Solid-Phase Speciation and Solubility of Phosphorus in an Acid

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Solid-Phase Speciation and Solubility of Phosphorus in an Acid Sulfate Paddy Soil during Soil Reduction and Reoxidation as Affected by Oil Palm Ash and Biochar Worachart Wisawapipat, Kamolchanok Charoensri, and Jirawat Runglerttrakoolchai J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b03925 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Journal of Agricultural and Food Chemistry

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Solid-Phase Speciation and Solubility of Phosphorus in an Acid Sulfate

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Paddy Soil during Soil Reduction and Reoxidation as Affected by Oil Palm

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Ash and Biochar

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Worachart Wisawapipat*, Kamolchanok Charoensri, and Jirawat Runglerttrakoolchai

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Department of Soil Science, Faculty of Agriculture, Kasetsart University,

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Bangkok 10900, Thailand

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ABSTRACT

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Understanding phosphorus (P) speciation and how redox conditions control P solubility in

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acid sulfate paddy soils with limited P availability is crucial for improving soil P availability.

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We examined P speciation and extractability in an acid sulfate paddy soil incorporated with

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oil palm ash (OPA) and biochar (OPB) during soil reduction and subsequent oxidation.

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Phosphorus K-edge X-ray absorption near edge structure (XANES) spectra of the soil

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samples revealed that P in the soil mainly occurred as P adsorbed to ferrihydrite and P

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adsorbed to gibbsite. During soil reduction, gibbsite-bound P was transformed into variscite,

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which was back-transformed to gibbsite-bound P during soil reoxidiation. Sequential

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extraction results confirmed the dominance of Fe/Al (hydr)oxides-bound P (average 72%) in

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the soils. The OPA incorporation increased the exchangeable P pool concurring with the

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decrease in gibbsite-bound P. The OPB incorporation enhanced the dissolved P from the

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residual pool presumably due to electron shuttling of biochar with Fe(III) minerals during soil

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reduction. Our results highlight P dynamics in paddy soils, which are of immense importance

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for effective P-management strategies in rice cultivation.

*

Corresponding Author

W.W. E–mail: [email protected], Telephone: +66 2 9428104, Fax: +66 2 9428106 1 ACS Paragon Plus Environment


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INTRODUCTION

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Acid sulfate soils occur extensively worldwide, covering around 7.5 million ha in the

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Tropics.1 In Thailand, these soils are widespread throughout the Central Plain, 95% of which

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are being used for paddy rice cultivation. The rice yield from these soils is, however, limited

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due to strong acidity, high Fe/Al toxicity and low P availability.2 Owing to the scarcity of

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water supplies for agriculture, intermittent flooding strategies have been introduced to

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enhance the productivity of irrigated paddy rice cultivation.3 This water management

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procedure greatly affects the biogeochemical cycling of redox sensitive elements (e.g., Fe,

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Mn, and S) and associated elements (e.g., P, and As).4 Reduction and reoxidation during soil

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flooding and drainage exert a strong influence on pH, Eh, speciation, phytoavailability, and

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solubility of these elements in soils.5, 6 Microbial respiration during soil flooding leads to the

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reductive dissolution of Mn(III, IV) and Fe(III) (oxyhydr)oxides in soils that enhances P

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availability by means of the release of P adsorbed to minerals.7,

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conditions during soil drainage or intermittent flooding promote the oxidative precipitation of

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Fe (oxyhydr)oxides, thereby reducing P solubility.9

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Conversely, oxidizing

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X-ray absorption near edge structure (XANES) spectroscopy is a powerful

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nondestructive technique that has been used to identify and quantify the speciation of P in a

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wide range of heterogeneous environmental samples, e.g., soils, sediments, sorbed solids and

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minerals.10-17 Using this technique requires the analysis of samples with high P contents,

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which limits the acquisition of good quality spectra for samples with low P contents.

