Interaction of Phosphorus Compounds with Anion-Exchange Membranes: Implications for Soil Analysis

Soil Chemistry Interaction of Phosphorus Compounds with Anion-Exchange Membranes: Implications for Soil Analysis

Anion exchange membranes are commonly used to measure readily exchangeable and microbial P in soil, yet there is Alexander W. Cheesman* little information on their interactions with organic and condensed inorganic P compounds, which can interfere with Soil and Water Science Dep. interpretation of the results. We addressed this by quantifying the sorption of a range of P compounds to a commonly 106 Newell Hall Univ. of Florida used anion exchange membrane (551642S, BDH-Prolabo, VWR International, Lutterworth, UK). Sorption and −2 Gainesville, FL 32611 recovery of orthophosphate by the membranes was complete up to 1.17 g P m . The membranes also completely recovered sodium pyrophosphate, glucose 6-phosphate, and adenosine 5´-monophosphate, as well as significant Benjamin L. Turner levels (20–60%) of 2-aminoethylphosphonic acid, sodium hexametaphosphate, and sodium phytate. Only sodium Smithsonian Tropical Research Inst. pyrophosphate, sodium hexametaphosphate, and d-glucose 6-phosphate were detected subsequently as molybdate- −1 Apartado 0843-03092 reactive P after elution with 0.25 mol L H2SO4, however, indicating their hydrolysis in the acid eluant. Solution Balboa, Ancon, Republic of Panama 31P nuclear magnetic resonance spectroscopy was used to confirm the stability of the tested compounds when exposed to the membrane and the absence of significant concentrations of orthophosphate as trace contaminants K. Ramesh Reddy in the compound preparations. Finally, for a series of tropical wetland soils from the Republic of Panama, we found Soil and Water Science Dep. negligible difference in eluted P concentrations determined by molybdate colorimetry and inductively coupled plasma 106 Newell Hall optical emission spectrometry for both unfumigated and hexanol-fumigated samples. We therefore conclude that Univ. of Florida although organic and condensed inorganic P compounds can be recovered by anion exchange membranes, this is Gainesville, FL 32611 likely to have limited impact on the analysis of soil samples.

Abbreviations: HDPE, high-density polyethylene; ICP-OES, inductively coupled plasma optical emission spectrometry; MDP, methylenediphosphonic acid; MRP, molybdate reactive P; NMR, nuclear magnetic resonance.

nion exchange membranes are commonly used to study P dynamics in soils Aand sediments (Saunders, 1964; Skogley and Dobermann, 1996; Myers et al., 2005). In particular, they are used to measure “readily exchangeable” phosphate in soils (Sibbesen, 1978) and, in conjunction with hexanol fumigation, to determine P contained within the soil microbial biomass (Kouno et al., 1995; Myers et al., 1999). Anion exchange membranes offer potential benefits over conventional soil P tests given their action as passive ion sinks analogous to biological uptake (Qian and Schoenau, 2002). Their use in the field can provide an integrated measure of P availability (Cooperband and Logan, 1994; Drohan et al., 2005; Meason and Idol, 2008) and has led to the development of commercial products designed to determine nutrient supply rates (e.g., PRS probes, Western Ag Innovations, Saskatoon, SK, Canada). Despite numerous assessments of the practical influence of experimental design on both batch and field-deployed membranes (Qian et al., 1992; Qian and Schoenau, 1997, 2002; Sato and Comerford, 2006; Mason et al., 2008), few studies have examined their interaction with organic and condensed inorganic P compounds. Yet there is a clear potential for sorption and recovery of organic P to both resin beads and membranes (Rubaek and Sibbesen, 1993; Cooperband et al., 1999; McDowell et al., 2008). Of particular note is the ~100% recovery of

Soil Sci. Soc. Am. J. 74:1607–1612 Published online 12 Aug.2010 doi:10.2136/sssaj2009.0295 Received 11 Aug. 2009. *Corresponding author ([email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

