Imidacloprid Sorption Kinetics, Equilibria, and Degradation in Sandy

May 4, 2015 - Soil and Water Science Department, Institute of Food and Agricultural Sciences, University of Florida, Post Office Box 110290,. Gainesvi...
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Imidacloprid Sorption Kinetics, Equilibria, and Degradation in Sandy Soils of Florida Jorge A. Leiva,† Peter Nkedi-Kizza,*,† Kelly T. Morgan,‡ and Jawwad A. Qureshi§ †

Soil and Water Science Department, Institute of Food and Agricultural Sciences, University of Florida, Post Office Box 110290, Gainesville, Florida 32611, United States ‡ Soil and Water Science Department, Southwest Florida Research and Education Center, and §Entomology and Nematology Department, Southwest Florida Research and Education Center, University of Florida, 2685 State Road 29 North, Immokalee, Florida 34142, United States ABSTRACT: Imidacloprid (IMD) is a neonicotinoid insecticide soil-drenched on sandy soils of southwest Florida for the control of Diaphorina citri Kuwayama or Asian citrus psyllid (ACP). The ACP vectors causal pathogens of a devastating citrus disease called citrus greening. Understanding the behavior of IMD in these soils and plants is critical to its performance against target pests. Samples from Immokalee fine sand (IFS) were used for sorption kinetics and equilibria experiments. IMD kinetics data were described by the one-site mass transfer (OSMT) model and reached equilibrium between 6 and 12 h. Batch equilibrium and degradation studies revealed that IMD was weakly sorbed (KOC = 163−230) and persistent, with a half-life of 1.0−2.6 years. Consequently, IMD has the potential to leach below the citrus root zone after the soil-drench applications. KEYWORDS: one-site mass transfer, soil partition coefficient, zero-order transformation, pesticide transport, Asian citrus psyllid, integrated pest management

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INTRODUCTION Imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolin2-ylideneamine (IMD)] is a systemic insecticide of the neonicotinoid class. Neonicotinoids are synthetic derivatives of nicotine, an alkaloid found in the leaves of many plants in addition to tobacco.1 IMD translocates through plant tissue after application, acting by contact and ingestion to control plant-sucking pests, such as psyllids and aphids, and some chewing insects, including termites and soil insects, and even fleas on pets.2,3 IMD targets post-synaptic nicotinic acetylcholine receptors in the insect nervous system.4 The binding affinity of IMD at the nicotinic receptors in mammals is much less than that of insect nicotinic receptors.5 IMD has been extensively applied to many crops, such as sugar beet, tomato, eggplant, cabbage, and mustard.1,6−8 In Florida, IMD is used to control the Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, the vector of the pathogens believed to cause the devastating huanglongbing or citrus greening disease.9 Insecticides, including IMD, are soil-drenched around the trunk of the citrus tree, so that they are absorbed by the roots and translocate to growing tender shoots, where the insects feed and reproduce.9,10 IMD data on the soil sorption coefficient normalized to the soil organic carbon content (KOC) range from 156 to 960, with a half-life of several weeks to years.11−14 The half-life increases with an increase in the initial applied concentration of IMD as observed with non-agricultural application rates (>50 μg g−1) to control termites.15 On the other hand, much shorter IMD halflives have been reported for agricultural application rates (e.g., 0.56 kg ha −1 around the root biomass of perennial commodities, such as citrus) or concentrations higher than 10 μg g−1.10,16,17 © 2015 American Chemical Society

Moderate to high leaching potential has been observed for IMD and most other neonicotinoids.18 From 1999 to 2005, the U.S. Geological Survey (USGS) detected IMD in 13% of groundwater monitoring wells located in citrus orchards of the Central Florida Ridge.19 This area is dominated by sandy, deep, and excessively drained Entisols.20 In southwest Florida, the dominant soils are Spodosols, which are sandy, low-lying, flat, and poorly drained, with shallow water tables.20,21 One example of these soils is Immokalee fine sand (IFS) that is excessively permeable, with a low organic carbon content in its A and E diagnostic horizons.20 It also has a poorly drained spodic layer (Bh), high in organic matter, with accumulation of Fe and Al.20 Currently, it is unclear how IMD reacts with these soils, and there are no data describing IMD transport and persistency in southwest Florida soils treated to control ACP. To address these questions, the study goals were to characterize IMD sorption and degradation in Florida sandy soils, specifically IFS diagnostic horizons A, E, and Bh. The data will provide a technical basis for IMD transport studies in the field regarding IMD application rates to citrus, leaching potential, and ACP control. The hypotheses tested were that (a) IMD has a small soil sorption coefficient (KD) and slow degradation rate (k) in IFS, (b) the spodic layer has a higher IMD sorption coefficient and faster degradation rate than the overlaying A and E horizons, because of its higher organic matter and clay contents, and (c) there is high leaching potential beyond the citrus root zone (0−45 cm) after IMD soil-drench applications (and Received: Revised: Accepted: Published: 4915

