013; Yan et al., 2014). Surface precipi-tation occurs when the inner-sphere surface complexes interact with metals released from the dissolution of the adsorbents, and form a separate three-dimensional solid phase on the surface (Yan et al., 2014; Arai and Sparks, 2007). Surface precipitation of metal phosphates is enhanced at greater PO4 loading rates, lower pH, higher temperature, and longer reaction times (Yan et al., 2014; Kim and Kirkpatrick, 2004; Lookman et al., 1994). Although we cannot rule out the possibility of metal-phosphate surface precipitates on the (hydr)oxides used here, their occur-rence may be less dominant than inner-sphere surface complexes of phosphate. For example, Khare et al. (2007) proposed that inner-sphere surface complexes of phosphate were indicated by the stronger white line intensity and weaker pre-edge feature in XANES spectra for PO4 adsorbed on ferrihydrite compared with phosphate in either pxl-Fe phosphate or strengite. We used similar materials, PO4 loadings, and experimental conditions as that of Khare et al. (2007), and our XANES spectra for Fe-bonded PO4 systems were similar. Distinguishing surface com-plexes on pxl-Al hydroxide is more difficult.

In the case of organic ligands, carboxylic compounds have also been shown to bind to surfaces of metal (hydr)oxides through bidentate or monodentate inner-sphere surface complexes and outer-sphere surface complexes, as well as hydrogen bonding be-tween the carboxylic groups of the compounds and the hydroxyl groups on oxide surfaces (Yeasmin et al., 2014; Vindedahl et al., 2016). Given that PO4 and carboxyl groups share common sorp-tion mechanisms, competition between PO4 and AVAIL for the same surface sites would be expected when simultaneously added to Fe- or Al-(hydr)oxide suspensions. Consistent with this hypothesis, sorption isotherms for PO4 and AVAIL carboxyl charge showed similar trends for pxl-Al(OH)3 over comparable regions of dissolved concentration, and sorption maxima were within 10% for the two sorbates (Fig. 1). In contrast, ferrihy-drite sorbed 35% more AVAIL than PO4 on a charge basis. If

each sorption site on the minerals could accommodate either 1 mol of H2PO4

- anion or 1 mol of car-boxylic acid functional group from AVAIL, then the greater sorption capacity of ferrihydrite for AVAIL implies that a maximum of 75% of polymer carboxyl groups are bind-ing on sorption sites. Considering the smaller size of PO4 ions rela-tive to that of AVAIL (average re-ported molecular weight of 3850 D-Verdesian Life Sciences, LLC, personal communication, 2015), it is likely that a smaller fraction of the AVAIL charge is effectively bound on the ferrihydrite compared with the Al(OH)3 surface, and the re-

maining unbound charge is dangling from the surfaces. This difference implies that a greater proportion of AVAIL carboxyl groups are complexed with binding sites on pxl-Al(OH)3.

Sorption of Co-Added Phosphate and AVAIlFor ferrihydrite and pxl-Al(OH)3 suspensions at pH 6.2,

dissolved PO4 increased with increasing inputs of co-added AVAIL or PO4 (Fig. 2). For both systems, trends in dissolved PO4 concentration with increasing AVAIL additions varied with co-added PO4 concentration, as indicated by our data being fit with quadratic upward, linear, or quadratic downward models for added PO4 concentrations of 30, 50, and 80% of the maxi-mum P sorption capacity (Fig. 2a). For a given input of AVAIL expressed as a percentage of PO4 sorption capacity, dissolved PO4 concentrations were generally ~2.5-fold greater for the pxl-Al(OH)3 system. Maximum dissolved concentrations of PO4 for the greatest inputs of AVAIL and PO4 were 1250 ± 0.3 mmol L-1 for pxl-Al(OH)3 and 466 ± 3 mmol L-1 for ferrihydrite (Fig. 2a), which represent 34 and 18% of added PO4, respectively (Fig. 2b). In essence, these trends indicate that AVAIL is more effective in inhibiting sorption of PO4 by pxl-Al(OH)3 than by ferrihydrite, and its effectiveness increases with increasing PO4 inputs for both sorbents. Considering that the maximum sorp-tion of AVAIL (in mmolc kg-1) was comparable to that of PO4 for pxl-Al(OH)3 but greater for ferrihydrite (Fig. 1a vs. 1b), our collective results suggest that a greater proportion of polymer carboxyl groups compete with aqueous H2PO4

- anions for binding sites in pxl-Al(OH)3. That is, the greater sorption capac-ity of ferrihydrite for AVAIL than for PO4 implies a lower bind-ing capacity of carboxylic groups (up to only 75%), and hence lower competitive advantage of AVAIL sorption over PO4 on the ferrihydrite compared with pxl-Al(OH)3.