SSSAJ: Volume 74: Number 5 • September–October 2010 1607 both phytic acid (myo-inositol hexakisphosphate, IP6) and glu- were then rinsed three times in deionized water to remove adhering solu- cose 6-phosphate by 204-U-386 membranes (Ionics, Watertown, tion. Bicarbonate was used as the counterion because it mimics biologi- MA) when exposed at 0.16 g P m−2 and loaded with a Cl− coun- cal uptake from the rhizosphere (Sibbesen, 1978; Qian and Schoenau, terion (Cooperband et al., 1999) 2002) and is considered preferable when studying P dynamics due to its Consideration of potential interactions between anion ex- lower affinity for exchange sites compared with other commonly used change membranes and organic and condensed inorganic P com- counterions (e.g., Cl−) (Skogley and Dobermann, 1996). pounds is important given the number of assumptions made dur- After exposure to a test solution, the membranes were rinsed in ing routine application of the exchange media to study P dynam- deionized water, shaken dry of excess water, and immersed in a conical −1 ics. First, if significant levels of organic and condensed inorganic P tube containing 50 mL of 0.25 mol L H2SO4 and shaken for 3 h, after are recovered by anion exchange membranes and transferred to the which a subsample of the eluant was decanted into a 20-mL scintilla- eluant solution, their inclusion in subsequent measurements will tion vial. The membranes were cleaned using a secondary acidic wash −1 lead to an overestimation of readily exchangeable orthophosphate. (0.25 mol L H2SO4 for 1 h) and rinsed with multiple changes of de- Second, when anion exchange membranes are used as an ion ionized water before regeneration with NaHCO3 as described above. sink during the measurement of microbial P (Kouno et al., 1995; Phosphorus recovery was unaffected by repeated use of the membrane Myers et al., 1999) the analysis of total P in the eluant is typically strips (data not shown) despite some discoloration following use with omitted because the difference between orthophosphate and soil samples. total P is assumed to be negligible (Brookes et al., 1984), as in fumigation–extraction procedures (Brookes et al., 1982; Hedley Phosphorus Determination et al., 1982). If organic or condensed inorganic P compounds re- Molybdate-reactive P, an operationally defined parameter, was leased from lysed microbial cells are adsorbed directly to anion determined in the extracts by automated colorimetry with detec- exchange membranes without conversion to orthophosphate by tion at 880 nm using a flow injection analyzer (Lachat Quickchem enzymatic or matrix induced hydrolysis, however, this may lead 8500, Hach Ltd., Loveland, CO). Total P was determined in the ex- to an underestimation of fumigation-released (microbial) P. tracts by ICP-OES (Optima 2100, PerkinElmer, Waltham, MA). Finally, if organic and condensed inorganic P are recovered Orthophosphate standards were prepared in the same matrix as the −1 by anion exchange membranes, both the nature of the eluant used samples (0.25 mol L H2SO4) for both analyses. for P recovery and the method of P detection may influence the P concentration determined as recovered from a sample. The use of Anion Exchange Membrane Exchange Capacity nonselective elemental analysis, such as inductively coupled plas- The response of the exchange membranes to increased orthophos- ma optical emission spectrometry (ICP-OES), or the hydrolysis phate concentrations was tested to determine the exchange capacity of the of P-containing compounds during elution by mineral acids may membranes and the range across which the dynamic anion exchange mim- erroneously attribute complex forms of P as bioavailable. ics that of an infinite sink. Replicate n( = 3) preloaded and rinsed mem- The aims of this study were to establish (i) the potential in- branes were placed in 250-mL HDPE bottles with 75 mL of orthophos- teractions between a commonly used anion exchange membrane phate standards between 0 and 2.5 mg P and shaken for 24 h. The adsorbed and a range of model organic and condensed inorganic P com- P was eluted and determined by molybdate colorimetry as described above. pounds, and (ii) the implications of any interaction during the routine application of anion exchange membrane procedures to Phosphorus Recovery by Anion wetland soils expected to contain high levels of soluble organic Exchange Membrane Strips and condensed inorganic P. A series of organic and condensed inorganic P compounds were prepared at an approximate concentration of 200 mg P L−1 in deion- MATERIALS AND METHODS ized water, with total P concentrations in the solutions determined Anion Exchange Membranes subsequently by ICP-OES (Table 1). Duplicate preloaded and rinsed The anion exchange membrane characterized in this study anion exchange membrane strips were loaded individually into 250-mL (551642S, BDH-Prolabo, VWR International, Lutterworth, UK) is HDPE centrifuge bottles with 70 mL of deionized water and 5 mL of used routinely in field and laboratory procedures (McLaughlin et al., standard P-containing solutions. After exposure and elution from the 1994; Myers et al., 2005; Roboredo and Coutinho, 2006; Turner and membranes (see above), the eluants were stored in 20-mL HDPE scin- Romero, 2009). The membranes (previously sold under the BDH brand tillation vials at 4°C until analysis by molybdate colorimetry within 72 h name) use a polystyrene–divinylbenzene copolymer base doped with and total P within 1 mo. quaternary ammonium as the ionogenic group and are supplied in 12.5- by 12.5-cm sheets preloaded with Cl− counterions. Purity and Stability of Organic and Condensed The anion exchange membranes were cut into 1.5- by 6.25-cm Inorganic Phosphates in Deionized Water strips, yielding a reactive area of 18.75 cm2 per strip. The strips were Standard solutions were analyzed by solution 31P nuclear mag- − charged with HCO3 counterions by shaking 25 strips in a 250-mL netic resonance (NMR) spectroscopy during 24 h to determine the high-density polyethylene (HDPE) bottle with three sequential chang- stability of the P compounds during exposure to anion exchange mem- −1 −1 es of 200 mL of 0.5 mol L NaHCO3 during a 24-h period. The strips branes. Solutions (~200 mg P L ) were mixed 1:1 with an internal

