Linking Phosphorus Sequestration to Carbon Humification in Wetland

Mar 19, 2012 - Department of Chemistry & Biochemistry, Florida State University, Tallahassee ... University of Florida, Gainesville, Florida 32611, Un...
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Linking Phosphorus Sequestration to Carbon Humification in Wetland Soils by 31P and 13C NMR Spectroscopy Rasha Hamdan,† Hasan M. El-Rifai,†,# Alexander W. Cheesman,‡ Benjamin L. Turner,‡ K. Ramesh Reddy,§ and William T. Cooper*,† †

Department of Chemistry & Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Republic of Panama § Soil and Water Science Department, University of Florida, Gainesville, Florida 32611, United States ‡

S Supporting Information *

ABSTRACT: Phosphorus sequestration in wetland soils is a prerequisite for long-term maintenance of water quality in downstream aquatic systems, but can be compromised if phosphorus is released following changes in nutrient status or hydrological regimen. The association of phosphorus with relatively refractory natural organic matter (e.g., humic substances) might protect soil phosphorus from such changes. Here we used hydrofluoric acid (HF) pretreatment to remove phosphorus associated with metals or anionic sorption sites, allowing us to isolate a pool of phosphorus associated with the soil organic fraction. Solution 31P and solid state 13C NMR spectra for wetland soils were acquired before and after hydrofluoric acid pretreatment to assess quantitatively and qualitatively the changes in phosphorus and carbon functional groups. Organic phosphorus was largely unaffected by HF treatment in soils dominated by refractory alkyl and aromatic carbon groups, indicating association of organic phosphorus with stable, humified soil organic matter. Conversely, a considerable decrease in organic phosphorus following HF pretreatment was detected in soils where O-alkyl groups represented the major fraction of the soil carbon. These correlations suggest that HF treatment can be used as a method to distinguish phosphorus fractions that are bound to the inorganic soil components from those fractions that are stabilized by incorporation into soil organic matter.

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INTRODUCTION Wetlands have long been recognized for their capacity to transform inorganic phosphate into various organic phosphorus pools that are largely immobile in soil.1 The ability of wetlands to retain phosphorus depends mainly on phosphorus chemical structure and soil physicochemical properties.2 These two factors determine the nature of phosphorus interactions with soil minerals, particularly with regard to sorbing onto anionic sorption sites or incorporation into stable soil humic complexes.3 These interactions determine whether sequestered phosphorus is resistant to changes in environmental conditions (e.g., pH, temperature water level).4 For example, Dunne et al. demonstrated that phosphorus release from marsh soils was affected by the length of flooding hydroperiods, in contrast to surrounding upland pasture soils that had lower levels of organic matter, moisture, bulk density, and nutrients.5 The accretion of phosphorus associated with stable organic matter is an important mechanism for phosphorus retention in wetlands.6 The nature of the organic material originating from plant and microbial remains therefore plays a major role in determining long-term stability of phosphorus.7 Carbon compounds in plant tissue degrade at markedly different rates, so information on soil organic carbon structure can provide important clues about organic matter decomposition or stabilization in © 2012 American Chemical Society

soils. For example, most plant derived carbohydrates are readily degraded,8 whereas more refractory compounds become stabilized and may persist in the soil for thousands of years.9 Linking phosphorus speciation and soil organic carbon composition is, therefore, a key aspect of any effort to understand soil phosphorus stabilization.10 Assessment of soil phosphorus chemistry often begins by partitioning phosphorus into labile and refractory components, especially to index phosphorus availability for plant growth.11 Phosphorus isotope techniques have been used to study phosphorus dynamics,12 but the short-half-life of 32P and the high cost of 33P limit the use of such experiments. Another approach for defining phosphorus pools is by addition of phosphatase enzymes to isolate biologically degradable phosphorus, although this is usually restricted to soluble pools of phosphorus.13 Gressel et al. used a combined approach to characterize and link the biochemical forms of both phosphorus and carbon using 31P and 13C NMR techniques, revealing correlations between phosphorus mineralization and litter Received: Revised: Accepted: Published: 4775