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However, identification of P speciation in samples with low P concentrations (∼50 mg kg–1)

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can be achieved using beamline 8 (BL8) at the Synchrotron Light Research Institute (SLRI),

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Thailand. 18

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The sequential extraction procedure is one of the invaluable techniques differentiating

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soil P into diverse operationally defined pools, 19 which assists in improving the mechanistic 2 ACS Paragon Plus Environment

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understanding of P solubility and its transformation in soils. Although this technique has

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inherent chemically artificial limitations, e.g., solubility of non-target phases and

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modification of the oxidation states of redox sensitive elements, it brings insight to

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quantitative knowledge on soil solution and exchangeable pools. These pools are not only the

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most available fractions to plants but also relevant to terrestrial environments, which are

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overlooked by advanced spectroscopic techniques, e.g., X-ray adsorption spectroscopy.

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Combining XANES techniques with a sequential extraction procedure is thus a critical

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prerequisite to quantitatively identify soil P speciation and extractability. Moreover, present

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knowledge on P speciation and solubility in acid sulfate soils is very limited.

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Currently, the land use on some of the acid sulfate soils in Thailand has changed from

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irrigated paddy rice cultivation to oil palm production. The palm oil industry produces

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appreciable amounts of agricultural wastes, e.g., oil palm ash (OPA) and oil palm shell which

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can potentially be used as soil amendments. The OPA is an alkaline material containing many

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plant nutrients (e.g., Si, K and P),

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soils. Also, oil palm-derived biochar (OPB) derived from oil palm shell has been documented

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as an important sorbent for plant nutrients and contaminants.21-23 Moreover, biochar can act

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as an electron shuttle between organisms and constituents in soils,

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transfers in redox reactions. 25, 26 Therefore, we hypothesized that these two materials should

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have different impacts on the solubility mechanisms of P in paddy soils under varying redox

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conditions.

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which is capable of improving the P availability in acid

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facilitating electron

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Irrigated paddy rice cultivation frequently experiences continuous flooding or

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intermittent flooding conditions, which play pivotal roles in changing the soil pH and Eh,

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subsequently affecting the transformations of P species in soils. The aim of this research was

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therefore to identify speciation and the solubility of P in an acid sulfate paddy soil amended

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with OPA and OPB under reduction and oxidation conditions.

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MATERIAL AND METHODS

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Sampling and Characterization

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A batch of an acid sulfate paddy topsoil (0–30 cm depth), classified as a Sulfic

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Endoaquept, was collected from a rice paddy field on the Thailand Central Plain. The bulk

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soil sample was air-dried, gently pulverized and then sieved to a particle size < 2 mm using a

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stainless steel sieve before analysis and the incubation experiment. Physicochemical

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properties were determined using standard procedures. 27

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The studied soil had a clayey texture with an extremely acidic pH of 4.4 (in water).

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The pH value in H2O2 (pH = 2.2) was lower than that in water, suggesting the studied soil

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may contain oxidizable pyrite and other sulfur materials, which can release sulfuric acid

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during chemical treatment.

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cmolc kg−1) reflects the dominance of 2:1 clay minerals 29, and is consistent with the presence

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of large amounts of soil organic matter (66 g kg−1). The total soil P concentration, as

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estimated by X-ray fluorescence, was very high (672 mg kg−1) but a moderate level of Bray-II

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extractable P was observed (13 mg kg−1) corresponding to only 1.9% of the total soil P.

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The high value for the cation exchange capacity (CEC) (35

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The OPA and oil palm shell materials were provided by a palm oil factory in Eastern

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Thailand. The OPA is commonly acquired from the combustion of the oil palm fruit fibers

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and shells used to produce thermal energy during the streaming of fresh fruit bunches. For oil

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palm biochar (OPB) production, the shell material was loaded into a 200 L traditional kiln,

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and charred using slow pyrolysis (24 h) at an operating temperature of 270‒350 oC. The

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biochar was ground to a particle size of < 2 mm prior to chemical analysis and the incubation

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experiment. Chemical characterization of the OPA and OPB materials was determined using

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standard procedures. 27

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The analysis of OPA and OPB revealed that the OPA was strongly alkaline (pH =

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12.1) with a relatively high electrical conductivity of 13.6 mS cm−1. The amount of Bray-II


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extractable P in the OPA was extremely high (798 mg kg−1). The OPB was slightly acidic

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(pH = 6.5), with a high amount of Bray-II extractable P (198 mg kg−1). However, the CEC for

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the studied OPB (6.7 cmolc kg−1) was moderately low, indicating low chemically reactive

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surfaces.