January 28, 2015 March 26, 2015 May 4, 2015 May 4, 2015 DOI: 10.1021/acs.jafc.5b00532 J. Agric. Food Chem. 2015, 63, 4915−4921

Article

Journal of Agricultural and Food Chemistry Table 1. Some Physical and Chemical Properties of IFS at the SWFREC horizon A E1 E1 E2 Bh a

pHa

depth (cm) 0−15 15−30 30−45 45−60 60−75

4.4 4.5 4.4 4.4 4.6

SOCb (g g−1) 8.0 1.9 1.0 8.8 1.9

× × × × ×

CECc (cmolc kg−1)

sand (%)

silt (%)

clay (%)

7.63 0.74 0.33 2.56 6.85

93.8 97.2 98.4 98.0 96.4

5.0 2.7 0.5 1.2 2.6

1.2 0.1 1.1 0.8 1.0

−3

10 10−3 10−3 10−3 10−2

pH in water at a soil/solution ratio of 10:25. bSOC = soil organic carbon. cCEC = cation-exchange capacity. The highest initial concentration used in the kinetics and batch studies was based on the label-recommended application rate for young nonbearing citrus trees in Florida, where IMD is soil-drenched around the tree trunk. Blank samples (no added IMD) were also prepared in 0.01 M CaCl2. The centrifuge tubes (and soil samples) were equilibrated in the dark using an Eberbach horizontal shaker, for 2, 6, 14, and 24 h. After shaking, the tubes were centrifuged at 6000 rpm for 20 min, and then the supernatant was filtered with Whatman 42 filter paper. The filtrate was refiltered with a Fisherbrand 0.45 μm nylon sterile syringe filter before analysis by high-performance liquid chromatography with ultraviolet detection (HPLC−UV). Batch Sorption Kinetics Model. The sorption kinetics phenomena in soils may be caused by the hydrophobicity or hydrophilicity of the organic chemical, the natural heterogeneity of sorption sites, and the diffusion pathways in the porous media.25 In this study, the kinetic parameters were optimized using the one-site mass transfer model (OSMT; eqs 1−3). This model assumes that all sites in the soil have rate-limited sorption or type 2 sites.26,27 The model is a special case of the two-site non-equilibrium (TSNE) model28,29

subsequent irrigation and rainfall events), during ACPintegrated pest management programs.

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MATERIALS AND METHODS

Soil Sampling. The soil used in this study was IFS classified as sandy, siliceous, hyperthermic, Arenic Alaquods.21 Soil samples were taken at the Southwest Florida Research and Education Center (SWFREC), University of Florida (UF), Immokalee, FL (latitude, 26° 27.75′ N; longitude, 81° 26.83′ W). The soil organic carbon (SOC) content was determined using the Walkley−Black method,22 and the soil pH was determined with electrometric methods using a soil/ solution ratio of 10:25 in water.23 Soil cation-exchange capacity (CEC) and texture data were obtained from the Florida Soil Characterization Database of the Institute of Food and Agricultural Sciences (IFAS), UF.24 The selected physical and chemical properties of IFS are presented in Table 1. In a SWFREC experimental plot, planting beds for young citrus trees were built to conduct studies on IMD leaching and ACP control. During planting bed construction, soils from the A horizon (surface) and E horizon (albic, subsurface) were somewhat mixed. Soil samples from the planting beds were collected from five depths using a bucket auger with dimensions of 5.0 cm (inner diameter) × 15.0 cm. The soil samples from the planting beds were used for the sorption kinetics study (Table 2) to generate transport parameters to model IMD movement in the field during ACP control.

A E1 E1 E2 Bh

0−15 15−30 30−45 45−60 60−75

α 0.67 0.97 0.95 0.60 0.83

(0.2) (0.2) (0.2) (0.1) (0.1)

KD 0.18 0.30 0.08 0.19 1.07

(0.01) (0.01) (0.01) (0.01) (0.02)

