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Journal of Environmental Quality

TECHNICAL REPORTS TECHNICAL REPORTS ORGANIC COMPOUNDS IN THE ENVIRONMENT

Sulfamethazine Sorption to Soil: Vegetative Management, pH, and Dissolved Organic Matter Effects Bei Chu, Keith W. Goyne,* Stephen H. Anderson, Chung-Ho Lin, and Robert N. Lerch

S

ulfamethazine (4-amino-N-[4, 6-dimethyl-2-pyrimidinyl] benzenesulfonamide) (SMZ) is one of the sulfonamide veterinary antibiotics (VAs) commonly used for livestock and aquaculture disease treatment, disease prevention, and growth promotion (Sarmah et al., 2006). In the United States, approximately 3.63 × 105 kg of SMZ is used annually as a feed additive for cattle and swine production (Mellon et al., 2001). Like most VAs, SMZ is poorly absorbed and metabolized after entering a human or animal body, and the typical SMZ concentration in manure and manure slurry is on the order of 3 to 35 μmol kg−1 (Haller et al., 2002; Kumar et al., 2005; Burkhardt et al., 2005; Shelver et al., 2010). Sulfamethazine concentrations in soil fertilized with manure range from nondetectable to several micromoles per kilogram (Hamscher et al., 2005; Aust et al., 2008). In surface runoff from manured plots, SMZ concentrations can reach 2.5 to 6.8 μmol L−1 (Burkhardt et al., 2005; Kreuzig et al., 2005). Additionally, 1 to 4% of the initial quantity of sulfonamides applied to test plots can be lost via short (1000 Da dissolved organic matter (DOM>1000 Da) on sulfamethazine (SMZ) behavior in soil. Sorption experiments were performed over a range of SMZ concentrations (2.5–50 μmol L−1) using samples from three soils (Armstrong, Huntington, and Menfro), each planted to one of three vegetation treatments: agroforestry buffers strips (ABS), grass buffer strips (GBS), and row crops (RC). Our results show that SMZ sorption isotherms are well fitted by the Freundlich isotherm model (log Kf = 0.44–0.93; Freundlich nonlinearity parameter = 0.59–0.79). Further investigation of solid-to-solution distribution coefficients (Kd) demonstrated that vegetative management significantly (p < 0.05) influences SMZ sorption (ABS > GBS > RC). Multiple linear regression analyses indicated that organic carbon (OC) content, pH, and initial SMZ concentration were important properties controlling SMZ sorption. Study of the two most contrasting soils in our sample set revealed that increasing solution pH (pH 6.0– 7.5) reduced SMZ sorption to the Armstrong GBS soil, but little pH effect was observed for the Huntington GBS soil containing 50% kaolinite in the clay fraction. The presence of DOM>1000 Da (150 mg  L−1 OC) had little significant effect on the Freundlich nonlinearity parameter; however, DOM>1000 Da slightly reduced SMZ Kd values overall. Our results support the use of vegetative buffers to mitigate veterinary antibiotic loss from agroecosystems, provide guidance for properly managing vegetative buffer strips to increase SMZ sorption, and enhance understanding of SMZ sorption to soil.

Copyright © American Society of Agronomy, Crop Science Society of America, and 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.

B. Chu, K.W. Goyne, and S.H. Anderson, Dep. of Soil, Environmental and Atmospheric Sciences, Univ. of Missouri, 302 ABNR Building, Columbia, MO 65211; C.-H. Lin, Center for Agroforestry, Univ. of Missouri, 203 ABNR Building, Columbia, MO 65211; R.N. Lerch, USDA–ARS, Cropping Systems and Water Quality Research Unit, 269 Ag. Eng. Bldg., Columbia, MO 65211. Assigned to Associate Editor K.G. Karthikeyan.

J. Environ. Qual. 42:794–805 (2013) doi:10.2134/jeq2012.0222 Supplemental data file is available online for this article. Received 30 May 2012. *Corresponding author ([email protected]).

Abbreviations: ABS, agroforestry buffers strip; CBD, citrate–bicarbonate– dithionite; DOM, dissolved organic matter; GBS, grass buffer strip; HPLC, Highperformance liquid chromatography; OC, organic carbon; RC, row crop; SMZ, sulfamethazine; VA, veterinary antibiotic.