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Incubation Experiment

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To investigate the impacts of the application of agricultural wastes from the palm oil

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industry (OPA and OPB) on P speciation and extractability in an acid sulfate paddy soil

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under varying redox conditions, three different incubation series were performed in triplicate.

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The series consisted of a mixture of the solid soil incorporated with OPA and OPB at the

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rates of 0 and 0.64% w/w. This corresponded to field application rates of 0 and 12.50 t ha‒1

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(calculated from 15 cm depth and a bulk density of 1.3 g cm‒3). For each series, 40 g of dry

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soil sample was placed into a 120 mL septum vial and mixed with different rates of oil palm

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wastes. Subsequently, 80 mL of deionized water were added, and a supplementary C source

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of 5 mM sodium lactate was used to stimulate microbial activity. The incubation vials were

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shaken in contact with air for 2 days in the so-called equilibration phase (EQ). The headspace

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of septum vials was subsequently purged with N2 gas and the soil suspensions were kept

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under anaerobic conditions. The soil suspensions were incubated for 40 days at room

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temperature with daily shaking in the so-called reduction phase (RED). Afterward, the lids

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were opened and O2 gas was allowed to diffuse into the soil suspensions for 28 days in the

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so-called reoxidation phase (REOX). For the REOX phase, the incubation vials were shaken

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only for the first 2 days after the lids had been opened. All redox-sensitive experimental steps

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and all sample preparations were performed in a plastic glovebox under N2 flow conditions to

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decrease the oxidation of soil constituents. After defined periods within the EQ phase (day 2),

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the RED phase (day 1, 5, 10, 15, 20, 30, and 40), and the REOX phase (day 2, 7, 14, and 28),

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respectively; the soil aliquots were transferred into centrifuge tubes and centrifuged at 5 ACS Paragon Plus Environment


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2,205×g for 10 min. The supernatants were filtered through 0.2-µm polytetrafluorethylene

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filters (VertiClean; Vertical Chromatography, Thailand) and preserved for further analysis.

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The unfiltered supernatants were collected for pH and Eh measurements. Aliquots of the

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filtered supernatants were stored in the freezer for the determination of P and sulfate (SO42-)

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using a spectrophotometer. Acidified aliquots (1% v/v of concentrated HCl) were stored in

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the freezer for the determination of major cations using a microwave plasma-atomic emission

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spectrometer (MP-AES; 4100 MP-AES; Agilent technologies, USA).

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Phosphorus K-edge XANES Spectroscopy

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Phosphorus reference standards and selected soil samples from all incubation series

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(untreated soil, 1d RED, 40d RED, 28d REOX) were analyzed using P K-edge (2,145.5 eV)

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XANES spectroscopy at beamline 8 of the Synchrotron Light Research Institute (SLRI),

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Thailand. 18 The beam energy was 1.2 GeV and the beam current varied from 70 to 150 mA.

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The X-ray photon energy was scanned using an InSb(111) double-crystal monochromator

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with an energy resolution of 0.4 eV. The XANES spectra were collected in fluorescence

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mode in an He chamber at room temperature using a 13-channel germanium element

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detector. The wet soil samples were freeze-dried, finely ground and placed between Kapton

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or polypropylene tapes for P XANES measurements. A range of P reference compounds was

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analyzed using XANES spectroscopy to support the interpretation of soil spectra. Spectra of

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P adsorbed to gibbsite, P adsorbed to ferrihydrite and variscite (AlPO4·2H2O) were provided

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by Dr. Dean L. Hesterberg (North Carolina State University). Description of the sample

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preparation was reported in Khare et al.

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phosphate rock) was provided by Dr. Chanida Charanworapan (Land Development

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Department) with a detailed description of the sample given in Charanworapan et al.