R = 1 + (m/v)KD

(2) (3) −1

where Ct is the solute concentration in solution (μg mL ) at any time, Co is the initial IMD solution concentration (μg mL−1), m is the mass of soil (g), v is the volume of soil solution (mL), R is the expression for retardation, β represents the fraction of retardation as a result of kinetic sites, α is the kinetic rate coefficient for desorption from type 2 sites (h−1), t is the time (h), and KD is the sorption coefficient at equilibrium (mL g−1). The parameters α and KD in the OSMT were optimized using nonlinear regression procedures.30 Soil Sorption Equilibria. The batch slurry equilibrium study was conducted using triplicated soil samples for each soil depth (0−15, 15−30, 30−45, 45−60, and 60−75 cm) and each equilibrium concentration. Triplicate 10 g samples of air-dried soil were weighed in 50 mL polycarbonate centrifuge tubes (Nalgene, Thermo Scientific). The soil samples were equilibrated with 10 mL of 0.01 M CaCl2 solution spiked with IMD initial concentrations of 2, 4, 6, and 8 μg mL−1 (Co; eq 4). Blank samples were also equilibrated in 0.01 M CaCl2. All samples were equilibrated for 24 h in an Eberbach horizontal shaker. After equilibration, the tubes were centrifuged for 20 min at 6000 rpm and then filtered with Whatman 42 filter paper. Before analysis by HPLC−UV, the aqueous filtrate was refiltered with a Fisherbrand 0.45 μm nylon sterile syringe filter. The measured solution concentration was considered as equilibrium concentrations (Ce, μg mL−1; eq 4). The IMD-sorbed equilibrium concentration (Se, μg g−1) is given by eq 4 based on mass balance

OSMT horizon

(1)

β = 1/R

Table 2. OSMT Model Parameters (α and KD) Optimized for IMD Sorption Kinetics Using an Initial Concentration (Co) of 2.5 μg mL−1 Equilibrated in IFS Diagnostic Horizons A, E, and Bha depth (cm)

Ct ⎛ 1 1⎞ = + ⎜1 − ⎟exp[− αRt ] ⎝ Co R R⎠

isotherm KDb (mL g−1) 0.14 0.24 0.07 0.18 0.78

a

Soil samples are from citrus planting beds. Standard deviations are in parentheses. bKD values are from 24 h batch sorption equilibrium isotherms.

The batch equilibria and degradation studies were conducted on IFS samples taken from a site with secondary forest regrowth. The soil samples were collected from a pit, from five depths divided in 15 cm increments from the soil surface. The five depths were representative of the A, E, and Bh soil diagnostic horizons of IFS (Table 1). Soil Sorption Kinetics Experiments. A kinetic batch study was conducted to estimate equilibration times for the IMD concentration in solution using samples from disturbed (bedded) flatwood soils, from IFS diagnostic horizons A, E, and Bh. Triplicate 5 g samples of air-dried soil (m) were weighed in 50 mL centrifuge tubes (Nalgene, Thermo Scientific). The equilibration volume (v) was 10 mL of 0.01 M CaCl2 spiked with three IMD concentrations of 2.5, 12.5, and 25 μg mL−1, representing three initial soil solution concentrations (Co; eq 1).

Se =

4916

V (Co − Ce) m

(4)

Se = KDCe

(5)

K OC = KD/OC

(6) DOI: 10.1021/acs.jafc.5b00532 J. Agric. Food Chem. 2015, 63, 4915−4921

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Journal of Agricultural and Food Chemistry

and k (eq 9) were used in this study to estimate IMD half-life (eq 10) for each soil depth.

where v is the volume of solution added (mL) and m is the mass of oven-dry soil (g). The sorption coefficient KD (mL g−1) was calculated using simple linear regressions (eq 5) and then normalized to account for the influence of OC (soil organic carbon, g g−1) on the sorption phenomena for hydrophobic organic compounds, such as IMD (eq 6). Soil Degradation Study. IFS soil samples were maintained at the initial moisture content found in the field for the degradation study, to keep microbial communities as intact as possible. The samples from five depths (0−15, 15−30, 30−45, 45−60, and 60−75 cm) were thoroughly mixed, and the moisture content was measured. Triplicate 10 g samples of soil were weighed into polycarbonate centrifuge tubes (Nalgene, Thermo Scientific) and kept at room temperature (21 °C). The samples were spiked with an IMD solution in deionized water and homogenized with vortex for 2 min. The intended initial spiking concentration So was 10 μg of IMD/g of oven-dry soil. The tubes were stored in the dark, loosely capped, and maintained at a constant field capacity (FC) of 10% moisture content on a gravimetric basis. The FC water content (or mass) for each tube was checked every other week during the entire study period (514 days), by replacing any lost moisture with deionized water using a dropper. The final soil mass and moisture contents were checked before sample extraction. The IMD degradation experiments consisted of soil samples from 0 to 15, from 15 to 30, and from 30 to 45 cm depths that were extracted at 9, 52, 120, 241, and 514 days after spiking. The soil samples from 45 to 60 and from 60 to 75 cm were extracted roughly every 12 weeks at 85, 168, 246, 329, and 434 days. Also, alongside the degradation experiment samples, autoclaved soil samples were spiked with the same solution, kept in a freezer, and then extracted to estimate IMD extraction efficiency as a function of time. The spiked samples were extracted using a soil/solution ratio of 1:2, adding 20 mL of acetonitrile/water (80:20) to the centrifuge tubes. The samples were shaken for 2 h, centrifuged for 20 min at 6000 rpm, and then filtered with Whatman 42 filter paper. Before the analysis by HPLC−UV, the filtrates were diluted with HPLC-grade water (1:1) and refiltered with Fisherbrand 0.45 μm nylon sterile syringe filters. The fraction of organic solvent in the extracting solutions of acetonitrile/water was based on the solvophobic theory, which states that the sorption coefficient for hydrophobic organic compounds (HOCs) decreases exponentially as the fraction of organic co-solvent increases in the extracting mixture.31,32 Extraction Efficiency (ε), Degradation Rate (k), and Half-Life (t1/2). The extraction efficiency was assumed constant during the degradation experiment for all soil depths. The initial IMD concentration in the soil sample was defined as S*o (μg g−1), and the concentration at a given time was defined as St* (μg g−1). The asterisk (∗) denoted the measured extracted concentration not corrected for extraction efficiency ε (eq 7). The ε was the absolute extraction efficiency estimated for each soil depth at each extraction date, using autoclaved control samples to eliminate microbial degradation. After autoclaving, these samples were spiked with the same solution (to obtain an approximate initial concentration of 10 μg g−1), and were frozen alongside unspiked samples. We assumed that the baseline to calculate ε was the concentration from the autoclaved frozen samples extracted about 9 days after spiking, because no IMD metabolites were observed from the autoclaved samples during analysis by HPLC−UV. This was conducted during the first 8 months of the degradation experiment (241 days), to ensure analytical consistency15 and to avoid estimation errors for IMD degradation rates. Our study showed that IMD degradation followed a zero-order process for soil samples from all depths (eqs 7 and 8).