1608 SSSAJ: Volume 74: Number 5 • September–October 2010 Table 1. Phosphorus compounds tested on anion exchange membranes. Stock solution concentrations determined by inductively coupled plasma optical emission spectrometry analysis. Compound Chemical formula Standard conc. mg P L−1

Potassium dihydrogen phosphate KH2PO4 100.0 Sodium hexametaphosphate (NaPO3)n ∙ Na2O 171.1 Sodium pyrophosphate, decahydrate Na4P2O7 ∙ 10H2O 207.6 d-Glucose 6-phosphate, disodium salt hydrate C6H11O9PNa2 ∙ xH2O 169.0 Adenosine 5´-monophosphate, monohydrate C10H14N5O7P ∙ H2O 157.5 2-Aminoethylphosphonic acid C2H8NPO3 195.6 Phytic acid†, dodecasodium salt C6H6O24P6Na12 183.6 RNA, Type VI from torula yeast (8.5% P) NA‡ 177.5

DNA, from salmon testes (A260 = 17.5) NA 250.7 † myo-Inositol hexakisphosphate, dodecasodium salt (sodium phytate). ‡ NA, not appropriate. standard (methylenediphosphonic acid; MDP) (~50 mg P L−1) made crobial P when adjusted for extraction efficiency, and microbial biomass up in deionized water. Of the resulting mixture, 0.9 mL was added to not killed by fumigation (k factor) (Kouno et al., 1995). Given problems 0.1 mL of deuterium oxide, the solution was vortexed, and then loaded with both correction factors, however, some researchers have preferred into a 5-mm-diameter NMR tube. Solution 31P spectra were acquired simply to use uncorrected fumigation-released values (e.g., Bunemann et after 15 min and 24 h using a Bruker Avance 500 Console, Magnex al., 2008). For this study, uncorrected total and molybdate-reactive P in 11.75 T/54 mm magnet fitted with a 5-mm BBO probe. Acquisitions the eluants of both unfumigated and fumigated samples were compared were run at a stabilized 25°C with a 2.47-µs (~30°) pulse length and a (mg P kg−1 dry weight). 2-s recycle delay. An average 2000 scans (run length, 1 h 21 min) were acquired and the resulting spectra referenced against internal MDP RESULTS set as 17.46 ppm, determined with reference to an externally held 85% Anion Exchange Membrane Capacity