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Vivianite (Kerch, Peninsula, Russia) was provided by Dr. Seriwat Saminpanya

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and Liu and Hesterberg


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. Apatite (Ratchaburi

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(Srinakharinwirot University). Strengite (FePO4·2H2O), brushite (CaHPO4·2H2O), archerite

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(K2HPO4), phytic acid and lecithin were purchased from Sigma.

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All data were analyzed using the software package ATHENA. The spectra were baseline-

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corrected by subtracting a linear regression through the pre-edge region (−25 to −10 eV

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relative to E0 set at the value of white line energy for each sample and standard), and was

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background-corrected using a quadratic function through the post-edge region (+30 to +45

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eV).16,

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combination fits with reproducible P speciation results as recently proposed by Werner and

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Prietzel.33 Linear combination fitting analysis (LCF) was performed across an energy range

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from −10 eV to +30 eV relative to E0 to investigate the P speciation.

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Phosphorus Solubility by Sequential Extraction

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These small normalization ranges have been shown to provide accurate linear

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The P solubility in the acid sulfate paddy soil treated with the OPA and OPB was

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determined using a five-step sequential extraction modified from Hedley et al. (1982). 19 This

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modified method allows for differentiation and quantification of five major P pools: (i) soil

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solution P, (ii) exchangeable P, (iii) Fe/Al (hydr)oxides-bound P, (iv) detrital P and (v)

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residual P. In brief, 1 g samples of wet soil paste were weighed into 50 mL centrifuge tubes,

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and equilibrated sequentially with: (F1) 0.01 M CaCl2, (F2) 0.5 M NaHCO3, (F3) 0.1 M

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NaOH, (F4) 1 M HCl and followed by (F5) aqua regia digestion (Table S1). The P

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concentration extracted from each extractant was determined using the standard molybdate

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blue method. 34

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RESULTS AND DISCUSSION

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Dynamics of Eh, pH, and Major Dissolved Species

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The dynamics of Eh, pH, P, and major cations in the solution during the reduction-

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reoxidation had similar trends in all incubation series (Figure 1). Values of the solution pH 7 ACS Paragon Plus Environment


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slightly increased from 4.4 to 4.9 (OPB), 5.5 (control) and 5.6 (OPA) after 5 days of soil

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reduction, and stabilized at about pH 5.5 during the entire reduction phase. The increase of

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solution pH resulted from consumption of H+ required for nitrate, Mn(III, IV) and Fe(III)

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reduction.

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attributed to the acidic and alkaline properties of the OPB and OPA materials, respectively.

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Solution Eh varied between ‒150 and ‒50 mV in the RED phase indicating reducing

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conditions. However, a clear increase in dissolved Mn and Fe in the RED phase as a function

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of time was not apparent, suggesting the complete microbial reduction of Mn(III/IV)- and

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Fe(III)-(oxyhydr)-oxides. A rapid decrease in dissolved sulfate over time demonstrated the

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period of sulfate reduction (Eh ‒100 mV) with the soluble sulfate completely consumed

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within 10 days. The decreasing dissolved Zn concentration with sulfate depletion in the RED

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phase likely indicated the precipitation of sphalerite (ZnS) colloids, which was consistent

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with a report indicating the formation of ZnS in the solution of contaminated paddy soils with

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excess amounts of sulfates. 36

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The variation in pH values in the OPB- and OPA-incubation series can be

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In all incubation series, dissolved P in solution was found in very small concentrations

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(~1‒2 µmol L−1), which is much lower than the critical P content of 6.46 µmol L−1 in soil

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solution required by most plants.

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soil reduction phase (1.0‒1.7 µmol L-1) was observed, which indicated the release of sorbed P

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from reductive dissolution of Fe/Mn-(hydr)oxides. The increase of aqueous P in the OPA-

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incubation series (4.3 µmol L-1) was noticeable from the first day of soil reduction, but it

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rapidly decreased to 1 µmol L−1 within 5 days of soil reduction showing a strong capability

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for P sorption by soil constituents such as Fe and Al (hydr)oxides.