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other sulfonamides can be absorbed by vegetable crops produced on manure-amended soils, thereby resulting in VA entry into the food chain (Dolliver et al., 2007). The environmental fate and transport of organic contaminants is strongly affected by compound interactions with soil. Sulfonamides are weak organic acids, with pKa values in the same range as the pH of most soil solutions (pH 4.5– 7.5). Due to their ionic nature, sulfonamides convert to anionic species as pH increases, resulting in greater repulsion from negatively charged soil surfaces and less sorption (Kurwadkar et al., 2007). The reported sorption coefficient (Kd) values for sulfonamides range from 0.9 to 10 L kg−1. Therefore, VAs in this class are likely to be highly mobile in soil (ThieleBruhn, 2003), and it is necessary to identify conservation and land management techniques that reduce the transport of sulfonamides and other VAs to water resources. Vegetative buffer strips are a conservation management practice commonly used in agriculture. Agroforestry buffer strips (ABS) (a combination of trees, shrubs, and grasses) and grass buffer strips (GBS) can improve soil hydraulic properties (Seobi et al., 2005) and reduce surface runoff (Udawatta et al., 2002). These buffers also have the potential to reduce agricultural pollutant losses from croplands, including VAs (Chu et al., 2010; Lin et al., 2010; Lin et al., 2011), although work investigating the use of vegetative buffer strips to reduce VA transport is limited. Mechanisms of pollutant removal by vegetative buffer strips include improving flow hydraulics, physical filtration by vegetation, pollutant adsorption to roots, biodegradation in the root zone, direct plant uptake, and enhanced sorption and transformation from improved soil properties (Barling and Moore, 1994; Burken and Schnoor, 1997; Mandelbaum et al., 1995). In particular, this study focuses on sorption of SMZ to vegetative buffer strip soils. Although previous studies investigating sulfonamide sorption to soils (Accinelli et al., 2007; Boxall et al., 2002; Chu et al., 2010; Lertpaitoonpan et al., 2009; Thiele-Bruhn et al., 2004) have provided useful information on the interactions of these drugs with soils, few studies have examined the effects of dissolved organic matter (DOM) on sulfonamide sorption to soil or clay minerals (Thiele-Bruhn et al., 2004; Essington et al., 2010). Dissolved organic matter has been demonstrated to enhance the transport of various organic compounds in soils (Kulshrestha et al., 2004; Zhang et al., 2012; Seol and Lee, 2000). In manured agroecosystems, manure-derived DOM and VAs are likely to be present in surface water runoff or shallow subsurface solution (e.g., soil solution moving laterally over a restrictive subsurface soil horizon) flowing into a vegetative buffer strip. Thus, elucidating DOM effects on VA sorption is particularly important when considering the use of vegetative buffer strips to mitigate VA transport. The objectives of this study were (i) to measure the sorption and retention of SMZ to soil materials collected from three different soils each planted to ABS, GBS, and row-crops (RC) and determine the soil physical and chemical properties governing antibiotic sorption to these soils and (ii) to investigate changes in VA sorption and retention to vegetative buffer strip and cropland soils in the presence of manure-derived DOM.

www.agronomy.org • www.crops.org • www.soils.org

Materials and Methods Sulfamethazine Sulfamethazine (>99% purity) (Fig. 1) was obtained from Sigma-Aldrich, and 14C-labeled sulfamethazine [phenyl-ring14 C(U)] was purchased from American Radiolabeled Chemicals, Inc. Sulfamethazine has a molecular mass of 278.33 g mol−1 and an octanol-water partition coefficient (log Kow) of 0.89 (Tolls, 2001). The pKa values of SMZ are 2.07 and 7.49 (Qiang and Adams, 2004), and SMZ may exist in solution as four different species: cationic SMZ (SMZ+), neutral and zwitterionic SMZ (SMZo), and anionic SMZ (SMZ−) (Fig. 1) (Sakurai and Ishimitsu, 1980).