H3PO4 standard (chemical shift 0 ppm). Orthophosphate recovered from a noncompetitive environment was within a 5% error for the majority of the exposure Extraction of Phosphorus Compounds from concentrations tested (Fig. 1). Only at >1.17 g P m−2 was re- Wetland Soils by Anion Exchange Membranes covery unacceptable, defined here as <95%. Taking a conserva- A comparison of total and molybdate-reactive P in the membrane tive working maximum of 1.00 g P m−2 of membrane and the eluants from wetland soil extractions was performed to determine the utilized membrane area/soil ratio of 5.6, this would equate to potential for organic and condensed inorganic P recovery when anion an orthophosphate sorption capacity of 560 mg P kg−1—well exchange membranes are used to measure readily exchangeable and microbial P. A batch process using anion exchange membranes prepared as described above was applied to 27 soils collected from a nutrient gra- dient in the freshwater San San Pond Sak wetland, a domed peatland in Bocas del Toro province, western Panama (Phillips et al., 1997). The soils were all high in organic matter (total C 41–50%) but contained a range of total P concentrations (388–1028 mg P kg−1). Two fresh samples of each soil (3.5 g dry weight equivalent) were weighed into 250-mL HDPE centrifuge bottles and sample-specific volumes of deionized water were added to bring the total water con- tent to 75 mL. Both samples received deionized water and a single anion exchange membrane strip (1.5 by 6.25 cm), with one subsample also receiving 1 mL of hexanol (95%, Sigma Aldrich Chemical Co., St Louis, MO). After 24 h of shaking, the anion exchange membrane strips were removed, rinsed, and eluted and the resulting solution analyzed for total and molybdate-reactive P as described above. A significant difference between total and molybdate-reactive P would indicate the retention of organic or condensed inorganic P compounds that were not hydrolyzed to molybdate-reactive P during elution in Fig. 1. Exchange capacity of anion exchange membrane (AEM) strips. Values are plotted alongside limits of reasonable recovery (95–105%) −1 0.25 mol L H2SO4. of calculated exposure. A typical exposure level (dashed line A) of up In normal application of the method, the difference in molybdate- to 0.25 g m−2 equates to 469 mg P kg−1 of exchangeable phosphate reactive P between the fumigated and unfumigated samples would be when using one strip (6.25 × 1.5 cm) with 1 g of soil. The limit of reasonable recovery rate (dashed line B) equates to 1.17 g P m−2 in a attributed to “fumigation-released P.” This provides an estimate of mi- noncompetitive environment.

SSSAJ: Volume 74: Number 5 • September–October 2010 1609

above the maximum labile P likely to be encoun- tered in the natural environment.

Organic and Condensed Phosphorus Recovery by Anion Exchange Membrane Strips The total P concentrations in the eluants demonstrated that most of the P compounds interacted with the anion exchange membranes (Fig. 2). Three compounds were recovered completely (sodium pyrophosphate, glucose 6-phosphate, and adenosine 5´-monophosphate), while three others showed recoveries between 20 and 60% (sodium hexametaphosphate, 2-aminoethylphosphonic acid, and phytate). The macromol- ecules RNA and DNA showed very limited recovery (~10 and 0.2%, respectively), although it was unclear if this represented true ion exchange at the ionogenic sites or physical adherence of the macromolecule to the polymeric membrane. Under the concentrated conditions of the experiment, a “sheen” was observed on the membrane strips exposed to DNA and RNA. We therefore considered that while DNA recovery was negligible, the limited RNA recovery observed was inconclusive in the context of this experiment. Fig. 2. Recovery of P compounds by anion exchange membrane (AEM) strips. Average Of the compounds showing significant values (n = 2) are shown, with all repeats within 2.3%. Recovered P plotted as molybdate- interaction with the membranes, some were re- reactive P (MRP) or as non-molybdate-reactive P (difference between total P and MRP). covered as molybdate-unreactive P (adenosine 5´-monophosphate, 2-aminoethlphosphonic acid, and phytate), while large proportions of others were recovered as molybdate-reactive P (Fig. 2). For example, of the ~100% of pyrophosphate and glucose 6-phosphate recovered by the anion exchange membrane, 36 and 69%, respectively, were detected as molybdate-reactive P.