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Nonetheless, a small increase in dissolved P during the

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After aeration of soil suspensions for 2 days, the solution pH rapidly increased from

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5.5 to 7.5 (OPA), 7.0 (control), and 6.4 (OPB), respectively. The soil solution Eh rapidly


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increased from ‒50 mV to 90 mV, with the Eh subsequently stabilized at approximately 100

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mV during the 28 days of the REOX phase. Several dissolved elements (Mn, Fe, Al, Si, and

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Zn) showed a marked increase at 7 days of the REOX phase, but displayed a rapid overall

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decrease during the entire REOX phase. This was due to the aeration causing the oxidative

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precipitation of dissolved Fe and the rapid formation of freshly formed Fe colloids that

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subsequently precipitated on the surfaces of the soil suspension.

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Changes in P Speciation by X-ray Absorption Spectroscopy

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Phosphorus XANES spectra for P mineral standards and sorbed solids provided clear

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spectral features in the pre-edge, white-line peak intensity and post-edge regions, which

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allowed for the identification of P species in the soil samples (Figure 2). The spectra for P

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associated with FeIII oxides (i.e., strengite and P adsorbed to ferrihydrite) had a clear pre-edge

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feature, which is an indication of P coordinated with FeIII atoms in the second shell.

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pre-edge feature was absent for FeII phosphate minerals (e.g., vivianite). Apatite and brushite

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(CaHPO4·2H2O) showed a post-white-line shoulder at 2158 eV that provides a descriptive

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indication for CaP compounds.38 Moreover, the presence of a secondary post-white line peak

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at 2164 eV indicated different degrees of crystallinity of the P compounds.39 XANES spectra

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for P sorbed solids including P adsorbed to gibbsite and ferrihydrite showed a more

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pronounced intensity of the white-line peak, which is an indication of P sorbed species,

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compared to P minerals (e.g., variscite, strengite, or apatite).30

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This

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To gain molecular-level insight into P speciation changes during soil reduction and

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reoxidation, we used P K-edge XANES spectroscopy to acquire direct information on the

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solid-phase speciation of P changes—a procedure that has not been used previously for P

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speciation in acid sulfate soils. Amounts of the various P species in the untreated soil and in

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the soils from the incubation experiment were measured using LCF of the P XANES spectra.



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The LCF results showed that 0–32% of the total P species in the soils occurred as variscite,

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7–40% occurred as P adsorbed to gibbsite (or similar sorbents), 50–62% occurred as P

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adsorbed to ferrihydrite (or other Fe [hydr]oxides), and 0–9% occurred as lecithin depending

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on the treatments and redox conditions (Figure 3, Table S2). On average, Al (hydr)oxides

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were estimated to host 39% of the total soil P in sorbed and mineral forms, whereas Fe oxides

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were estimated to host 56% of the total soil P. The P adsorbed species accounted for 83% of

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the total soil P. However, it should be noted that P adsorbed to gibbsite or ferrihydrite could

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be P adsorbed to other species of Al or Fe (oxy)hydroxides.

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In the untreated soil sample, P species were comprised P adsorbed to ferrihydrite

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(52%), P adsorbed to gibbsite (32%), variscite (6%), and lecithin P (9%). After the first day

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of soil reduction, in the control series, the amounts of P adsorbed to gibbsite rapidly reduced

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to 14% of the total soil P, which quantitatively transformed into variscite (22%).

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Subsequently, the freshly authigenic variscite inversely completely transformed P adsorbed to

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gibbsite at 28 days of the REOX phase. Interestingly, the clear transformations of P adsorbed

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to ferrihydrite, the most dominant P species in the soils, was not observed during the

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reduction phase. We postulate that Al (hydr)oxides play a more important role than Fe

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(hydr)oxides in controlling the dynamics of P in the acid sulfate soils. This could be

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attributed to Al (hydr)oxides in the soil perhaps occurring as non-crystalline Al-hydroxides,

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which had greater P sorption capacity (0.23 mol P mol–1 Al) than that of poorly crystalline Fe