Dissolved Organic Matter Extraction Turkey litter (manure and bedding materials) was obtained from an organic turkey farm where SMZ was not administered to livestock. Turkey litter was selected because poultry production is common and economically important in many midwestern U.S. states and because litter from these operations is often applied to row-crops and pastures. Dissolved organic matter was extracted by adding 250 g (oven-dry mass) of turkey litter to 2 L of ultrapure water. The mixture was shaken on a platform shaker at high speed for 2 h in dark, followed by centrifugation at 34,400 g for 20 min

Fig. 1. (a) Chemical structure of sulfamethazine (SMZ) and (b) distribution of cationic (SMZ+), neutral and zwitterionic (SMZ0), and anionic (SMZ−) species in aqueous solution as a function of pH. 795

and filtration through a tandem glass fiber prefilter (Millipore AP20 and AP15) and a 0.45-μm nominal pore size Durapore membrane filter (Millipore Corp.). After filtration, the DOM solution was placed in dialysis tubing (Spectra/Por RC 1000 Da molecular weight cutoff ; Spectrum Laboratories Inc.) and dialyzed against ultrapure water for 3 d. The 1000-Da molecular weight cutoff was chosen because previous work shows that lowermolecular-weight DOM has less binding capacity for organic contaminants (Chin et al., 1997; Seol and Lee, 2000). To minimize DOM degradation, dialysis was performed at 4°C in the dark. The dialyzed DOM solution was freeze-dried, thoroughly mixed, and stored in a desiccator. Details regarding characterization and properties of the >1000 Da DOM (DOM>1000 Da) can be found in the Supplemental Material section.

Sample Collection and Characterization Soil samples were collected in 2006 from grass and agroforestry buffer strips and croplands from three locations in Missouri. The three vegetative treatments were present at each sampling site, and the soil differed at each site. Soil collected at the University of Missouri (MU) Greenley Memorial Research Center (40°01′ N, 92°11′ W) was a somewhat poorly drained, Armstrong silt loam (fine, smectitic, mesic, Aquertic Hapludalfs). Clay minerals in this soil consisted of 63% smectite, 33% illite, and 4% kaolinite (techniques used to identify and semiquantitate clay mineralogy are provided in the Supplemental Material section). Row cropping at this site consists of no-till planting to a maize (Zea mays)–soybean (Glycine max) rotation. Primary species in the upland contour GBS were redtop (Agrostis gigantean Roth), bromegrass (Bromus spp.), and birdsfoot trefoil (Lotus corniculatus L.). Upland, contour ABS were planted to oak trees (Quercus sp.) and the same ground species as found in the GBS. Trees were approximately 9 yr old and 3 m in height at the time of sampling. Soil collected at the MU Horticultural and Agroforestry Research Center (39°01′ N, 92°45′ W) was a well drained Menfro silt loam (fine-silty, mixed, superactive, mesic, Typic Hapludalfs) with clay minerals consisting of 43.9% smectite, 39.4% illite, and 16.7% kaolinite. Tall fescue (Festuca arundinacea Schreb; Kentucky 31) was the primary species found in the GBS,

