Validation of Soil Phosphate Removal by Alkaline and ...

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Jun 8, 2015 - Validation of Soil Phosphate Removal by Alkaline and Acidic Reagents in a Vertosol Soil using XANES. Spectroscopy. T. I. McLAREN,1,2 C. N. ...
Communications in Soil Science and Plant Analysis

ISSN: 0010-3624 (Print) 1532-2416 (Online) Journal homepage: http://www.tandfonline.com/loi/lcss20

Validation of Soil Phosphate Removal by Alkaline and Acidic Reagents in a Vertosol Soil using XANES Spectroscopy T. I. McLaren, C. N. Guppy, M. K. Tighe, C. R. Schefe, R. J. Flavel, B. C. C. Cowie & A. Tadich To cite this article: T. I. McLaren, C. N. Guppy, M. K. Tighe, C. R. Schefe, R. J. Flavel, B. C. C. Cowie & A. Tadich (2015) Validation of Soil Phosphate Removal by Alkaline and Acidic Reagents in a Vertosol Soil using XANES Spectroscopy, Communications in Soil Science and Plant Analysis, 46:16, 1998-2017, DOI: 10.1080/00103624.2015.1048252 To link to this article: http://dx.doi.org/10.1080/00103624.2015.1048252

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Accepted author version posted online: 08 Jun 2015. Published online: 08 Jun 2015.

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Date: 28 July 2016, At: 04:42

Communications in Soil Science and Plant Analysis, 46:1998–2017, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 0010-3624 print / 1532-2416 online DOI: 10.1080/00103624.2015.1048252

Validation of Soil Phosphate Removal by Alkaline and Acidic Reagents in a Vertosol Soil using XANES Spectroscopy T. I. McLAREN,1,2 C. N. GUPPY,2 M. K. TIGHE,2 C. R. SCHEFE,3* R. J. FLAVEL,2 B. C. C. COWIE,4 AND A. TADICH4 Downloaded by [Indian Inst of Soil Science] at 04:42 28 July 2016

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Soils Group, School of Agriculture, Food and Wine and Waite Research Institute, University of Adelaide, Urrbrae, South Australia, Australia 2 School of Environmental and Rural Science, University of New England, Armidale, New South Wales, Australia 3 Future Farming Systems Research Division, Department of Primary Industries, Rutherglen Centre, Victoria, Australia 4 Australian Synchrotron, Victoria, Australia There is a paucity of information on the soil phosphorus (P) forms removed by alkaline and acidic reagents in Vertosols. The first aim of this study was to identify which soil phosphates are removed by a two-step sequential fractionation (0.1 M NaOH and 1 M HCl) and by a dilute acid extractant (0.005 M H2SO4; Bureau of Sugar Experiment Stations (BSES) soil P test) on an “untreated” Vertosol using P K-edge x-ray absorption near-edge structure (XANES) at the Australian Synchrotron. There was supporting evidence that the 0.1 M sodium hydroxide (NaOH), 1 M hydrochloric acid (HCl), and 0.005M sulfuric acid (H2SO4) extractants remove soil phosphates according to the chemical solubility of known P minerals. The XANES spectra revealed the 1 M HCl and 0.005 M H2SO4 extractants remove calcium (Ca) phosphates from Vertosols, suggesting the latter extractant could be used as an alternative for a rapid and cost-effective measure of Ca phosphates in Vertosols. Keywords Near-edge x-ray absorption fine structure, NGR, phosphorus mineral, slowly available P, Vertisol

Introduction Phosphorus (P) fertilizer use efficiencies have been highly variable and typically low in cotton systems of the northern grains region (NGR) of eastern Australia, which range from 0 to 67 percent (Dorahy, Rochester, and Blair 2004). The bicarbonate soil P test (i.e., the method of Colwell (1963)) is routinely used to predict soil P fertility in cropping systems (Rayment 1993; Speirs et al. 2013). However, long-term studies in the NGR have questioned the reliability of the Colwell method to predict plant-available P (Lester, Dowling, and Birch 2003; Wang et al. 2007). Wang et al. (2007) suggested that the Received 30 May 2014; accepted 3 December 2014 *Present address: Schefe Consulting, 59 Sheridan Court, Rutherglen, Vic. 3685, Australia. Address correspondence to T. I. McLaren, Soils Group, School of Agriculture, Food and Wine and Waite Research Institute, University of Adelaide, Urrbrae 5064 SA, Australia. E-mail: [email protected]