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(hydr)oxides, that is, ferrihydrite (0.34 mol P mol–1 Al).30,

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phosphate minerals was not observed during the soil reduction, which was likely due to the

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high solubility of vivianite in acidic environments or insufficient amounts of dissolved P for

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precipitation of ferrous phosphate minerals.40 The amounts of P adsorbed to gibbsite in the

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OPB and OPA incubation series also decreased on the first day of the RED phase with the

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OPB treatment containing the lowest amount of the P adsorbed to gibbsite species (7%). This 10 ACS Paragon Plus Environment

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The formation of ferrous

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suggested that biochar can enhance the formation of Al phosphate minerals under anoxic

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conditions. Over 28 days of soil reoxidation, the OPA application slightly lowered the

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formation of gibbsite-bound P species compared with the control, suggesting that the OPA

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may decrease the available sites of Al hydroxides for P sorption, thereby increasing the plant-

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available P. This was consistent with an increase of exchangeable pool P during soil

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reoxidation as discussed in the following section.

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Changes in P Extractability by Sequential Extraction

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Changes in P extractability in the studied acid sulfate soil samples treated with OPA

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and OPB during reduction and reoxidation were characterized by a five-step sequential

252

extraction (Figure 4). Details of the sequential extraction results are provided in Table S3.

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The sequential extraction results clearly showed that these soils had a different distribution of

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the main P bearing phases (i.e., soil solution P, exchangeable P, Fe/Al (hydr)oxides-bound P,

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detrital P and residual phases), which are very important for considering readily available P

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and long-term availability of P in the samples and its potential risk to terrestrial

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environments.

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The sequential extraction results demonstrated that Fe/Al (hydr)oxides-bound P (F3)

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was the dominant pool of P in the studied soil in all incubation series with values varying

260

from 3.94 to 5.69 µmol g−1 (58–77% of total extracted P). This fraction is also considered to

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be P associated with poorly crystalline Fe and Al (hydr)oxides. The residual P fraction (F5)

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represented the second largest P pool in the studied soil, with average amounts of 1.05–1.96

263

µmol g−1 (17–26% of total extracted P). Exchangeable P (F2), as readily available P, was

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present at trace to moderate levels varying from 0.26 to 1.82 µmol g−1 (4.1–21% of total

265

extracted P) depending on both the reduction and reoxidation periods as well as the

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incubation series.



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Detrital P (F4) was present as a small fraction of the soil P pool with amounts varying

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from 0.024 to 0.134 µmol g−1 (0.39–2.40% of total extracted P). This suggested that

269

terrestrial apatite and CaCO3-associated P, which are mainly derived from their parental

270

materials or former phosphate rock applications, contribute an insignificant fraction of P to

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the soil. The soil solution P (F1) as estimated by CaCl2 extraction which is thought to be the

272

most readily bioavailable P, was present at trace levels (0.021-0.046 µmol g−1; Figure 2),

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accounting for only 0.34–0.75% of the total extracted P. This was consistent with the

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observed P in solution indicating that the soil contained a very low amount of readily

275

available P.

276

During soil reduction, the fraction of the Fe/Al (hydr)oxides-bound P (F3) increased

277

from 1 day (58% and 69% of total extracted P) to 40 days (70% and 74% of total extracted P)

278

in the OPA and OPB series, respectively. However, there was no change in the Fe/Al

279

(hydr)oxides-bound P in the control treatment. This suggested that soil reduction enhanced

280

the dissolution of P associated with Fe/Al (hydr)oxides. Moreover, the electrostatic

281

adsorption between Al3+ ions and the negative surfaces of biochars or the surface

282

complexation of Al(OH)2+/Al(OH)2+ with hydroxyl and carboxyl functional groups of

283

biochars under acidic conditions

284

enhance P adsorbed to Fe/Al (hydr)oxides. Moreover, a small decrease in the residual P

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fraction (F5) from 1 day to 40 days was also found in the OPA (19.5% to 18.7% of total

286

extracted P) and OPB (23.4% to 19.1% of total extracted P) series. This demonstrated that

