Soil Chemistry
Phosphorus Solubility of Agricultural Soils: A Surface Charge and Phosphorus-31 NMR Speciation Study Tsutomu Ohno* Dep. of Plant, Soil, and Environmental Sciences Univ. of Maine Deering Hall Orono, ME 04469-5722
Syuntaro Hiradate National Institute for AgroEnvironmental Sciences 3-1-3 Kan-nondai Tsukuba, Ibaraki 305-8604, Japan
Zhongqi He USDA-ARS Southern Regional Research Center 1100 Robert E. Lee Blvd. New Orleans, LA 70124
We investigated 10 soils from six states in the United States to determine the relationship between potentiometric titration derived soil surface charge and the concentration of water-extractable P (WEP). Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy was used to determine the chemical speciation of soil P. The surface charge value at the native soil pH was correlated to the WEP concentration, indicating that electrostatic interactions are involved in determining soil phosphate solubility. The titration curves were fit to a two-site Langmuir model and analysis showed that the native pH surface charge was accounted for by the low pH Type 1 (S-OH2+) site, attributed to positively charged metal (oxy)hydroxides. The 31P NMR data indicated that 98% of the inorganic form of P was composed of orthophosphate species and 95% of the organic P was composed of the P monoester class compounds. The inorganic orthophosphate form of P was directly related to the total soil P content, suggesting that external fertilizer inputs control the level of this form of soil P. In contrast, P monoester class compound content was not related to total soil P content, suggesting that organic soil P is controlled by P cycling independent of external P inputs. The 31P NMR speciation data indicated that the inorganic orthophosphate, pyrophosphate, and DNA P concentrations in the soils were significantly associated with oxalate-extractable Al and Fe concentrations, which further demonstrates that metal (oxy)hydroxides are important surfaces where P species are interacting with soils. Abbreviations: EDTA, ethylenediaminetetraacetic acid; NMR, nuclear magnetic resonance; STP, soil test phosphorus; WEP, water-extractable phosphorus.
I
n agricultural systems, the low fraction of inorganic P fertilizer additions that remain available for plant uptake may be due to sorption and precipitation reactions on soil surfaces (Tisdale et al., 1993). The sparingly soluble nature of most soil P compounds (Lindsay, 1979) makes it difficult to supply sufficient P to sustain optimum crop yields without adding relatively high levels of P fertilizers. This standard practice comes with an environmental cost because high P fertilizer use can lead to eutrophication when soluble forms of P and P-rich soil particles erode from fields from which they may be transported offsite to reach surface waters. Studies have suggested that eutrophication is the primary cause of diminished water quality in the United States (USEPA, 1996). Although agricultural land is not the sole source of P involved in eutrophication, agricultural activities represent a significant nonpoint source of P input to many surface waters (Sharpley et al., 2000). The 2003 average annual P surplus in the United States was 33.6 kg P ha−1 (Sharpley et al., 2003). Concerns about increasing P loads in the environment have, in part, driven the effort to develop alternative farming practices reliant on ecological, rather than agrochemical, interactions for nutrient management (Karlen et al., 1994; National Research Council, 1993). The determination of soil test P (STP) levels has a long history in recommending sufficient P fertilizer levels to support high crop yields. Recently, STP methods have also been used to make environmentally sound P management decisions, either directly as an indication of P levels or indirectly as an input into a model designed to estimate whether P is likely to move from a particular field into Soil Sci. Soc. Am. J. 75:1704–1711 Posted online 4 Aug. 2011 doi:10.2136/sssaj2010.0404 Received 22 Oct. 2010. *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.