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Table 1. Sample Information location wetland type vegetation dominant species

tropical Raphia swamp

tropical sawgrass bog

subtropical marsh

boreal Spaghnum bog

Bocas del Toro, Panama peat dome forested Raphia taedigera

Bocas del Toro, Panama peat dome herbaceous/stunted trees Campnosperma panamensis, Cyrilla racemif lora

Florida, USA ephemeral herbaceous/open Pontedaria cordata var. lancifolia, Juncus ef fusus

Alaska, USA bog moss Sphagnum, Carex sp.

decomposition.10 Despite this, there remains little information on the mechanisms of association of phosphorus with soil organic matter in wetland soils. Detailed chemical characterization of soil organic matter can be accomplished with solid state 13C NMR spectroscopy using cross-polarization and magic angle spinning (CP-MAS 13C NMR).14 The technique provides information on functional groups within intact organic matter with little if any need for chemical or physical pretreatments. CP-MAS 13C NMR has clear advantages over conventional wet chemical procedures that can involve lengthy and often inefficient extractions that may alter the material under investigation.15 Here we address the link between phosphorus stability and organic carbon composition in a series of four contrasting wetlands soils. Our approach was to combine solution 31P and solid state 13C NMR spectroscopy of wetland soils before and after hydrofluoric acid treatment. Geochemists and soil chemists have long used hydrofluoric acid (HF) to concentrate organic matter in soils and remove paramagnetic elements prior to solid state 13C NMR spectroscopy.16 Our hypothesis is that phosphorus associated with metals (e.g., Fe) or in organic forms sorbed to anionic sorption sites will be removed during HF treatment and thus represent phosphorus fractions that would be affected by environmental changes that disrupt these ionic associations (e.g., changes in pH and redox conditions, flooding and/or rewetting).17 Conversely, phosphorus that persists following HF pretreatment must be part of the stable (e.g., humified) soil organic matter fraction and therefore represents a pool of sequestered organic phosphorus that will be resistant to remobilization as long as the soil organic matter remains stable.18

Table 2. Physiochemical Characteristics of Soilsa

bulk density (g cm−3) pH TOC before HF (%) TOC after HF (%) mass recovery after HF (%) alkyl/O-alkyl TN mg g−1 TP mg g−1 total metals mg g−1 Al Fe Mg Ca

Raphia swamp

sawgrass bog

marsh

Spaghnum bog

0.062 3.9 46 53 81 1.6 29 1.1

0.033 3.4 49 58 75 0.5 20 0.43

0.639 4.2 39 54 53 1.4 nd 0.55

0.076 5.0 36 49 83 0.4 18 1.1

1.8 4.7 0.50 1.4

0.6 1.2 0.44 0.6

2.1 1.2 0.77 4.5

3.2 12 0.76 2.6

a

Alkyl/O-alkyl ratio represents a humification factor introduced by Baldock et al.14

N content. All samples were analyzed in quadruplicate. For analysis of the total concentrations of soil Al, Fe, Mg, and Ca, samples were digested by nitric and hydrochloric acids. After they were cooled, the samples were made up to volume, were mixed, and were allowed to settle overnight prior to analysis. Digests were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Total P in soils prior to HF treatment was determined by combustion of soil at 550 °C in a muffle furnace for 4 h, dissolution of the ash in 6 mol L−1 HCl, and then detection of soluble reactive P (SRP) using a segmented flow analyzer (AAII Technicon, SEAL Analytical, U.K.) and standard molybdate colorimetry. Total P (after acid digestion) and inorganic P (before acid digestion) in NaOH-EDTA extracts in all samples before and after HF treatment were measured using standard molybdate colorimetry, and organic P calculated by difference. All phosphorus data are included in Supporting Information Table S1, including relative amount of P extracted by NaOHEDTA. Soil−water content was determined as weight loss following drying at 70 °C for 72 h. Soil pH was determined on a 1:2 soil to water suspension using a glass electrode and soil bulk density was calculated using the known sample volume and determined water content. Acid Pretreatment. Five grams of soil were placed into a 125 mL polyethylene centrifuge tube along with 10 mL of 10% HF prepared following the method of Robl and Davis.22 The soil/HF slurries were shaken end-to-end for 1 h, centrifuged at 1790 g for 10 min, and then filtered through Whatman grade GMF 2UM qualitative filter paper (Whatman International, England). The HF treatment was performed four times for 1 h, then twice for 24 h, with the supernatant being discarded each time. The residue was washed five times with ultrapure water