whereas the ABS consisted of eastern cottonwood trees (Populus deltoids Bortr. ex. Marsh.) planted into tall fescue. The trees were 5 yr in age and 4 to 6 m tall at the time of sampling. The row-crop sampling site was approximately 1.5 km from the MU Horticultural and Agroforestry Research Center. Historically, the row-crop site has been planted to a maize–soybean rotation and pasture, and the land was no-till planted to maize in the summer before sampling. Soil collected at the MU Agricultural Experiment Station Southwest Center (37°05′ N, 93°52′ W) was a well drained Huntington silt loam (fine-silty, mixed, active, mesic, Fluventic Hapludolls) with clay minerals consisting of 46.3% kaolinite, 45% illite, and 8.7% vermiculite. This site does not have vegetative buffers but contains similar vegetation types to the other sampling sites. The agroforestry sampling site consisted of 13-yr-old and 10-m-tall black walnut trees (Juglans nigra L.), and the primary ground vegetation species included orchardgrass (Dactylis glomerata L.), cheatgrass (Bromus tectorum L.), and henbit (Lamium amplexicaule L.). The grass sampling site consisted of a field of mixed grasses, with tall fescue being the predominant species. The cropland sampling area at this site was planted to a no-till maize-soybean rotation. Soil samples (0–10 cm depth) were randomly collected at multiple points from the GBS, ABS, and row-cropped areas present at each site. When sampling ABS subsites, samples were collected from multiple trees (three or four) at a distance of 30 to 50 cm from the base of the tree. Subsamples were bulked by vegetation type at each site, thus providing nine separate samples to be studied (Table 1). Collected soil samples were thoroughly mixed, air-dried, sieved to 1000 (150 mg L−1 OC) was conducted in a similar manner. However, Da mixtures of 14C-labeled SMZ and nonlabeled SMZ were used to achieve SMZ concentrations ranging from 2.5 to 50 μmol L−1. The specific activity of radiolabeled SMZ in each sample was 0.1 μCi mL−1. Dissolved organic matter was introduced into the system by dissolving freeze-dried DOM>1000 Da in CaCl2 background electrolyte solution and adding an aliquot of DOM>1000 Da solution to each reaction vessel. Desorption of SMZ from the Armstrong and Huntington GBS soils was performed immediately after sorption reactions conducted with 10 and 50 μmol L−1 initial SMZ concentrations in the presence and absence of manure-derived DOM>1000 Da. Desorption was achieved by adding a volume of background electrolyte solution equivalent to the volume of supernatant removed. Reaction vessels were agitated on an end-over-end shaker in the dark for 2 h. The samples were centrifuged, and supernatant solutions were removed for analysis. This procedure was repeated three additional times. Short-duration desorption steps were used in this study to simulate pulses of water moving through a VBS during multiple small runoff events that may occur as the intensity of precipitation changes throughout a storm event or during multiple small storm events occurring in succession. The sorption of SMZ to manure-derived DOM>1000 Da was investigated using the dialysis tube method (Carter and Suffet, 1982). In these experiments, the DOM is retained in the dialysis bag, and unbound SMZ is able to diffuse into the bag. Once entering into the bag, SMZ may remain unbound in solution or bind to the DOM. The concentration of unbound SMZ will be equivalent inside and outside of the bag after equilibrium. The www.agronomy.org • www.crops.org • www.soils.org

amount of SMZ bound to DOM can therefore be calculated as the difference in SMZ concentration inside and outside of the dialysis bag (Carter and Suffet, 1982). In brief, freeze-dried DOM>1000 Da was dissolved in background electrolyte solution (I = 0.01 mol L−1 CaCl2) to create solutions containing 25 and 150 mg L−1 OC. A 10-mL aliquot of DOM>1000 solution was transferred to dialysis tubing (Spectrum Spectra/ Da Por RC 1000 Dalton molecular cutoff ) with clip closures and dialyzed against ultrapure water for 24 h to remove any remaining DOM 1000 Da into 50-mL centrifuge tubes. A 40-mL aliquot of radiolabeled and nonlabeled SMZ mixture was added to the centrifuge tubes with CaCl2 background electrolyte and NaN3 as previously described. Ionic strength in this system may have been slightly reduced from 0.01 to 0.008 mol L−1 due to dialysis against ultrapure water to remove 1000 Da from dialysis tubing was less than 5% of the initial DOM>1000 Da concentration, and no sorption of SMZ to dialysis tubing was observed in preliminary trials. The range of SMZ concentration and ratios of radiolabeled and nonlabeled SMZ were the same as those used to investigate SMZ sorption to soil. The centrifuge tubes were wrapped in aluminum foil, placed on end-over-end shakers, and reacted in dark. Preliminary experiments examined an equilibrium time up to 5 d at pH 4.5, 6.0, and 7.5. Solutions from inside and outside of the dialysis tubes were transferred into scintillation vials for analysis of radioactivity, and the data were used to calculate the amount of SMZ bound to the DOM>1000 Da.