Evidence for a mechanism of competitive sorption between PO4 and AVAIL is apparent from plotting sorbed concentra-tions of each of these anionic components in Fig. 2 against each other (Fig. 3). Sorbed PO4 decreased with increasing concen-

Fig. 1. Sorption isotherms for a) phosphate and b) AVAIl on ferrihydrite (in red) and on poorly crystalline Al-hydroxide (in blue). The phosphate sorption isotherms are fitted to Freundlich models whereas the AVAIl isotherms on a charge basis are fitted to langmuir models.


tration of sorbed AVAIL, and linear regression slopes in Fig. 3 indicate that the effect became more pronounced with increas-ing PO4 input. For example, between PO4 inputs of 50 and 80% of maximum sorption capacity, the regression slope decreased from -0.06 to -0.19 and -0.16 to -0.29 mol sorbed PO4·molc

-1 sorbed AVAIL for ferrihydrite and pxl-Al(OH)3, respectively. However, for our lowest PO4 inputs (Fig. 3), regression slopes near zeros suggested minimal competi-tion between AVAIL and PO4 when surface binding sites are in excess of the co-added sorp-tives. These data for greater PO4 inputs are consistent with either preferential or more rapid binding of AVAIL blocking PO4 sorption, or displacement of any sorbed PO4 by AVAIL. At higher PO4 inputs, a greater fraction of PO4 may be bound less strongly compared with that at lower surface loadings, allow-ing the polymer to compete more effectively and displace more weakly bound PO4 ( Johnson and Loeppert, 2006). Ligand ex-change between inner-sphere surface complexes has been com-monly assumed for competitive sorption between PO4 and com-peting ligands (Violante, 2013). However, recent studies have shown that competition for weaker surface sites rather than for inner-sphere surface sites control competitive sorption between PO4 and organic ligands (Lindegren and Persson, 2009, 2010; Weng et al., 2008; Violante, 2013). For example, Lindegren and Persson (2009, 2010) showed that destabilization of PO4 sur-face complexes occur when high charge density organic ligands outcompete PO4 complexes for H-donor or H-acceptor sites on goethite surfaces. Similarly, Weng et al. (2008) found that elec-trostatic interaction plays a greater role over inner-sphere surface complexation on competitive sorption between PO4 and humic substances. In the case of PO4 and AVAIL, we hypothesize that competition of the two ligands for H-bonding and/or electro-static interactions might have contributed along with ligand exchange to increased PO4 solubilization, but such molecular mechanisms cannot be confirmed from our experiments.

Complexometric (or ligand-enhanced) dissolution is another surface mechanism that could affect sorption of PO4 in the pres-ence of AVAIL. Figure 4 shows that dissolved Fe increased with increasing AVAIL inputs for all inputs of PO4 in ferrihydrite sus-pensions. In contrast, there was no increase in dissolved Al in the pxl-Al(OH)3 suspensions. Presumably the dissolved Fe, which accounts for up to 5% of Fe in the ferrihydrite suspensions, was complexed with carboxylic acid groups on the AVAIL polymer. This evidence for greater complexometric dissolution of Fe rela-

tive to Al suggests that higher-affinity, inner-sphere complexation of carboxyl groups at ferrihydrite surfaces weakens the Fe-O bond at the oxide surface (Kleber et al., 2015). Johnson and Loeppert (2006) observed greater dissolved Fe/PO4 molar ratios when low molecular weight organic acids were present at lower pH, consis-tent with ligand-enhanced dissolution as mechanism for increased solubilization of PO4 presorbed on iron oxides. We found that the dissolved molar ratio of Fe/PO4 decreased as dissolved PO4 increased with increasing inputs of P, with a maximum ratio

Fig. 2. Dissolved phosphate as affected by level of co-added AVAIl and phosphate expressed in a) concentrations or b) fractions of initially added phosphate, for suspensions of ferrihydrite (left axis in a) or poorly crystalline Al hydroxide [pxl-Al(oh)3] (right axis in a). Phosphate inputs were 30, 50, or 80% (P30, P50, P80) of the maximum sorption capacity for each solid. Mean equilibrium ph values (± standard deviations) are indicated on the legend for each sorbent.