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EXPERIMENTAL METHODS Study Sites. Soils were collected from wetland sites chosen to represent a wide range of soil carbon sources and site nutrient status (Table 1). These were a boreal Sphagnum dominated peatland (Alaska), a tropical Raphia taedigera palm swamp and tropical ombrotrophic peat dome (Panama),19 and subtropical ephemeral wetlands within an agricultural landscape (Florida).20 All sites were highly organic peatlands except the Florida marsh, which has a relatively high mineral content (bulk density, Table 2) consisting of uncoated Quartz grains.21 In all of these soils, phosphorus dynamics are dominated by interactions with organic matter.21 At each site a single surface (0−10 cm) core (diameter 7.5 cm) was collected and transferred to the laboratory. Samples were homogenized and solid particles (>2 mm) and recognizable plant fragments removed by hand. Samples were oven-dried (70 °C, 72 h) and ground, with subsamples stored in airtight containers under ambient lab conditions until analysis. Elemental Analysis. A Thermo Finnigan Elemental Analyzer (Flash EA 1112) was used to determine the C and 4776

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duplicates; 9% for orthophosphate, 10% for total organic phosphorus, 16% for DNA. Pyrophosphate was totally removed by HF treatment. 13 C Solid State NMR Spectroscopy. Solid state CP-MAS 13 C spectra were acquired with high power proton decoupling on a Varian Unity-Inova 500 MHz spectrometer operating at 125 MHz for carbon. Depending on the density of the sample, approximately 500 mg of the treated and untreated dried soil from each site sampled were packed with 100 mg of an internal standard of sodium trimethyl silanolate (Na-TMS) into a 7 mm (outside diameter) solid state rotor with a Kel-F cap and spun at 5 kHz. Spinning sidebands were eliminated using the total suppression of sidebands (TOSS) sequence. Between 32000 and 45000 scans were accumulated using a 90° pulse of 6.5 μs pulse width and a 3 s pulse delay. According to inversionrecovery experiments, those delays were longer than five times the 1H spin−lattice relaxation time required to avoid saturation. From a series of variable contact time experiments, a 750 μs CP contact time was determined to yield the most representative spectra. Chemical shifts were externally referenced to the 41.5 ppm resonance of glycine. The internal standard Na-TMS was used to normalize the integrated intensity across each of the spectral windows of the different samples. The quantitative reliability of the solid state NMR experiments was evaluated using spin counting to calculate an “observability factor”. Our spin counting experiments differed from those used previously, in that we employed an internal standard instead of an external reference sample, an approach that avoids variations in sample properties and NMR conditions. The intensity of signal per unit mass of carbon for each spectrum was determined from the intensity of the internal standard Na-TMS peak, which is well-resolved from the rest of the signals in these spectra. The intensity of the entire spectrum was then integrated, the intensity of the internal standard subtracted, and the resulting corrected spectral intensity used to calculate the “observed” amount of carbon. This observed carbon was then compared to the carbon determined by combustion analyses, and the resulting ratio (as %) is the observability factor. These observability factors for untreated and HF-treated soils are summarized in Supporting Information Table S3. The relative distribution of carbon groups in different structures was determined by integrating the signal intensities over defined chemical shift windows. These spectral windows and the structures they represent were 0−50 ppm (alkyl C), 50−110 ppm (O-alkyl C), 110−160 ppm (aromatic C), and 160−220 ppm (carbonyl C found in carboxylic acids, esters amides ketones and aldehydes).27