Analytical Techniques High-performance liquid chromatography (HPLC) with ultraviolet detection (Beckman Corp.) and a silica-based Columbus C8 column (4.6 mm × 250 mm; 5 μm; 110 Å pore size) (Phenomenex) was used to determine SMZ concentration in aqueous samples that did not contain 14C-labeled SMZ. The mobile phases for HPLC analysis consisted of 0.1% H3PO4 buffer (pH 2.2) and 100% acetonitrile, and the gradient method used a flow rate of 1 mL min−1. The ultraviolet absorbance at 254 nm was used for detection and quantification of SMZ in solution. Sulfamethazine calibration curves showed good linearity (r2 > 0.99). No detection interference was observed from the dissolved constituents, and the limit of detection for the HPLC analysis was 0.38 μmol L−1. For radiolabeled samples, 1 mL of solution was mixed with 4 mL scintillation cocktail (Ultima GoldTM AB, PerkinElmer), and sample radioactivity was analyzed using a Beckman LS 6000SC liquid scintillation counter with an analysis time of 1 min. The limit of detection for liquid scintillation counter analysis was 61.9 nmol L−1. A comparative study between the HPLC and LSC methods was performed, and the difference between the two techniques was 1000 Da in the reaction vessel. Adsorption and desorption data were fitted by the Freundlich isotherm model (q = KfCN), where Kf (Freundlich sorption coefficient) and N (measure of isotherm nonlinearity) are positive-value adjustable fitting parameters, and C (μmol L−1) is equilibrium SMZ concentration in solution after the adsorption or desorption reaction period. Freundlich parameters were obtained using nonlinear fitting methods developed by Bolster and Hornberger (2007) and Bolster and Tellinghuisen (2010).

Analysis of variance with Duncan’s multiple range test was used to analyze the sorption data using SAS statistical analysis software (SAS Inst., 2005). Treatments included three soils (Armstrong, Huntington, and Menfro) and three vegetative management systems (RC, GBS, and ABS) in a factorial design (two factors each with three levels). Statistical differences were tested at α = 0.05. Prediction equations for log Kd of SMZ as a function of soil properties and initial SMZ concentration were estimated using stepwise multiple regression analysis. A t test was used to compare differences in the percentage of SMZ remaining on the soils after one and four desorption cycles.

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Results and Discussion Influence of Vegetative Management and Soil on Sulfamethazine Sorption When SMZ only was present in the system, sorption isotherms (Fig. 2) were generally well fitted by the Freundlich equation (Table 2). The Freundlich sorption coefficient, log Kf, values ranged from 0.44 to 0.93, and the nonlinearity parameter (N) ranged from 0.59 to 0.79, indicating that all isotherms exhibited nonlinearity. A greater degree of isotherm nonlinearity often

Fig. 2. Sulfamethazine (SMZ) sorption to soil in the presence and absence of >1000 Da dissolved organic matter (DOM) (150 mg L−1 organic C) to (a) Armstrong row-crop (RC), (b) Armstrong grass buffer strip (GBS), (c) Armstrong agroforestry buffer strip (ABS), (d) Huntington RC, (e) Huntington GBS, (f) Huntington ABS, (g) Menfro RC, (h) Menfro GBS, and (i) Menfro ABS soils. Error bars represent the 95% confidence interval. 798

Journal of Environmental Quality

indicates heterogeneity of adsorption sites in a soil as well as specific compound interactions with soil organic matter (SOM) functional groups and mineral surfaces (Essington, 2004; ThieleBruhn et al., 2004). The range of values observed for log Kf and N were comparable to other SMZ studies (Kurwadkar et al., 2007; Lertpaitoonpan et al., 2009). The calculated Kd values for SMZ only (Fig. 3) ranged from 0.66 to 6.73 L kg−1, and these values were similar to previous studies (Boxall et al., 2002; ThieleBruhn et al., 2004; Sukul et al., 2008). Due to differences in the Freundlich N values, we were unable to directly compare Kf values (Chefetz et al., 2006; Chen et al., 1999; Gunasekara and Xing, 2003). Therefore, Kd values for SMZ sorption (Supplemental Table S1) were used to elucidate treatment differences. Sorption data were analyzed using an ANOVA model, and the results are shown in Table 3 (additional details regarding main factor effects can be found in Supplemental Fig. S1). Vegetation was a significant factor affecting Kd values for all concentrations studied. Duncan’s multiple range test results demonstrate that Kd values for SMZ were significantly greater (p < 0.05) for ABS than GBS soils and that Kd values associated with GBS soils were significantly greater than RC soils. Soil was also a significant factor affecting Kd values at all initial SMZ concentrations, with the exception of 18 μmol L−1. Additionally, Kd values for the Huntington soil were significantly greater than the Menfro soil, with the exception of 18 and 50 μmol L−1 data. The soil by vegetation interaction was also statistically significant (p < 0.05), indicating that both soil and vegetation type should be used to interpret Kd data; this interaction is shown in Fig. 3. Over the range of SMZ concentrations investigated, Kd values were greatest for GBS in the Armstrong soil, and Kd values for the ABS were significantly greater than GBS and RC for the Huntington and Menfro soils. Overall, our sorption data

indicated that the presence of perennial vegetation (i.e., GBS or ABS) enhanced SMZ sorption to soil. Greater sorption to soils under perennial vegetation likely contributed to the 70% reductions in SMZ transport when surface runoff was passed through vegetative buffers strips (Lin et al., 2011).