287

both materials are capable of promoting the dissolution of P from stable minerals during soil

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reduction. The OPB treatment released more P from the residual fraction than the OPA

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treatment. This may be attributed to the electron shuttling properties of biochar26 that can

290

accelerate the reductive dissolution of P incorporated into the structure of crystalline Fe,

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which was not dissolved in mild NaOH solution as this extractant (F3) mainly targets poorly

41, 42

may increase the positively charged surfaces, which


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crystalline Fe oxides and minor amounts of organic constituents. 43, 44 In addition, biochar can

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stimulate growth for FeIII-reducing bacteria, which affect the transformation of minerals or

294

contaminants (e.g., pentachlorophenol and DDT) in paddy soils under anoxic conditions.45-48

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The OPA application increased P in fraction F2 from the first day of the RED phase

296

considerably (1.82 µmol g−1, 21% of total extracted P) but this exchangeable pool of soil P

297

decreased rapidly with soil reduction (40 days = 0.75 µmol g−1, 9.2% of total extracted P).

298

This was consistent with the increase of the Fe/Al (hydr)oxides-bound P fraction (F3) in the

299

RED phase. Upon the first day of soil reoxidation, the exchangeable P pool of the soil

300

continuously decreased to 0.28 µmol g−1 (4.4 % of total extracted P) but this fraction

301

considerably increased over 28 days of soil aeration (0.72 µmol g−1, 11% of total extracted P)

302

compared to the control series (0.31 µmol g−1, 5.0% of total extracted P). This concurred with

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the decrease of gibbsite-bound P and the increase of variscite (observed using XANES) in the

304

OPA series. This indicated that variscite (AlPO4·2H2O) may contribute an exchangeable P

305

pool, as plant-available P, to the soil under aerated conditions. Also, the OPA- (0.123 µmol

306

g−1, 1.40% of total extracted P) and the OPB- (0.062 µmol g−1, 1.0% of total extracted P)

307

treated series increased P in fraction F4 (P associated with CaCO3 and apatite) compared to

308

the control series (0.034 µmol g−1, 0.57% of total extracted P) suggesting that the OPA may

309

contain some CaCO3 minerals.

310

IMPLICATIONS

311

The present work has quantitatively identified the changes in P species in acid sulfate

312

soils under reduction and reoxidation conditions using the XANES spectroscopic technique

313

and sequential extraction. The P XANES results revealed that P adsorbed to Fe and Al

314

(hydr)oxides were the major P species in the studied acid sulfate soil. Aluminum

315

(hydr)oxides are likely to play a more important role than Fe (hydr)oxides in the dynamics of

316

P in the strongly acidic soil, as P adsorbed to gibbsite was the only phase transformed upon 13 ACS Paragon Plus Environment


317

reduction and reoxidation conditions with no transformation of P adsorbed to ferrihydrite

318

being observed. This may suggest that P adsorbed to other P-bearing Fe(III) (hydr)oxide

319

species could be present. To acquire a better estimation of the ferric-Fe bound P species in

320

soils, more references of P adsorbed to a wide range of naturally occurring Fe(III)

321

(hydr)oxides are required, or the XANES results should be complemented by other

322

techniques. The P sequential extraction results confirmed Fe/Al (hydr)oxide-bound P was the

323

major P pool in the studied soils. Moreover, this extraction technique effectively provided the

324

most labile P pools (i.e., soil solution and exchangeable P), the most relevant fractions to P

325

mobility and bioavailability in the environment and in agricultural systems, which cannot be

326

quantified by XANES. Therefore, combining XANES and the sequential extraction technique

327

substantially improved the qualitative and quantitative P speciation. Our findings shed light

328

on the importance of redox conditions in acid sulfate soils for the cycling and fate of P.