(18 MΩ), lyophilized, and then stored in glass vials until further analysis. Organic carbon concentrations in soil samples were measured before and after HF treatment. Total carbon in the treated samples was compared to that in the untreated soils and, after accounting for mass losses due to removal of inorganic species, 97%, 95%, 90%, and 99% of the organic matter remained intact after the treatment in Raphia swamp, sawgrass bog, marsh, and Sphagnum bog samples, respectively. Elemental Al, Fe, Mg, and Ca in one sample, Raphia swamp, were measured before and after HF. In this sample, 62% of Al, 81% of Fe, 100% of Mg, and 71% of Ca was removed by the HF extraction. These data are summarized in Supporting Information Table S2. NaOH−EDTA Extraction and Solution 31P NMR Spectroscopy. NaOH−EDTA-extractable phosphorus was obtained before and after the HF pretreatment. Phosphorus was extracted by shaking 2 g of air-dried or acid pretreated soil with 40 mL of extracting solution (a 50:50 mixture of 0.5 M NaOH and 0.1 M EDTA) for 24 h at room temperature.23 Samples were centrifuged at 1790 g for 10 min and the supernatant drawn off for lyophilization. The freeze-dried soil extract was ground in a mortar and pestle to optimize their subsequent dissolution. Two hundred milligrams of the ground extract was mixed with 1 mL of a mixture of D2O and 1 M NaOH. The base was used to increase the pH of the reconstituted extract to greater than 13, ensuring chemical shifts in the NMR spectra consistent with those in the literature and maximizing the shifts of different species.24 The solution was centrifuged (1300 g) for 20 min to remove particulate matter and the supernatant was transferred to a 5 mm NMR tube for analysis. Solution 31P NMR spectra were obtained using a Bruker 600 MHz high resolution NMR spectrometer operating at 242.9 MHz for phosphorus. A 45° pulse was implemented to allow for shorter total experiment time. The pulse delay was 2 s, which was at least five times the T1 value of the orthophosphate resonance determined in preliminary inversion−recovery experiments. Recycling delays of 2 s, when added to the acquisition time (410 ms), are reasonable for using a 45° pulse.25 Peak areas were determined by integration over predetermined spectral regions using Bruker proprietary software. All chemical shifts are expressed in parts per million and referenced to an external standard of 85% H3PO4. An internal standard of methylene diphosphonic acid (MDP) was used for quantification.26 The chemical shift of MDP (18.2 ppm) was determined relative to 85% phosphoric acid (0.0 ppm). The internal standard solution was placed in a coaxial stem insert inside the 5 mm NMR sample tube to position it in the magnetic field without mixing with the sample. Peak assignments followed those of Turner et al.24 The precision of the 31P NMR analysis and combined HF treatment and 31P NMR analysis procedure was assessed by extracting a sample soil before and after HF treatment in duplicate and analyzing each extract separately. The mean concentration of total phosphorus in the soil before HF determined as the sum of all signals based on the MDP internal standard was 930 ± 15.6 mg P kg−1. The coefficient of variation for concentrations of individual compounds was smallest for pyrophosphate (3%) and largest for DNA (16%). When calculated as the proportion (%) of the extracted P, coefficients of variation were 6% for orthophosphate, 8% for total organic phosphorus, 10% for DNA, and 2% for pyrophosphate. The coefficients of variation were generally larger for the HF treated

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RESULTS AND DISCUSSION Phosphorus Speciation before and after HF Treatment. Chemical properties of soils are summarized in Table 2. All soils were acidic, with pH values ranging from 3.4 to 5.0. It is largely accepted that Al and Fe oxides are the predominant phosphorus absorption sites in acidic to neutral soils, whereas sorption is predominantly on Ca, Mg, and carbonate sites in soils with pH > 7.28 Because all the soils in this study were acidic, phosphorus sorption should be controlled by Al and Fe. Bulk densities were generally low, typical of ombrotrophic and spagnum moss peat systems,29 but significantly higher for the Florida marsh soil, which was attributed to a greater sand content. Total phosphorus concentrations were higher in the Raphia swamp and Sphagnum bog soils relative to those from the sawgrass bog and marsh sites. Total organic carbon 4777