Purity and Stability of Phosphorus Compounds in Deionized Water Comparison of the 31P NMR spectra from test compounds with a known orthophosphate standard (Fig. 3) showed neither contamination of the original compounds with orthophosphate nor orthophosphate released by hydrolysis in deionized water during a period of 24 h. The pres- ence of molybdate-reactive P in the eluant solutions was therefore considered to be due entirely to acidic hydrolysis during elution of P from the membranes and subsequent storage before colorimetric analysis. Hydrolysis during colorimetric

Fig. 3. Solution 31P nuclear magnetic resonance spectra of P compounds measured after analysis itself (<1-min contact time) was likely to 24 h in deionized water. Spectra were acquired using a 2.47-μs (~30°) pulse length and be negligible given the relatively long period of a 2-s recycle delay, with approximately 2000 scans required to achieve suitable signal to −1 elution and storage in 0.25 mol L H2SO4. noise. Spectra are presented using 8-Hz line broadening and referenced to an internal methylenediphosphonic acid standard (17.46 ppm).

1610 SSSAJ: Volume 74: Number 5 • September–October 2010 Application to Wetland Soils for Exchangeable ly used anion exchange membrane method. Of these compounds, and Fumigation-released Phosphorus glucose 6-phosphate and the condensed inorganic P compounds For the total and molybdate-reactive P recovered by the an- pyrophosphate and hexametaphosphate were recovered partially ion exchange membranes in a standard extraction from wetland as molybdate-reactive P. Given that orthophosphate contamina- soils, paired t-tests showed significant P( < 0.05) differences be- tion in the original compounds was negligible and that the com- tween molybdate-reactive P and total P determined in both un- pounds were stable in deionized water during a 24-h period, we fumigated and fumigated samples; however, the average differ- conclude that these compounds were hydrolyzed to orthophos- −1 ences between molybdate-reactive and total P were only 3.4 and phate during elution from the membranes in 0.25 mol L H2SO4 1.3% within unfumigated and fumigated samples, respectively. and storage before colorimetric analysis. The potential for acidic Such small relative differences were below the margin of error hydrolysis of organic and condensed inorganic P is a recognized due to calibration. We conclude, therefore, that there was no source of error in the determination of orthophosphate by mo- evidence that acid-stable P forms (i.e., adenosine 5´-monophos- lybdate colorimetry (Dick and Tabatabai, 1977; Worsfold et al., phate, 2-aminoethylphosphonic acid, and phytate) were recov- 2005), although hydrolysis during the acidic molybdate reaction ered in appreciable amounts by the anion exchange membranes. (<1-min contact time) in the flow injection analysis procedure used here was considered to be negligible given the relatively long −1 DISCUSSION period of elution and storage in 0.25 mol L H2SO4. The orthophosphate exchange capacity for the studied anion Similar studies examining the hydrolysis of standard organic exchange membranes agrees well with the published information P during extraction in strong acids reported variable results. For on the performance of similar membranes, including the linear re- example, Ivanoff et al. (1998) detected 38% hydrolysis of para- sponse observed to 0.221 g P m−2 (Schoenau and Huang, 2001) nitrophenyl phosphate during a 3-h extraction in 1 mol L−1 HCl, and the total exchange capacity of commercially available anion ex- whereas Bowman (1989) reported “negligible” hydrolysis of a se- change membrane products. For example, The P exchange capacity ries of organic P compounds (para-nitrophenyl phosphate, glyc- of PRS probes is reported to be 3.014 g P m−2 based on the charge erophosphate, phytic acid, and bis-para-nitrophenyl phosphate) −1 capacity (www.westernag.ca/innov/technical_1.php; verified during a short extraction in 1.08 mol L H2SO4, including an 14 July 2010). Our window of acceptable recovery appears more initial exposure to concentrated H2SO4. Further analysis using robust than that demonstrated by Mason et al. (2008), who, assum- nondestructive direct monitoring by 31P NMR spectroscopy ing that a 6.26- by 2.5-cm strip has a reactive surface of 31.3 cm2, would be required to establish the impact of pH-dependant hy- found a linear response to only 0.046 g P m−2. While this may be drolysis on model compounds during eluant holding times. due to their use of a counterion (Cl−) with a higher affinity for the For a series of wetland soils with contrasting P contents, no membrane exchange sites, it is also likely to have been influenced by detectable molybdate-unreactive P was recovered by the anion ex- their conservative interpretation of acceptable recovery (S. Mason, change membranes (Fig. 4). The organic nature of the samples sug- Univ. of Adelaide, personal communication, 2008). gested that they might contain relatively high organic P concentra- Several organic and condensed inorganic P compounds in- tions, while fumigated samples were expected to contain high con- teracted with, and in some cases were fully recovered by, a routine- centrations of both organic and condensed inorganic P from lysed