Fig. 3. Sorbed phosphate vs. AVAIl on ferrihydrite or poorly crystalline Al hydroxide [pxl-Al(oh)3] for co-added AVAIl and phosphate at concentrations corresponding to 30, 50, or 80% (P30, P50, P80) of the maximum phosphate sorption capacity of each sorbent. The slopes (b) of the linear relationships are denoted, and coefficients of determination (r2) were as follows: P30 = 0.58, P50 = 0.88, and P80 = 0.98 for ferrihydrite, and P30 = 0.54, P50 = 0.91, and P80 = 0.97 for poorly crystalline Al hydroxide.


near 12 occurring for our lowest PO4 inputs (Fig. 4d). This trend implies that dissolved PO4 increased more than dissolved Fe with increasing AVAIL input (Fig. 4a–d and 2a), indicating that com-plexometric dissolution is secondary to competitive sorption as a mechanism of AVAIL enhancing dissolved PO4 in ferrihydrite systems, particularly for greater PO4 inputs.

XANES AnalysisExamples of P K-edge XANES spectra are shown on Fig. 5

for PO4 sorbed on ferrihydrite and pxl-Al(OH)3, alone or in 1:1 (w/w) physical mixtures, and for varying levels of added AVAIL. The XANES spectra showed a strong white-line peak near 2155 eV

for all samples, along with a pre-edge peak between 2146 and 2152 eV for ferrihydrite suspensions that is characteristic of PO4 bonded with Fe(III) (Hesterberg et al., 1999; Khare et al., 2004). No such pre-edge peak is evident for PO4 bonded with Al(III) in samples of pxl-Al(OH)3 alone. The combination of a strong white line and weak pre-edge peak in the ferrihydrite system (Fig. 5a) is consistent with PO4 adsorbed as inner-sphere surface complexes rather than precipitated as Fe-phosphates (Khare et al., 2005; 2007).

Figure 5b shows that the pre-edge peak for PO4 bonded in Fe/Al-hydroxide mixtures, with or without AVAIL, was more similar to PO4–Al bonding than PO4–Fe bonding. Linear-combination fitting (LCF) analysis of the pre-edge region of Fe/

Fig. 4. Dissolved Fe or Al concentrations in ferrihydrite or poorly crystalline Al hydroxide [pxl-Al(oh)3] suspensions in relation to level of co-added AVAIl at phosphate inputs of a) 30% (P30), b) 50% (P50), and at c) 80% (P80) of maximum P sorption capacities of each sorbent. Exponential (for P30 and P50) and quadratic curves (for P80) are meant to show trends. d) Molar ratios of dissolved Fe-to-P in relation to dissolved P concentration at each level of added P.


Al-hydroxide mixtures with corresponding spectra from indi-vidual sorbents indicates the proportions of PO4 bonded with Fe(III) vs. Al(III) in the mixtures (Khare et al., 2004, 2007). In our ferrihydrite/pxl-Al(OH)3 mixtures, LCF results indicated that 72 to 86% of the sorbed PO4 was bonded with Al(III) vs. 14 to 28% bonded with Fe(III) (Fig. 6). Additions of AVAIL had <15% effect (within uncertainty of LCF results) on the PO4 bonding distribution between Fe (III) or Al(III). Based on the greater PO4 sorption capacity of pxl-Al(OH)3 over ferrihydrite (Fig. 1), we expected that only 58% of the PO4 would be bonded with Al in a 1:1 (w/w) mixture if there was no bonding prefer-ence for either sorbent (Khare et al., 2004, 2005). However, pre-vious work with poorly crystalline Fe/Al-hydroxide mixtures and coprecipitates indicated that PO4 can bind with Al(III) bonded on surfaces of Fe-hydroxide domains (Liu and Hesterberg, 2011), which explains greater Al(III)-bonded PO4 in such mix-tures than is predicted if individual components in the mixture behaved independently. In any case, we found no evidence that the AVAIL co-polymer affected the interactions between PO4, pxl-Al(OH)3, and ferrihydrite in the mixtures.