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Figure 1. 31P NMR spectra of NaOH-EDTA extract of tropical Raphia swamp soil before and after HF treatment. Intensities are scaled so that the absolute intensities of the internal standard peak at 18.2 ppm are equal. Slight changes in peak locations due to variations in pH of extracts due to HF treatment.

matter pasture soils, in which HF treatment removed >98% of the inorganic orthophosphate.30 However, our results are consistent with a previous report of orthophosphate associated with organic matter in NaOH extracts of high organic matter wetland soils from the Florida Everglades.31 Inorganic phosphorus stabilization in acidic soils occurs mainly through sorption on the surface of soil minerals if any Fe or Al oxide coatings are present.32 HF breaks Si−O bonds, leading to the solubilization of these minerals. Phosphorus adsorbed to the minerals will then be released and lost during the treatment.33 The most surprising aspect of our data is the amount of orthophosphate retained in the carbon matrix in these soils. Orthophosphate was most retained in the subtropical marsh soil after the HF treatment (55% retained) when compared to the tropical Raphia swamp (32%), tropical sawgrass bog (14%) and boreal Sphagnum bog (17%) soils. Turner et al. also saw considerable association of orthophosphate with organic matter (as precipitated humic acids) in NaOH extracts of similar soils following HCl pretreatment.31 While it is possible that the persistent appearance of orthophosphate is the result of degradation of organic phosphorus during the alkaline extraction, data presented by Smernik and Dougherty and Cheesman et al. suggest that little if any orthophosphate is formed during this extraction, even when a large amount of organic phosphorus is present.34,26a Organic Phosphorus. After HF treatment the amounts of both phosphonates and phosphate monoesters (O-mono) decreased only slightly in the Raphia swamp and Florida marsh soils. In contrast, significant decreases in these phosphorus

increased significantly after HF treatment in all soils (Table 2). This increase was more significant in the marsh and Alaskan bog sites and is most likely due to the higher mineral contents of these soils. Trends in phosphorus speciation are best described by comparing the amounts of the various phosphorus species identified by 31P NMR before and after acid treatment. Figure 1 includes such spectra of NaOH−EDTA extracts from the tropical Raphia swamp. Intensities of peaks in these spectra are normalized to the internal standard peak at 18.2 ppm. Since the same amount of MDP was used for all samples, intensities in the spectra of Figure 1 are quantitatively representative of the individual phosphorus species observed. For this tropical Raphia swamp, a relatively small decrease in orthophosphate was observed, while phosphonates and phosphate monoesters increased as a result of the concentration of stable organic matter in the soil after HF treatment. Inorganic Phosphorus. Figure 2 summarizes the effects of the acid treatments on phosphorus speciation in soils from the four sites. Here we have reported phosphorus concentrations relative to the initial mass of soil so as to account for mass losses of inorganic components that accompany HF treatment. Pyrophosphate was completely removed by the acid pretreatment, clearly indicating associations with anionic sorption sites or presence in live microbial cells. Inorganic orthophosphate was not completely removed, decreasing by between 45 and 83%, indicating that relatively large amounts were physically/ chemically stabilized through associations with soil organic matter. This differs markedly from a study of low organic 4778

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Figure 2. Distributions of phosphorus species before and after HF treatment in soils from (a) tropical Raphia swamp, (b) tropical sawgrass bog, (c) subtropical marsh, and (d) boreal Sphagnum bog sites. All values are expressed relative to the initial amount of soil in the extract.