Fig. 4. Comparison of total and molybdate-reactive P detected in anion exchange membrane eluants from wetland soil extractions. Unfumigated (X) and hexanol fumigated (O) samples show significant differences (pairedt -test P < 0.05) between P determined by inductively coupled plasma optical emission spectrometry and colorimetric analysis. Linear correlations are: unfumigated y = 0.98x + 0.36, R2 = 0.999; fumigated y = 0.96x − 4.08, R2 = 0.997.

SSSAJ: Volume 74: Number 5 • September–October 2010 1611 microbial cells, although it is probable that much of these would phosphorus with anion-exchange membranes. Soil Sci. Soc. Am. J. 58:105–114. Dick, W.A., and M.A. Tabatabai. 1977. Determination of orthophosphate in be hydrolyzed to orthophosphate during fumigation (Brookes et aqueous solutions containing labile organic and inorganic phosphorus al., 1982). This study demonstrates that, given the use of an acidic compounds. J. Environ. Qual. 6:82–85. eluant, there is no significant difference between microbial P -re Drohan, P.J., D.J. Merkler, and B.J. Buck. 2005. Suitability of the plant root simulator probe for use in the Mojave Desert. Soil Sci. Soc. Am. J. 69:1482–1491. covered by anion exchange membranes as determined from total Espinosa, M., B.L. Turner, and P.M. Haygarth. 1999. Preconcentration and P analysis (ICP-OES) and operationally defined molybdate colo- separation of trace phosphorus compounds in soil leachate. J. Environ. Qual. rimetric analysis. This does not rule out the inclusion of acid-labile 28:1497–1504. organic and condensed inorganic P within the molybdate-reactive Hedley, M.J., J.W.B. Stewart, and B.S. Chauhan. 1982. Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by P, especially because both types of compounds have been reported laboratory incubations. Soil Sci. Soc. Am. J. 46:970–976. in soil solutions (Espinosa et al., 1999). Yet the absence of any Ivanoff, D.B., K.R. Reddy, and S. Robinson. 1998. Chemical fractionation of molybdate-unreactive P in these samples suggests that acid-labile organic phosphorus in selected Histosols. Soil Sci. 163:36–45. Kouno, K., Y. Tuchiya, and T. Ando. 1995. Measurement of soil microbial biomass compounds are likely to be negligible, although the use of anion phosphorus by an anion-exchange membrane method. Soil Biol. Biochem. exchange membranes with a more benign eluant (e.g., NaHCO3 27:1353–1357. or KCl) would be needed to confirm this assumption. Mason, S., R. Harnon, H. Zhang, and J. Anderson. 2008. Investigating chemical constraints to the measurement of phosphorus in soils using diffusive In studies that have reported the recovery of organic P by ex- gradients in thin films (DGT) and resin methods. Talanta 74:779–787. change media, an assumption was made, due to their mode of action, McDowell, R.W., L.M. Condron, and I. Stewart. 2008. An examination of potential that resin-extractable organic P provided an estimate of labile soil or- extraction methods to assess plant-available organic phosphorus in soil. Biol. ganic P (Rubaek and Sibbesen, 1993; McDowell et al., 2008). A pre- Fertil. Soils 44:707–715. McLaughlin, M.J., P.A. Lancaster, P.G. Sale, N.C. Uren, and K.I. Peverill. 