1704
SSSAJ: Volume 75: Number 5 • September–October 2011
surrounding surface waters. Using STP as an indicator of crop P needs or as an indicator of potential environmental risk is possible because STP is generally well correlated with levels of WEP. Studies have shown that WEP is a good predictor of the potential loss of dissolved phosphate from soils (Torrent and Delgado, 2001). There is a need for continuing research on soil factors that influence the ability of soil chemical extracts to predict WEP. Sensitivity analysis of plant P uptake models has shown that soil solution phosphate concentration is an important factor in P uptake by plants (Silberbush and Barber, 1983; Hettiarachchi et al., 1997). The plant uptake of P is a function of the “free” phosphate species rather than the total P concentration in solution (Hendrix, 1967; Sentenac and Grignon, 1985). These modeling and experimental studies suggest that a good understanding of sorption reactions that control the amount of phosphate species present in soil solution are important to improving the utilization of soil P. Phosphate typically chemisorbs to soil surfaces through a binuclear bridging mechanism with surface hydroxyl functional groups present in oxyhydroxides, aluminosilicate edge sites, and organic matter (McBride, 1994; Sparks, 2003). These functional groups control the net surface charge as a function of pH and are important factors in the interactions of nutrient anions such as phosphate with soils ( Jiao et al., 2007, 2008). The sorption of organic matter ligands onto the solid-phase mineral components will alter their surface charge characteristics. Incorporation of C-rich amendments to build up levels of soil organic matter also have the potential to alter the surface charge characteristics by direct addition of charged organic functional groups as well as masking inorganic mineral charged sites through direct involvement in a ligand exchange reaction with the organic ligand or physical masking of the charged site. Changes in surface charge would be expected to alter the sorption of agronomically and environmentally important anions such as phosphate. Soil test P methods do not provide chemical-level information on the speciation of soil P. Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy can differentiate P chemical species present in a sample based on chemical shift of resonance signals relative to a reference signal from a known compound in the sample (Turner et al., 2003; Doolette et al., 2009). A team of USDA-ARS scientists from eight locations across the continental United States has been conducting a nationally coordinated research project to develop, validate, and use predictive relationships that quantify the impacts of key soil, environmental, and environmental factors on manure N mineralization (Honeycutt et al., 2011). Those locations represent climatic diversity with mean annual temperatures from 17.7°C (Mississippi State, MS) to 5.2°C (Orono, ME) and mean annual precipitation from 1453 mm (Mississippi State, MS) to 439 mm (Pendleton, OR). Thus, these agricultural soils provide an opportunity to investigate soil problems of national significance. The objectives of this study were to: (i) relate how the surface charge affects the P solubility of soils; and (ii) use 31P NMR to determine the soil P speciation using a soil set representing a range of agricultural soils
SSSAJ: Volume 75: Number 5 • September–October 2011
collected from across the United States. These studies will provide detailed information on how the soil chemical environment affects soil P reactions such as sorption and solubility, which are of vital importance to the environmental and economic sustainability of agricultural systems.
MATERIALS AND METHODS Soil and Soil Chemical Analysis Ten field soil samples were collected from USDA-ARS research centers across the United States to evaluate the P chemistry of soils that have received minimal fertilization in the recent past. All soil samples were taken in areas that had not received N or P fertilizer inputs for at least 5 yr and no manure additions for at least 10 yr. Those soil samples were collected from grass, sod, cropped, or fallowed areas. Soils at each location were randomly sampled by collecting 12 cores using a sliding drop hammer to sample the 0- to 15-cm depth. The soil pH was measured using a 1:1 deionized, distilled (DI) water/soil ratio with 10-min intermittent stirring. The Mehlich 3 extraction was used to determine STP (Mehlich, 1984). Acid ammonium oxalate extraction was used to determine oxalate-extractable Al and Fe (Iyengar et al., 1981). Twenty-five milliliters of 0.2 mol L−1 ammonium oxalate adjusted to pH 3.5 was added to 0.5 g of soil in a 50-mL centrifuge tube and shaken for 4 h in the dark. The tubes were then centrifuged and filtered. The water-extractable P, Ca, Mg, K, and Al were obtained by adding 30.0 mL of DI water to 3.00 g of soil in 50-mL centrifuge tubes. The tubes were shaken on an orbital shaker for 1 h, and then centrifuged at 900 × g for 25 min. The supernatant was decanted and vacuum filtered through a Whatman Nuclepore polycarbonate 0.4-μm membrane filter. All solutions were analyzed for P, Al, and Fe by inductively coupled plasma–atomic emission spectrometry (ICP–AES). The geochemical code Visual MINTEQA2 was used to speciate the ions in the soil/water extract. Stability plots for Al and Ca phosphates were based on data presented in Lindsay (1979).