species were observed in the sawgrass bog and Sphagnum bog soils. Interestingly, despite differences in phosphorus retention between the Raphia swamp and Sphagnum bog soils, they have similar bulk densities (∼0.07 g cm−3) and showed an enhancement in the total organic carbon fraction after HF treatment (Table 2). However, the organic matter fractions of these soils reacted differently to the HF treatment. The 13C NMR spectra of the Raphia swamp soil before and after treatment were quite similar (Figure 3a), with only a slight increase in carbon

intensities of all carbon functional groups (Figure 3b). This increase in signal intensity is greater than can be attributed to the concentration effect alone (Figure 4d) and indeed we observe an increase in carbon “observability” from 94 to 102% for this sample. It should be noted that the Sphagnum bog soil also has a much higher Al and Fe content than the Raphia swamp. The effects of paramagnetic metals on 13C NMR spectra have been the subject of numerous studies, some of which concluded that such metals effect only selected functional groups within soil organic matter, while others concluded that the effects were spread throughout the organic matter matrix. Preston and Newman, for example, attributed rapid relaxation of carbohydrates and aromatic components in de-ashed humin to association with iron.35 Schilling and Cooper identified carboxyl, hydroxyl, and carbohydrate copper binding sites in a peat soil through loss of signal intensities when copper was added to the soil.36 Smernik and Oades observed both nonselective and selective signal losses that they attributed to either (1) field inhomogeneity because of bulk magnetic properties or (2) interactions between nuclear and electronic spins, respectively.37 Schöning et al. also observed both nonselective and selective losses in signal intensities and noted that their data contradicted that of some previous studies.38 They suggested that the selective effects of paramagnetics may be masked in soils with high organic carbon to iron ratios, which is certainly true in the Sphagnum bog sample here Phospholipids were detected in very low concentrations after HF treatment. Because of the rapid hydrolysis of some phospholipids and RNA in alkaline extracts, the proportion of organic phosphorus determined as phosphodiesters is likely to be underestimated.24 A significant portion of organic phosphorus might be associated with relatively refractory organic compounds. Paing et al. observed that humic-bound phosphorus accounted for about 30% of organic phosphorus in sediments from a eutrophic lagoon.39 The stability of the organic phosphorus

Figure 3. 13C NMR spectra of (a) tropical Raphia swamp and (b) boreal Sphagnum bog soils before and after HF treatment.

observability (e.g., spins observed per unit carbon) from 95 to 97% (Supporting Information Table S3). The slight increase in the NMR signal is thus due almost entirely to the concentration of organic matter after HF. In contrast, the spectra of the Sphagnum bog soil showed a more significant increase in 4779

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Figure 4. Distribution of major organic carbon species before and after HF treatment in soils from (a) tropical Raphia swamp, (b) tropical sawgrass bog, (c) subtropical marsh, and (d) boreal Sphagnum bog sites. Carbon concentrations are normalized for actual amount of soil and do not take into account enrichment due to removal of inorganic material after HF treatment.

groups represented only ∼30% of soil carbon. The alkyl to O-alkyl ratio for these two soils was >1 (Raphia swamp = 1.6, Florida marsh = 1.4), which indicates a preferential loss of carbohydrates and humification of the organic matter.14 In contrast, the soils in which organic phosphorus was significantly reduced by HF treatment, tropical sawgrass bog and boreal Sphagnum bog, contained organic matter dominated by O-alkyl groups (∼54% of the total 13C NMR signal for the tropical bog soil and ∼64% of the signal for the boreal bog soil). Relatively minor contributions from alkyl groups to the total organic carbon fraction were noted for these soils. For tropical sawgrass bog and boreal Sphagnum bog soils the alkyl to O-alkyl ratio was 0.5 and 0.4, respectively, reflecting weakly decomposed and less stable organic matter. We have introduced HF pretreatment as a method for differentiating phosphorus chelated to metals or sorbed to anionic sorption sites from phosphorus incorporated into soil organic matter. Combined results from solution 31P and solid 13 C NMR revealed different behavior for soils sharing similar physical properties. Relatively little phosphorus was removed after HF treatment in soils dominated by alkyl and aromatic groups compared to soils dominated by O-alkyl groups. These differences support the hypothesis that phosphorus will form associations with humified organic matter consisting primarily of alkyl and aromatic functional groups. Conversely, in soils dominated by carbohydrates, phosphorus appears to be predominantly bound to free metals or minerals. Metal- and mineral-phosphorus complexes are highly influenced by the oxidation/reduction state of the metals and by water content.17b Furthermore, a recent study reported a correlation between organic matter content of marsh soils and release of P after flooding.5 Our data are consistent with those results and suggest that sequestration of phosphorus in wetlands may be tightly linked to soil carbon humification, the process that converts labile biopolymers into stable geopolymers. Once sequestered in this way, release of phosphorus into overlying