Implications for AVAIl Effectiveness in SoilsThe effectiveness of AVAIL in boosting crop yields in the field

has been questioned because of inconsistent crop responses. In a review reported by Chien et al. (2014), crop responses to AVAIL ranged from approximately 15% decrease to roughly 20% increase in yield relative to crops that did not receive AVAIL-treated PO4 fertilizers. Our findings suggest that AVAIL could potentially en-hance solubilization of co-added PO4 from fertilizers mainly by competitive sorption on pxl-Fe and Al hydroxides, which are con-sidered to be major PO4 sorbents in soils. Given that the effective-ness of AVAIL in inhibiting PO4 sorption increases with increas-

ing PO4 loading on Fe and Al hydroxides (Fig. 2), we hypothesize that the polymer would be more effective when PO4 is concentrat-ed in a fertilizer band (i.e., PO4 granules applied or concentrated to a smaller volume of contacted soil). Moreover, the competitive sorption mechanism implied by our research suggests that AVAIL effectiveness would also depend on soil sorption capacity (e.g., the contents of poorly crystalline Fe and Al oxides as major PO4 sor-bents). Soils with greater sorption capacity are likely to limit the

Fig. 5. a) Stacked P K-edge XANES spectra and b) the XANES pre-edge region (not stacked) for different levels of AVAIl co-added with phosphate at 450 mmol P kg-1 of sorbent in either 1:1 (w/w) mixtures or single-sorbent suspensions of ferrihydrite and poorly crystalline Al-hydroxide [pxl-Al(oh)3]. Figure 5b shares the same legend as Fig. 5a.

Fig. 6. Proportions (fractions) of P bonded with Al(III) [vs. Fe(III)] in mixtures of ferrihydrite and poorly crystalline Al-hydroxide for varying concentrations of AVAIl co-added with 450 or 1350 mmol P kg-1 sorbent. Error bars indicate uncertainties in fitted proportions calculated by the Athena program.


mobility and plant availability of added PO4, which could be one factor contributing to the reported inconsistencies of AVAIL ef-fectiveness. For example, no response to AVAIL was indicated on acid to near neutral soils (Karamanos and Puurveen, 2011, Cahill et al., 2013, Heiniger, 2008), where PO4 availability could be con-trolled largely by sorption to Fe and Al oxides (Hesterberg, 2010).

Additionally, given the presence of other ions in the soil, it is also likely that differences in natural abundance of other li-gands led to different degrees of competition with AVAIL for the same surfaces, potentially contributing to the inconsistencies of AVAIL effectiveness between soils. The effectiveness of AVAIL may also decrease if certain cations complex with the dissolved polymer and diminish the ability of the polymer to sorb and block P sorption sites in soils. However, we did not investigate these multi-component effects, and their influence on AVAIL effectiveness is still unclear. Nevertheless, to effectively mini-mize PO4 sorption on these Fe and Al oxides, the rate of applied AVAIL relative to PO4 may need to be increased and tailored to specific soil properties. It is, however, important to note that the concentrations of AVAIL used in our study, which aimed to as-sess mechanisms of interactions of AVAIL with PO4 on isolated Fe and Al hydroxides that compose only a fraction of a soil, are orders of magnitude greater than the current recommended ap-plication rate of AVAIL in the field.