Phosphorus-31 NMR Spectroscopy Five grams of soil was extracted with 100 mL of 0.25 mol L−1 NaOH in 0.05 mol L−1 ethylenediaminetetraacetic acid (EDTA) for 16 h at room temperature (Cade-Menun and Preston, 1996). The extracts were centrifuged for 30 min at 900 × g and filtered through Whatman no. 42 filter paper. A small aliquot was taken for P analysis by ICP–AES and the remaining solution was frozen and freeze-dried. For the NMR spectra acquisition, 100 mg of the sample was dissolved in 0.6 mL of 1 mol L−1 NaOH in 10% D2O. Solution 31P NMR spectra were collected on an Alpha 600 FT NMR spectrophotometer ( JEOL, Tokyo) with a 5-mm probe. Spectra were recorded at 242.85 MHz using a pulse width of 10.00 μs (90°), an acquisition time of 0.4522 s, and a pulse delay time of 2.0000 s, with broadband proton decoupling at 30°C. The pulse delay time of 2 s was based on the methods of Turner et al. (2007). Absolute peak areas are sensitive to delay times used for species with longer T1 times, which may not allow full relaxation within the recycle period used. The comparisons of relative peak areas between soils should be valid, however, for spectra collected under identical conditions. Each spectrum was scanned 30,000 times and a broadening factor
1705
of 5.00 Hz was used in the Fourier transform procedure. Chemical shifts (ppm) were determined with respect to 85% H3PO4 solution (0 ppm). The 31P NMR spectra were divided into four representative P chemical compound classes: orthophosphate (typically observed at around 6.2 ppm), pyrophosphate (−4.1 ppm), DNA phosphate (−0.4 ppm), and phosphate monoesters (7.5–3.4 ppm as a mixture of signals with extreme broadening). The total signal intensity and the fraction contributed by each of the four classes of P compounds were calculated by integration of the spectral signals using Alice 2 for Windows version 5.1.1 ( JEOL, Tokyo). The integrated NMR signals were designated into chemical classes manually.
2
2
i 31
i 31
Q 3º Q i 3º
Q max,i K i [OH # ] 1! K i [OH # ]
[1]
where Q is the quantity of OH− adsorbed (cmol kg−1) at the hydroxide concentration [OH−] (mol L−1), Qmax,i is the adsorption maximum for the ith site, and Ki is the affinity constant for the ith site. The custom equation fitting option of the MATLAB Curve Fitting toolbox was used to fit the nonlinear two-site model (Eq. [1]) to the surface charge data.
RESULTS AND DISCUSSION Soil Phosphorus Characteristics
Surface Charge Determination and Modeling
The 10 soils investigated spanned a range of climate regions across the United States and included six of the 10 USDA soil textural classes. The Mehlich 3 STP values ranged from 20 to 313 mg P kg−1 soil, with a mean of 93 mg P kg−1 soil (Table 1). Sims et al. (2002) proposed classification guidelines for STP Mehlich 3 values as: 100, above optimum; and >150, environmental degradation. The soils used in this study include four classified as below optimum, three as optimum, two as above optimum, and one as indicating potential environmental degradation. Although the guidelines of Sims et al. (2002) were developed specifically for use with mid-Atlantic state soils, they provide a reference that can be used to guide the classification of other soils into P level categories. Our soil set was biased toward the lower end of P content, which was chosen to allow insight into how soil surface charge and P speciation may affect P solubility in soil environments that have not received excessive P loadings in the past. Total soil P of the soils ranged from 82 to 1610 mg P kg−1 soil, with a mean of 854 mg P kg−1 soil, and WEP ranged between 2.5 to 14.8 mg P kg−1, with a mean of 6.6 mg P kg−1 (Table 1). Sharpley et al. (2004) and Dou et al. (2009) have suggested that heavy manure amendment to soils can affect P soil chemistry by (i) precipitation of sparingly soluble P reaction products, (ii) affecting the P sorption–desorp-