Correlation between Sulfamethazine Sorption and Soil Properties To explore the factors influencing SMZ sorption, stepwise multiple regression analysis was used to model the relationship between SMZ log Kd values and initial SMZ concentration, clay content, organic carbon content, pHs, CEC, percent base saturation, extractable Al and Fe content, and the percent of illite, vermiculite, smectite, and kaolinite found within the clay fraction. The resulting model indicated that that OC content, soil salt pH (pHs), and initial SMZ concentration (SMZ Conc.) were the most important variables controlling SMZ adsorption: log Kd = 0.706 + 0.0316 (OC) – 0.160 (pHs) – 4.42 (SMZ Conc.) (r2 = 0.78). The Pearson correlation coefficients between log Kd and OC, pHs, and initial SMZ concentration were 0.590, −0.338, and −0.343, respectively (p < 0.05), indicating that OC was more important than the other factors for predicting Kd. Figure 4 shows that this regression model can effectively predict experimentally determined Kd values. Thiele-Bruhn et al. (2004) and Lertpaitoonpan et al. (2009) also observed positive correlation between OC content and negative correlation with pH and Kd values. The dependence of SMZ sorption on OC content and pH fits well with proposed SMZ sorption mechanisms. Enhanced sorption with greater OC content likely arises due to increased hydrophobic interactions as SOM content increases (Essington

Table 2. Freundlich model parameters for sulfamethazine adsorption to row crop, grass buffer strip, and agroforestry buffer strip soils in the presence and absence of >1000 Da dissolved organic matter (150 mg L−1 organic carbon). Soil

Vegetation†

Phase‡

log Kf ± 95% CI§

N¶ ± 95% CI

SSE#

RC

SMZ SMZ + DOM SMZ SMZ + DOM SMZ SMZ + DOM SMZ SMZ + DOM SMZ SMZ + DOM SMZ SMZ + DOM SMZ SMZ + DOM SMZ SMZ + DOM SMZ SMZ + DOM

0.48 ± 0.16 0.43 ± 0.09 0.63 ± 0.14 0.55 ± 0.08 0.61 ± 0.11 0.46 ± 0.05 0.44 ± 0.28 0.31 ± 0.10 0.57 ± 0.12 0.35 ± 0.05 0.93 ± 0.11 0.70 ± 0.04 0.56 ± 0.07 0.31 ± 0.03 0.59 ± 0.12 0.39 ± 0.13 0.67 ± 0.11 0.52 ± 0.05

0.74 ± 0.12 0.74 ± 0.06 0.79 ± 0.11 0.78 ± 0.06 0.66 ± 0.09 0.78 ± 0.04 0.63 ± 0.18 0.72 ± 0.07 0.59 ± 0.08 0.75 ± 0.03 0.61 ± 0.09 0.75 ± 0.03 0.67 ± 0.05 0.85 ± 0.03 0.68 ± 0.09 0.83 ± 0.09 0.69 ± 0.09 0.80 ± 0.04

68.0 33.0 115 47.7 50.6 14.7 109 25.4 35.9 7.75 157 24.8 17.6 83.0 56.9 78.2 65.5 17.7

Armstrong

GBS ABS Huntington

RC GBS ABS

Menfro

RC GBS ABS

† ABS, agroforestry buffer strip; GBS, grass buffer strip; RC row crop. ‡ DOM, dissolved organic matter; SMZ, sulfamethazine. § Confidence interval (n = 15). ¶ Freundlich nonlinearity parameter. # Sum of squares due to error. www.agronomy.org • www.crops.org • www.soils.org

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Fig. 3. Sulfamethazine solid-to-solution distribution coefficients (Kd) at (a) 2.5 μmol L−1, (b) 10 μmol L−1, (c) 18 μmol L−1, (d) 25 μmol L−1, and (e) 50 μmol L−1 initial concentrations for Armstrong, Huntington, and Menfro soils under row crop (RC), grass vegetative buffer strip (GBS), and agroforestry buffer strip (ABS). Error bars represent the 95% confidence interval.