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large concentrations of acid-soluble P often found in Vertosols may be supplying or replenishing the readily available P pool as measured by the method of Colwell (1963). Hence, predicting crop response to P fertilizer in these systems may be difficult because of the supply of acid-soluble P to the soil solution not accounted for by the method of Colwell (1963) (McLaren, Guppy et al. 2014). Wang et al. (2007) reported that the 1 M hydrochloric acid (HCl)–P and residual-P fractions of a modified Hedley, Stewart, and Chauhan (1982) sequential fractionation decreased by 50 and 20 percent respectively over eighteen crop cycles in a Vertosol of the NGR. The chemical composition of the former (and latter) is unknown but is considered to comprise calcium (Ca) phosphates (Hedley, Stewart, and Chauhan 1982; Williams, Mayer, and Nriagu 1980). This is likely to include Ca phosphates of low and high thermodynamic stability (e.g., dicalcium phosphate and apatite, respectively), which could influence the rate of P supply to the soil solution (Fixen, Ludwick, and Olsen 1983; McLaren, Guppy et al. 2014). Consequently, a major problem with chemical fractionation procedures is that soil phosphates are identified according to gross solubility with various reagents rather than small differences in solubility and kinetic release rates affected by chemical composition (Hedley, Stewart, and Chauhan 1982; Williams, Mayer, and Nriagu 1980). A dilute acid extractant (a modified Truog (1930) soil P test known as the Bureau of Sugar Experiment Stations (BSES) extractant (Kerr and Von Stieglitz 1938)) has received much attention as an estimate of “reserve” or slowly available P in the Vertosols of the NGR (Conyers and Moody 2009; McLaren et al. 2013, 2014a; Moody et al. 2013). However, there is a lack of information on the soil P forms removed by the BSES extractant in Vertosols and it is assumed to be largely that of Ca phosphates (Conyers and Moody 2009; McLaren, Guppy et al. 2014). In general, acidic reagents primarily act as solvents dissolving soil P minerals in descending order of Ca phosphates > aluminium (Al) phosphates > iron (Fe) phosphates (Thomas et al. 1973). It is also likely the acidic anion of the BSES extractant will competitively desorb phosphate associated with Al and Fe oxy-hydroxides (Rayment 1993; Truog 1930). Information is needed on the forms of soil P removed by the BSES soil P test and how this compares to that removed by the 1 M HCl extractant of a chemical fractionation procedure, and the latter is not commercially available in the NGR of eastern Australia. The two most commonly used reagents in chemical fractionation procedures are the 0.1 M sodium hydroxid (NaOH) and 1 M HCl extractants (Pierzynski, McDowell, and Sims 2005). The 0.1 M NaOH reagent is designed to estimate the quantity of Al and Fe phosphates, sorbed P, and the more stable organic P that would not be removed by a preceding bicarbonate reagent (Hedley, Stewart, and Chauhan 1982; Thomas et al. 1973). The 1 M HCl extractant is designed to estimate the quantity of Ca phosphates, and the difference between this measurement and total P is referred to as “residual P” (Hedley, Stewart, and Chauhan 1982). However, there has been no direct validation of the soil phosphates removed by the 0.1 M NaOH and 1 M HCl extractant in untreated soils (i.e., soil prior to chemical treatment and no added P fertilizer), and it is unknown to what extent these chemical reagents may alter the soil chemical environment, and the speciation of soil P (Kar et al. 2011; Kruse and Leinweber 2008). Only three studies have attempted to directly validate the soil phosphates removed by the 0.1 M NaOH and 1 M HCl extractants (Kar et al. 2011; Kruse and Leinweber 2008; McDowell, Condron, and Mahieu 2003). Using solid-state 31P nuclear magnetic resonance (NMR) spectroscopy on a silt loam, McDowell, Condron, and Mahieu (2003) concluded that Al and Ca phosphates were removed by the 0.1 M NaOH and 1 M HCl extractants,

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respectively. Residual P was thought to contain Al and Fe phosphates (by total element ratio) representing approximately 75 and 24 percent of total P respectively. However, McDowell, Condron, and Mahieu (2003) reported a number of limitations to the study. These included (1) indirect determination of Fe phosphates via elemental ratios, (2) reduced spectral resolution due to paramagnetic ions [e.g., Fe and magnesium (Mg)], (3) ambiguity in P mineral peak assignment, and (4) possible peak assignment shifts induced by the 0.1 M NaOH reagent. Dougherty, Smernik, and Chittleborough (2005) has also shown that the observability of soil P forms using solid-state 31P NMR spectroscopy is extremely low and is unlikely to be representative of the total soil P. Synchrotron-based studies by Kruse and Leinweber (2008) reported no significant change between the P K-edge x-ray absorption near-edge structure (XANES) spectra before and after each step of the sequential fractionation procedure on an untreated peat soil. However, Kruse and Leinweber (2008) suggested that Al, Ca, and/or Mg phosphates were possibly removed by the acid extractant step of a sequential fraction procedure due to slight changes in the P K-edge XANES spectra on the soil residues before and after acid extraction. Using a similar methodology to Kruse and Leinweber (2008), Kar et al. (2011) reported that soil phosphates removed by a sequential fractionation procedure were similar to those thought to be removed based on chemical solubility (Hedley, Stewart, and Chauhan 1982). However, as Kar et al. (2011) collected strip-mine spoiling from a reclamation site which had received very high cumulative inorganic and organic P amendments, the validation of the sequential fractionation procedure in this context is likely to be heavily influenced by the inorganic and organic P fertilizers applied. Hence, the results of this study are more likely to reflect the solubility of the added fertilizer rather than that of the native soil phosphates. Advances in synchrotron x-ray techniques have been successfully applied to soil P studies using XANES spectroscopy on the P K edge, although almost exclusively on soils with very high cumulative inorganic and organic P amendments (Ajiboye et al. 2008; Beauchemin et al. 2003; Lombi et al. 2006). Soil P XANES spectroscopy is an increasingly used technique in agricultural and environment studies, although this has not been carried out before at the Australian Synchrotron (Ingall et al. 2011; Shober et al. 2006). X-ray absorption near-edge structure spectroscopy has been recommended as the best available technique for identifying the chemical structure of soil inorganic phosphates, whereas solution 31P NMR spectroscopy is suited to that of soil organic P (Doolette and Smernik 2011). The aim of this study was to identify the soil phosphates removed by a two-step sequential fractionation procedure using the commonly used alkaline (0.1 M NaOH) and acidic (1 M HCl) extractants in an untreated Vertosol from the NGR. The aim of this study was to also investigate the soil phosphates removed by the BSES extractant and how they relate to the soil phosphates removed by the stronger 1 M HCl extractant used in a two-step sequential fractionation procedure.