within the humic−phosphorus complex depends mainly on the stability of the organic matter and degree of diagenesis of the accreted soil.6,40 A recent incubation study to determine the effect of a single dry-rewetting cycle on the mineralization of carbon, nitrogen and phosphorus showed that the changes in available carbon and phosphorus were highly variable between thirty two soils, and this was attributed to the dependence of mineralization on the composition and chemical nature of organic material.41 Baldock et al., using CP-MAS 13C NMR, assumed that advanced decomposition of soil organic matter results in a relative preservation of alkyl C.42 Zech et al. showed that aliphatic structures such as long chain fatty acids may be selectively preserved during SOM decomposition.43 Conversely, Schmidt et al. and Preston et al. observed that some carbohydrates are easily solubilized and removed from soils and sediments by circulating water.44 Schilling and Cooper analyzed HF treated mineral-rich soils (Ultisols and Spodosols) by solid state 13C NMR and reported a relative decrease in signal intensity in the O-alkyl C (carbohydrates) chemical shift region, with a simultaneous relative increase in signal intensity in the regions assigned to carboxylic C and aromatic C.16 This was attributed to the loss of water-soluble carbohydrates during the HF treatment and an increase in signals of carboxyl and aromatic C due to the removal of paramagnetic species associated with these carbon groups. Our results from the solid state 13C NMR before and after HF treatment for the four soils are in general agreement with those findings. Taken together with the organic phosphorus data, it appears that the behavior of organic phosphorus is tightly linked to the composition of organic matter in these soils. Figure 4 includes the abundances of different carbon groups before and after acid treatment for the four soils. The tropical Raphia swamp and subtropical marsh soils in which the organic phosphorus is largely unaffected by the HF treatment are both dominated by alkyl functional groups that represent about 50% of the total soil organic carbon before the HF treatment, whereas O-alkyl 4780