et al., 2010; Gao and Pedersen, 2010; Lertpaitoonpan et al., 2009). Molecular mechanics computational modeling performed by Schwarz et al. (2012) indicated that sulfonamides may sorb to

SOM via hydrogen bonding and dipole–dipole interactions. The influence of pH on SMZ sorption can be attributed to increased presence of anionic SMZ in solution as pH rises, thus decreasing

Table 3. Analysis of variance results indicating the effects of soil, vegetative management, >1000 Da dissolved organic matter, and interactions of these factors on sulfamethazine solid-to-solution distribution coefficients at varying initial concentrations. Initial concentration μmol L 2.5 10 18 25 50

P>F Soil

Veg.†

Soil × Veg.

DOM‡

Soil × DOM

Veg. × DOM

0.008 0.024 0.077 0.009 0.007

1000 Da, as noted in our dialysis bag experiments. It is also possible that the amount of DOM>1000 Da sorbed to soils in the system (Supplemental Table S2) was insufficient to alter the nonlinearity parameter. The latter is in agreement with findings illustrating no discernible change in isotherm linearity when SMZ was reacted with smectites loaded with two levels of humic acid (Gao and Pedersen, 2010). The Kd values for SMZ sorption conducted in the presence of DOM>1000 Da ranged from 0.74 to 3.96 L kg−1 (Supplemental Table S1). This range was comparable to but narrower than the range observed in the absence of DOM>1000 Da. In contrast to the general lack of significant DOM>1000 Da effects on the Freundlich nonlinearity parameter, DOM>1000 Da had a statistically significant (p < 0.05) and slight negative effect on SMZ Kd values except at the 18 μmol L−1 initial concentration (Table 3). The observation of significant differences in Kd values when DOM was present may be due to the greater sensitivity of Kd values to small changes in sorption, whereas sensitivity of the N parameter may be diminished due to integration of data over the entire isotherm. Considering that SMZ did not sorb to DOM>1000 Da extracted from poultry litter, we attribute decreased SMZ sorption in the presence of DOM>1000 Da to competitive interactions between SMZ and DOM>1000 Da for sorption sites. Thiele-Bruhn and Aust (2004) observed a similar effect on sulfonamide sorption to soil when swine manure slurry was introduced into the system. Other studies have also postulated or observed evidence of competitive sorption interactions between DOM and organic pollutants, such as herbicides and insecticides (Ertli et al., 2004; FloresCéspedes et al., 2006; Spark and Swift, 2002). In contrast to our findings, Essington et al. (2010) observed enhanced SMZ sorption to montmorillonite in the presence of DOM at pH ~4.5. The interaction of DOM with montmorillonite via cation or water bridging (Chorover and Amistadi, 2001) likely resulted in increased organic matter on the mineral surface, thereby increasing sorption of the neutral SMZ species (Essington et al., 2010). However, SOM was inherent in the soils studied here (12–30 g kg−1 OC), and the pH of our systems was generally much less acidic (pH 4.8–7.1). We postulate that DOM>1000 Da sorbed to the soils may have blocked SMZ sorption sites on SOM and mineral surfaces Journal of Environmental Quality

Fig. 6. Sulfamethazine (SMZ) desorption isotherms at initial SMZ concentrations of 10 and 50 μmol L−1 for (a) Armstrong grass buffer strip (GBS) soil reacted with SMZ only, (b) Armstrong GBS soil reacted with SMZ and >1000 Da dissolved organic matter (DOM) (150 mg L−1 OC), (c) Huntington GBS soil reacted with SMZ only, and (d) Huntington GBS soil reacted with SMZ and >1000 Da DOM (150 mg L−1 OC).

and that increased surface charge on DOM sorption may have repelled anionic SMZ from mineral surfaces at neutral pH. It is also possible that SMZ was binding to indigenous fractions of OM released into solution during the batch reaction process, thus reducing SMZ sorption to soil. More detailed studies are required to thoroughly evaluate the influence of DOM on SMZ sorption, and DOM derived from other types of manures should be investigated due to variations in chemical properties (Ohno and Crannell, 1996).