Degryse et al. (2013) found increased dissolved P concen-trations close to AVAIL-coated fertilizer granules in a laboratory study with samples from an acid Oxisol, but they observed no such increase in samples from a calcareous Inceptisol. Because the levels of AVAIL applied in the soil near granules correspond-ed to 100 to 1000 times the current AVAIL manufacturer appli-cation rate (based on fertilizer granule coatings), Degryse et al. (2013) concluded that similarly high application rates are likely not economically viable for enhancing P solubility from fertil-izers. We attempted to predict the dissolved P concentrations (as % of added P) reported by Degryse et al. (2013) by assum-ing that their oxalate-extractable soil Fe and Al corresponded to Fe and Al-hydroxides with identical sorption characteristics as those synthesized in our study. For AVAIL inputs ranging from 0 to 2.5 g g-1 fertilizer (Degryse et al., 2013), our predictions overestimated dissolved P for the Oxisol sample by 3- to 12-fold for inputs up to 0.83 g g-1, and underestimated dissolved P by twofold for the greatest input of 2.5 g g-1. Our predictions were less promising for the Inceptisol, for which we overestimated dis-solved P by twofold for the zero-AVAIL treatment of Degryse et al. (2013), but underestimated dissolved P by orders of magni-tude for all other treatments. Our predictions for the Oxisol, at least, were reasonable given the following assumptions made in connecting results from our single-sorbent model systems to the more complex soil systems: (i) only Fe and Al oxides implied by oxalate extractions were relevant for phosphate sorption in the soil samples of Degryse et al. (2013); (ii) our freshly synthesized Fe and Al (hydr)oxides had the same sorption capacities and af-finities as the soil oxides; (iii) there were no effects of compet-ing natural organic matter or other ions in the soils, and (iv) our

P solubilization results could be extrapolated to the high P and AVAIL loading rates near the fertilizer granules in the study of Degryse et al. (2013). Moreover, our calculations also did not account for any interaction effects between the pxl Fe and Al ox-ides when both are present in a mixed system, as has been shown by Liu and Hesterberg (2011) and suggested in our discussion of the P K-edge XANES spectra above. Furthermore, the high water/solids ratios used in our experiments with aqueous suspen-sions could enhance the competitive sorption of AVAIL for PO4 binding sites, thereby overestimating the effectiveness at the low water/solid ratios of the moist soils used by Degryse et al. (2013). In summary, our prediction of the soils data suggests that a more consistent experimental approach may be needed to bridge the knowledge gap between pure model system studies and soils.

CoNCluSIoNSOur macroscopic results showed that dissolved PO4 concen-

trations in aqueous suspensions of ferrihydrite and pxl-Al(OH)3 at pH 6.2 increased with increasing additions of PO4 and co-added AVAIL, with greater effects shown for pxl-Al(OH)3. The (hydr)oxides evaluated are models of high-capacity phosphate sorbents in soils. The primary mechanism for enhanced PO4 solubiliza-tion by AVAIL, as deduced from our data, is competitive adsorp-tion between the co-polymer carboxyl groups and H2PO4

- for either ferrihydrite or pxl-Al(OH)3. Complexometric dissolution of surface Fe(III) could contribute to enhanced PO4 solubiliza-tion in ferrihydrite suspensions. The mechanism of competitive adsorption on high P-sorbing solids implies that the effective rates of AVAIL co-applied with phosphate fertilizers would depend on soil contents of poorly crystalline Fe and Al (hydr)oxide minerals (and other sorbents), which are often indicated by oxalate-extract-able soil Fe and Al. A greater enhancement of dissolved PO4 by AVAIL in our model systems at higher PO4 inputs suggests that a fertilizer-AVAIL co-application that concentrates both materi-als in a smaller soil volume (e.g., a banded fertilizer application) would enhance plant-availability of P. However, given that soils contain mixtures of Fe and Al oxides along with other ions and organic materials, extrapolating our results from model sorbents to predicting AVAIL effectiveness in soils would be improved by evaluating other chemical and physical factors that influence the interaction of AVAIL and PO4.

ACKNoWlEDGMENTSWe appreciate technical assistance from support staff at Beamline BL8 at the Synchrotron Light Research Institute (SLRI) in Nakhon Ratchasima, Thailand, where part of this research was conducted. We also thank Dr. Owen Duckworth for equipment use, and Dr. Megan Andrews and Samuel O’Connor for laboratory assistance at North Carolina State University (NCSU). This work was performed in part at the Analytical Instrumentation Facility (AIF) at NCSU, which is supported by the State of North Carolina and the National Science Foundation (Award No. ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). Financial and materials support for this research were provided by Verdesian Life Sciences, LLC. Results reported here do not imply endorsement of any commercial products used in this study.


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