Materials and Methods Soil Collection and Classification A Vertosol soil (Isbell 1996), similar to Vertisols as described in IUSS Working Group WRB (2006), naturally high in P (i.e., from parent material) was collected (0- to 10-cm depth interval) from central Queensland, Australia. The naturally high P Vertosol was collected from a field that had been cropped intermittently for the past 50 years. In

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Table 1 Some chemical and physical properties of the high P Vertosol used in this study Soil property pHa Organic C (%)b Organic P (mg kg−1)c Total P (mg kg−1)d Clay (FT)e

High P Vertosol 7.8 1.1 57 8922 Light medium clay

a

Measured using method 4A1 as described by Rayment and Lyons (2011). Measured using method 6B3 as described by Rayment and Lyons (2011). cMeasured as described by Walker and Adams (1958). d Microwave aqua regia digestion as set out by Tighe et al. (2004). e FT refers to the field texture test as set out by McDowell, Condron, and Mahieu (2003).

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general, this consisted of a crop every 18 months due to a long preceding fallow period designed to increase soil moisture, as is typical in the NGR of eastern Australia. In the past 10 years, the site started to receive minimal P input, which typically consisted of 3 kg P ha−1 as mono- or diammonium phosphate prior to crop establishment. Therefore, the contribution of P fertilizer on the total soil P of the high P Vertosol would be negligible. After drying, samples were ground using a porcelain mortar and pestle to pass through a 0.25-mm sieve and used in subsequent chemical and spectroscopy analyses. Some chemical and physical properties of the high P Vertosol are presented in Table 1. Two additional Vertosol soils containing 987 mg total P kg−1 (low P Vertosol) and 2424 mg total P kg−1 (medium P Vertosol) were also sourced and prepared as before (see Supplementary Material Tables SI-1 and SI-2). The low-P Vertosol and the soil residues of the medium-P Vertosol after chemical extraction were less than the detection limit of P kedge using XANES spectroscopy on the soft x-ray beamline at the Australian Synchrotron, and so will not be further discussed. For example, the P K-edge XANES spectra of soil residues after chemical extraction for the medium P Vertosol is shown in Supplementary Material, Figures SI-1, SI-2, and SI-3. For the high-P Vertosol, the term “untreated” refers to the soil prior to chemical extraction and without any P amendment prior to chemical analysis and XANES spectroscopy. Two-Step Sequential Fractionation Procedure A two-step sequential fractionation procedure was carried out on the high-P Vertosol to determine which soil phosphates were removed by the 0.1 M NaOH and 1 M HCl extractants. The preliminary steps of resin and bicarbonate extraction in a normal sequential fractionation was not performed because P in these extractions are presumably completely recovered in the NaOH extract (Hedley, Stewart, and Chauhan 1982; Thomas et al. 1973); and their removal is unlikely to be detected using P K-edge XANES spectroscopy (Kar et al. 2011; Kruse and Leinweber 2008). This is in part due to the small quantity of P removed by these two steps as a proportion of total soil P and the lack of strong spectral features in comparison to the large fractions of mineral P (Beauchemin et al. 2003). Three replicates of 0.50 g (± 0.02 g) of soil were weighed into 50-mL centrifuge tubes. Each tube received 30 mL of 0.1 M NaOH solution and was shaken on an

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end-to-end shaker for 16 h at 15 revolutions per minute at 23 °C. The supernatant of each tube was then collected after centrifuging at 900g for 15 min. Two 30-mL aliquots of distilled water were then added to each tube to rinse the NaOH solution from the soil as recommended by Condron and Newman (2011). After each 30 mL water addition, the samples were placed on an end-to-end shaker for 30 min and subsequently centrifuged at 900g for 15 min, with the supernatant discarded. Similarly, 30 mL of 1 M HCl solution was added after the 0.1 M NaOH extraction on an additional three soil replicates (0.1 M NaOH + 1 M HCl) and subsequent washing with distilled water as occurred with the extraction with 0.1 M NaOH alone. The three replicate soil residues that were extracted with 0.1 M NaOH and 0.1 M NaOH + 1 M HCl solution treatments were then oven dried for 3 days at 60 °C. Soils were reground to powder (