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(12) Di, H. J.; Condron, L. M.; Frossard, E. Isotope techniques to study phosphorus cycling in agricultural and forest soils: A review. Biol. Fert. Soils 1997, 24 (1), 1−12. (13) Turner, B. L.; McKelvie, I. D.; Haygarth, P. M. Characterisation of water-extractable soil organic phosphorus by phosphatase hydrolysis. Soil Biol. Biochem. 2002, 34 (1), 27−35. (14) Baldock, J. A.; Oades, J. M.; Nelson, P. N.; Skene, T. M.; Golchin, A.; Clarke, P. Assessing the extent of decomposition of natural organic materials using solid-state 13C NMR spectroscopy. Aust. J. Soil Res. 1997, 35 (5), 1061−1083. (15) Schnitzer, M. The in situ analysis of organic matter in soils. Can. J. Soil Sci. 2001, 81 (3), 249−254. (16) Schilling, M.; Cooper, W. T. Effects of chemical treatments on the quality and quantitative reliability of solid-state 13C NMR spectroscopy of mineral soils. Anal. Chim. Acta 2004, 508 (2), 207− 216. (17) (a) Celi, L.; Lamacchia, S.; Marsan, F. A.; Barberis, E. Interaction of inositol hexaphosphate on clays: adsorption and charging phenomena. Soil Sci. 1999, 164 (8), 574−585. (b) Moore, P. A.; Reddy, K. R.; Fisher, M. M. Phosphorus flux between sediment and overlying water in Lake Okeechobee, Florida: Spatial and temporal variations. J. Environ. Qual. 1998, 27 (6), 1428−1439. (18) Jordan, S.; V. S., Zeitz, J. The influence of degree of peat decomposition on phosphorus binding forms in fens. Mires Peat 2007, 2, 1−10. (19) Sjögersten, S.; Cheesman, A. W.; Lopez, O.; Turner, B. L. Biogeochemical processes along a nutrient gradient in a tropical ombrotrophic peatland. Biogeochemistry 2011, 104 (1−3), 147−163. (20) Cheesman, A. W.; Dunne, E. J.; Turner, B. L.; Reddy, K. R. Soil Phosphorus Forms in Hydrologically Isolated Wetlands and Surrounding Pasture Uplands. J. Environ. Qual. 2010, 39 (4), 1517− 1525. (21) Harris, W. G.; Rhue, R. D.; Kidder, G.; Brown, R. B.; Littell, R. Phosphorus retention as related to morphology of sandy coastal plain soil materials. Soil Sci. Soc. Am. J. 1996, 60 (5), 1513−1521. (22) Robl, T. L.; Davis, B. H. Comparison of the HF-HCl and HFBF3 maceration techniques and the chemistry of resultant organic concentrates. Org. Geochem. 1993, 20 (2), 249−255. (23) Cade-Menun, B. J.; Preston, C. M. A comparison of soil extraction procedures for P-31 NMR spectroscopy. Soil Sci. 1996, 161 (11), 770−785. (24) Turner, B. L.; Mahieu, N.; Condron, L. M. Phosphorus-31 nuclear magnetic resonance spectral assignments of phosphorus compounds in soil NaOH-EDTA extracts. Soil Sci. Soc. Am. J. 2003, 67 (2), 497−510. (25) Cade-Menun, B. J.; Liu, C. W.; Nunlist, R.; McColl, J. G. Soil and litter phosphorus-31 nuclear magnetic resonance spectroscopy: Extractants, metals, and phosphorus relaxation times. J. Environ. Qual. 2002, 31 (2), 457−465. (26) (a) Cheesman, A. W.; Turner, B. L.; Inglett, P. W.; Reddy, K. R. Phosphorus transformations during decomposition of wetland macrophytes. Environ. Sci. Technol. 2010, 44 (24), 9265−9271. (b) Turner, B. L. Soil organic phosphorus in tropical forests: an assessment of the NaOH-EDTA extraction procedure for quantitative analysis by solution P-31 NMR spectroscopy. Eur. J. Soil Sci. 2008, 59 (3), 453−466. (27) Kogel-Knabner, I. The macromolecular organic composition of plant and microbial residues as inputs to soil organic matter. Soil Biol. Biochem. 2002, 34 (2), 139−162. (28) Syers, J. K.; Harris, R. F.; Armstrong, D. E. Phosphate chemistry in lake sediments. J. Environ. Qual. 1973, 2, 1−14. (29) Yoshikawa, K.; Overduin, P. P.; Harden, J. W. Moisture content measurements of moss (Sphagnum spp.) using commercial sensors. Permafrost and Periglacial Processes 2004, 15 (4), 309−318. (30) Dougherty, W. J.; Smernik, R. J.; Bunemann, E. K.; Chittleborough, D. J. On the use of hydrofluoric acid pretreatment of soils for phosphorus-31 nuclear magnetic resonance analyses. Soil Sci. Soc. Am. J. 2007, 71 (4), 1111−1118.

water would be minimized as long as the soil organic matter was stabilized.

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ASSOCIATED CONTENT

S Supporting Information *

Tables showing elemental concentrations of Al, Fe, Mg, and Ca in Raphia swamp soils, phosphorus concentrations in wetland soils, and carbon observability factors for untreated and HF treated wetland soils. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 850-644-6875. Fax: 850-644-8281. Present Address #

Department of Chemistry & Physics, West Virginia University Institute of Technology, Montgomery, WV 25136, U.S.A. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by a grant from the U.S. Department of Agriculture (CSREES 2004-35107-14918). 31P and 13C NMR spectra were acquired in the Florida State University Department of Chemistry and Biochemistry NMR facility. The assistance of Dr. Tom Gedris, Staff Supervisor, is greatly appreciated.

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