329

Recycling of agricultural wastes into soils constitutes an effective strategy to improve

330

the P availability in soils for sustainable crop production. Our results highlighted that ash

331

derived from oil palm has potential for increasing exchangeable P pools (readily available P)

332

in paddy soils during flooded and aerated conditions through decreasing the reactive surfaces

333

of Fe/Al (hydr)oxides for P adsorption. Furthermore, biochar incorporation in the soils under

334

waterlogged conditions enhanced the dissolution of P from the residual fraction. This result

335

leads to the consideration of utilizing biochar to reclaim “legacy P” that is thought to be the

336

most stable form of inorganic orthophosphate accumulated in the soils through excessive P

337

fertilizer over time. However, further research is necessary to obtain a mechanistic

338

understanding of the role of biochar in retrieving accumulated legacy P in diverse soil types

339

under varying redox conditions. Greenhouse experiments should also be conducted to testify

340

the availability of soil legacy P to economic crops.

341

ASSOCIATED CONTENT


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Page 15 of 23

342


Supporting Information

343

Supporting Information Available: details of the five-step sequential extraction

344

procedure, P K-Edge XANES fitting results and further P sequential extraction results during

345

soil reduction and reoxidation. This material is available free of charge via the Internet at

346

http://pubs.acs.org. This material is available free of charge on the ACS Publications website.

347

ACKNOWLEDGMENTS

348

We gratefully acknowledge Dr. Wantana Klysubun and her staff for their support

349

during the XAS data collection at BL 8 of the Synchrotron Light Research Institute (SLRI),

350

Thailand. We are grateful to Dr. Dean Hesterberg (North Carolina State University) for

351

providing some of the P reference spectra and his suggestion on normalization procedure of P

352

XANES spectra. We thank Dr. Seriwat Saminpanya for providing vivianite and Dr. Chanida

353

Charanworapan for providing Ratchaburi phosphate rock. We appreciate the assistance of

354

Honorific Patrick Freeze (Washington State University) for English editing in an earlier draft.

355

This research was financially supported by the Agricultural Research Development Agency

356

(ARDR) Grant No. PRP5605021270.

357

REFERENCES

358 359

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473 Reduction

Re-oxidation

Reduction

8

100

Control Control OPA2 OPA OPB2 OPB

0

pH

Eh (mV)

7

-100

6 5

-200

4 0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

0

10

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30

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0

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0

10

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30

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70

0.8

Fe (mmol L-1)

12

Mn (µmol L-1)

Re-oxidation

8 4 0

0.6 0.4 0.2 0.0

0

10

20

30

40

50

60

70

70

5 4

P (µmol L-1)

Sulfate (mmol L-1)

3 2 1 0

2 1 0

0

10

20

30

40

50

60

70

50

60

70

1.5

5.0 4.0

Zn (µmol L-1)

Al (mmol L-1)

3

3.0 2.0 1.0

1.0 0.5 0.0

0.0 0

10

20

30

40

50

60

70

50

60

70

Incubation time (day)

474 475

Figure 1 Solution dynamics during soil reduction and reoxidation for the acid sulfate soil

476

treated with oil palm ash (OPA) and oil palm shell biochar (OPB).



477 478

Figure 2 Normalized phosphorus K-edge XANES spectra of selected organic P compounds,

479

inorganic P compounds, P sorbed solids, P minerals and studied soil sample. Peak “a”

480

indicates P associated with Fe (hydro)oxides; Peaks “b” and “c” indicates different Ca−P

481

species.


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Page 21 of 23


482

483

Figure 3 (a) Normalized bulk P K-edge XANES spectra with LCF model fits over the energy

484

range of 2140–2190 eV. Green lines indicate experimental data and gray dotted lines show

485

the LCF model fits. (b) P speciation based on XANES LCF of an acid sulfate soil treated with

486

oil palm ash (OPA) and oil palm biochar (OPB).



487

488

Figure 4 Results from the five-step P sequential extraction of selected soil samples in

489

control, oil palm ash (OPA) and oil palm shell biochar (OPB) incubation series during the

490

reduction, and reoxidation periods. The extracted fractions are shown as relative percentages

491

of the total soil P content determined by the sum of all extracted fractions (F1 to F5).


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170x85mm (150 x 150 DPI)

ACS Paragon Plus Environment

Solid-Phase Speciation and Solubility of Phosphorus in an Acid

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