1994. vious study of the nature of organic P in pasture soils using phospha- Comparison of cation–anion exchange resin methods for multielement tase hydrolysis, however, reported negligible concentrations of labile testing of acidic soils. Aust. J. Soil Res. 32:229–240. phosphomonoesters (including polyphosphoric compounds), which Meason, D.F., and T.W. Idol. 2008. Nutrient sorption dynamics of resin membranes and resin bags in a tropical forest. Soil Sci. Soc. Am. J. 72:1806–1814. was presumed to indicate a rapid hydrolysis of these compounds fol- Myers, R.G., A.N. Sharpley, S.J. Thien, and G.M. Pierzynski. 2005. Ion-sink lowing release into the soil solution (Turner et al., 2002). Further in- phosphorus extraction methods applied on 24 soils from the continental vestigation is needed to determine whether labile organic P may be USA. Soil Sci. Soc. Am. J. 69:511–521. Myers, R.G., S.J. Thien, and G.M. Pierzynski. 1999. Using an ion sink to extract recovered from soils by the anion exchange membranes tested here. microbial phosphorus from soil. Soil Sci. Soc. Am. J. 63:1229–1243. In conclusion, although anion exchange membranes can Phillips, S., G.E. Rouse, and R.M. Bustin. 1997. Vegetation zones and diagnostic interact with a diverse range of P species, this does not appear to pollen profiles of a coastal peat swamp, Bocas del Toro, Panama. Palaeogeogr. limit their application to the determination of readily exchangeable Palaeoclimatol. Palaeoecol. 128:301–338. Qian, P., and J.J. Schoenau. 1997. Recent developments in use of ion exchange phosphate or microbial P in soils, particularly for samples from nat- membranes in agricultural and environmental research. Recent Res. Dev. Soil ural environments. Care must be taken, however, in assigning P re- Sci. 1:43–54. covered by anion exchange membranes to “labile orthophosphate,” Qian, P., and J.J. Schoenau. 2002. Practical applications of ion exchange resins in agricultural and environmental soil research. Can. J. Soil Sci. 82:9–21. because acid-labile organic and condensed inorganic phosphates Qian, P., J.J. Schoenau, and W.Z. Huang. 1992. Use of ion-exchange membranes in may also be included. This could be especially problematic if mem- routine soil testing. Commun. Soil Sci. Plant Anal. 23:1791–1804. branes are deployed after the application of condensed inorganic P Roboredo, M., and J. Coutinho. 2006. Chemical characterization of inorganic phosphorus desorbed by ion exchange membranes in short- and long-term fertilizer (Bertrand et al., 2006). Although not fully resolved, the extraction periods. Commun. Soil Sci. Plant Anal. 37:1611–1626. adaptation of the anion exchange membrane procedure to include Rubaek, G.H., and E. Sibbesen. 1993. Resin extraction of labile, soil organic phosphorus. J. 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Storage-induced changes in extractable Soil organic phosphorus and microbial community composition as affected by inorganic nutrients during transport and storage of tropical forest soils. Soil 26 years of different management strategies. Biol. Fertil. Soils 44:717–726. Sci. Soc. Am. J. 73:1972–1979. Cooperband, L.R., P.M. Gale, and N.B. Comerford. 1999. Refinement of the anion Worsfold, P.J., L.J. Gimbert, U. Mankasingh, O.N. Omaka, G. Hanrahan, P.C.F.C. exchange membrane method for soluble phosphorus measurement. Soil Sci. Gardolinski, P.M. Haygarth, B.L. Turner, M.J. Keith-Roach, and I.D. McKelvie. Soc. Am. J. 63:58–64. 2005. Sampling, sample treatment and quality assurance issues for the determination Cooperband, L.R., and T.J. Logan. 1994. Measuring in situ changes in labile soil- of phosphorus species in natural waters and soils. Talanta 66:273–293.

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