A back-titration method was used to determine the surface charge of the soils (Duquette and Hendershot, 1993; Jiao et al., 2008). This was conducted by titrating 2-g soil samples in 30 mL of 0.01 mol L−1 KCl to pH 3 by the addition of 0.1 mol L−1 HCl in 0.1-mL increments under an N2 atmosphere. A continuous back titration to pH 10 with 0.02 mol L−1 NaOH was conducted at a rate of 0.25 mL min−1 using a syringe pump, and the pH was recorded at 30-s intervals. A reference titration was conducted on a solution with the same background ionic strength but in the absence of soil under an N2 atmosphere. After the acid titration step, the suspension was centrifuged for 20 min at 900 × g and filtered through a 0.4-μm polycarbonate filter into a preweighed tube. The tube with the filtered supernatant was weighed and the solution back titrated to pH 10 as described above to obtain the reference titration curve. The OH− consumption by the soil surface was calculated by subtracting the reference OH− consumption (corrected for the quantity of the entrained solution) from the sample OH− consumption between pH 3 and 10 at 0.1 pH unit intervals. The OH− consumed at each 0.1 pH unit interval was calculated using a sixth-order polynomial interpolation fit of the experimental titration curve using the Curve Fitting toolbox of MATLAB R2008b (The Mathworks, Natick, MA). The surface OH− adsorption isotherm was fitted to a two-site Langmuir model: Table 1. Selected physical and chemical properties of the soils. Particle size distribution Soil State
Soil series
Sand Silt
Clay
Oxalate extractable Textural class† pH
——— % ———
Total C g
kg−1
Al
Fe
——— mg
P
DPS‡
kg−1 ———
NaOH– WaterMehlichEDTATotal extractable extractable extractable P P P P —————–––——— mg kg−1 ————————
A
ME
Caribou
43
43
14
L
5.8
18.5
3800
7030
1350
16.3
1610
9.0
313
1138
B
NE
Sharpsburg
3
57
40
SiC
6.1
18.6
1630
2110
207
6.8
445
5.5
20
276
C
NE
Valentine
93
4
3
FS
6.0
4.3
197
108
49
17.1
82
3.3
23
63
D
ME
Newport
37
51
12
SiL
5.8
25.1
5550
6040
1100
11.3
1460
3.3
147
850
E
OR
Walla Walla
14
73
19
SiL
5.3
13.1
1050
2250
375
15.3
752
9.4
76
257
F
WI
Loyal
12
72
16
SiL
6.6
25.8
2080
6730
604
9.9
739
4.8
49
518
G
WI
Rosholt
30
61
9
SiL
5.0
14.4
1240
2310
346
12.8
470
2.5
62
337
H
IL
Catlin
11
58
31
SiCL
7.2
32.0
1680
2470
535
16.3
788
8.6
104
471
I
MS
Brooksville
19
41
40
SiC
6.5
20.5
1110
3660
617
18.7
1338
14.8
90
620
J
OR
Adkins
73
21
6
FSL
6.9
6.7
578
2850
355
15.8
860
4.7
46
119
† L, loam; SiC, silty clay; FS, fine sand; SiL, silt loam; SiCL, silty clay loam; FSL, fine sandy loam. ‡ Degree of P saturation, calculated as: DPS = [Pox]/([Alox] + [Feox]), where subscript ox denotes oxalate extractable. 1706
SSSAJ: Volume 75: Number 5 • September–October 2011
Fig. 1. Calcium–phosphate double-function plot for the 10 soils with the variscite–gibbsite solubility line added to determine potential phosphate equilibria with variscite, hydroxyapatite (HAP), tricalcium phosphate (TCP), and dicalcium phosphate dehydrate (DCPD).
tion processes, and (iii) saturation of the active P sorption sites. Under these circumstances, the role of surface charge and metal (oxy)hydroxides on P solubility would be expected to diminish. The soils used in this investigation received no manure amendment for a minimum of 10 yr and ranged in degree of P saturation using oxalate extraction from 6.8 to 18.7, indicating that they were far from P saturation. The Spearman rank order correlation coefficients between WEP and the soil chemical parameters shown in Table 1 were all