Sulfamethazine Desorption Studies Short-duration, multistep desorption studies were conducted using the two most contrasting soils in our sample set (Armstrong and Huntington GBS) to elucidate SMZ retention to soil (Fig. 6). We intentionally used short-duration desorption steps to simulate pulses of water moving through a vegetative buffer strip during a storm event with varying precipitation intensity or during multiple small runoff events occurring throughout a sequence of storms. Desorption results (Table 4) indicate that 46 to 88% of SMZ was retained on the soils after a single desorption

cycle and that a greater total proportion was desorbed after four desorption cycles (20–60% of initially adsorbed SMZ was retained). These data demonstrate that the soils studied can retain a large proportion of SMZ after a short desorption cycle, which could impede further hydrologic transport in the environment. After a single desorption cycle, significant differences existed between soils initially reacted with SMZ or SMZ + DOM>1000 Da, with the exception of Huntington soil, which reacted with 50 μmol L−1 SMZ (Table 4). Thus, the data suggest that the presence of DOM>1000 Da during the sorption phase increased SMZ retention. Navon et al. (2011) made a similar observation when carbamazepine was reacted with soils prereacted with DOM. Although several postulates were put forth by Navon et al. (2011) to explain this occurrence, the most applicable to our system is the possibility that DOM physically encapsulates the pharmaceutical in soil. However, this effect was no longer observed after four desorption cycles (Table 4).

Implications of this Research Vegetative buffer strips are a conservation practice commonly recommended to farmers and land managers by the United States Department of Agriculture, Natural Resources Conservation Service (USDA–NRCS, 2010). Vegetative buffer strips are well known for reducing runoff, sediment, nutrient, and herbicide export from agroecosystems (Udawatta et al., 2002; Krutz et al., 2005; Liu et al., 2008). More recently, studies have investigated the use of vegetative buffer strips to reduce the transport of VAs to surface water resources (Chu et al., 2010; Lin et al., 2010; Lin et al., 2011; Unger et al., 2013a). Specifically, the data presented here indicate that SMZ had a greater affinity for soil collected from vegetative buffer strips. Greater SOM content within soils planted to perennial vegetation may make buffer strips particularly effective for reducing SMZ transport in soils near neutral pH or those with kaolinitic mineralogy. In addition to enhanced SMZ sorption, differences in soil microbial function and community structure under vegetative buffer strips (Unger et

Table 4. Percentage of sulfamethazine retained on Armstrong and Huntington grass buffer strip soils reacted with initial sulfamethazine concentrations of 10 and 50 μmol L−1 without and with >1000 Da dissolved organic matter (150 mg L−1 organic carbon) after one desorption cycle (Desorption-1) and four desorption cycles (Desorption-4). Initial concentration μmol L−1 10 50

Phase

Desorption-1 Desorption-4 Desorption-1 Desorption-4

Armstrong† SMZ

Huntington SMZ+DOM

SMZ

SMZ+DOM

——————————————————— % ——————————————————— 61a‡ 76b 46a 63b 41a 51a 20a 31a 74a 88b 85a 87a 55a 58a 60a 55a

† DOM, dissolved organic matter; SMZ, sulfamethazine. ‡ Percentages within a row and for a particular soil followed by the same letter are not significantly different (α = 0.05). www.agronomy.org • www.crops.org • www.soils.org

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al., 2013b) may lead to improved SMZ degradation as observed by Lin et al. (2010). Other research indicates that soil microbial communities are fairly resilient to VA exposure, as noted by the 1000 Da and SMZ into a vegetative buffer strip may slightly reduce SMZ sorption, but the DOM>1000 Da may enhance SMZ retention for a short period of time. Overall, the results presented here and in our previous studies (Chu et al., 2010; Lin et al., 2010; Lin et al., 2011; Unger et al., 2013a) strongly suggest that establishing upland or riparian vegetative buffer strips in agroecosystems receiving animal manure is a strategy that could be used to reduce VA transport to water resources.

Acknowledgments This work was partially funded through the University of Missouri Center for Agroforestry under cooperative agreements 58-6227-2-008 and 58-6227-5-029 with the USDA-ARS. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture. The authors thank current and former faculty and staff of the University of Missouri Center for Agroforestry and University of Missouri Research Stations for developing and maintaining the experimental sites sampled for this work and the staff at the USDA Cropping Systems and Water Quality Research Unit for analytical assistance.

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