St* S* = o − kt ε ε

(7)

St* = So* − k*t

(8)

k = k*/ε t1/2 = So/2k

(9) (10)

Analytical Procedures and Statistics. Analytical-grade standards for IMD were obtained from ChemService, Inc., West Chester, PA. A 100 ppm standard stock solution was prepared in 0.01 M CaCl2, and mixed thoroughly. The sorption equilibrium batch spikes (2, 4, 6, and 8 μg mL−1) were serially diluted in 0.01 M CaCl2 using HPLC-grade water. The HPLC calibration curve for the batch samples had IMD concentrations of 0, 2.5, 5.0, and 10.0 μg mL−1. On each sampling date during the degradation experiment, freshly made calibration standards were prepared from a stock solution of 100 μg of IMD mL−1 in pure acetonitrile. The HPLC−UV calibration curve standards were serially diluted to solutions of 1.25, 2.5, 5.0, 10.0, and 20.0 μg of IMD mL−1 in a matrix of acetonitrile/water (40:60). The IMD soil extracts (degradation study) and equilibration solutions (sorption kinetics and sorption equilibria studies) were analyzed using an Agilent Infiniti 1260 HPLC−UV with a mobile phase of HPLCgrade acetonitrile/water (40:60), a Supelcosil LC-18 column (150 × 4.6 mm; Sigma-Aldrich Co.), an injection volume of 20 μL, a flow rate of 1 mL min−1, and an absorption wavelength of 270 nm. The analysis by HPLC−UV for the sorption and degradation studies had limit of detection and quantitation (LOD and LOQ) values of 0.40 and 1.25 μg mL−1, respectively (eqs 11 and 12) (11) LOD = sdt LOQ = 3LOD

(12)

where sd is the standard deviation of the recovered spikes and t is the one-tailed statistic for 99% confidence using six replications. All degradation extracts were diluted with HPLC-grade water to obtain a mixture of acetonitrile/water of 40:60. All solutions were filtered before analysis by HPLC−UV using Fisherbrand 0.45 μm non-sterile syringe filters. The retention time for IMD under the above conditions was 2.70 min. The KD values and IMD degradation rate (k) values were obtained by a simple linear regression conducted in JMP Pro 9.33 The linear regression fitted parameters were checked using analysis of variance (ANOVA) and t tests. Multiple comparison Tukey honestly significant difference (HSD) tests on the sorption coefficients (KD) and degradation rates (k) were conducted using an analysis of covariance (ANCOVA) procedure to assess the effect of the covariate (soil depth) on the linear regression slopes between soil depths.34

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RESULTS AND DISCUSSION IMD Sorption Kinetics. IMD sorption kinetics data were fitted properly by the OSMT model.28,35 The fitted parameters are summarized in Table 2 for each of the five soil depths. To avoid crowding the figures, the data presented correspond only to the 2.5 μg mL−1 initial concentration (Co). Data in Figure 1 show that the OSMT model provided a good fit for IMD kinetic data, with a sharp decrease in the solution concentration and then reaching equilibrium between 6 and 12 h when the concentration in solution remained relatively constant. This trend was observed in every combination of Co and soil depth. IMD has been reported to have a rapid equilibration time of 6 h or less in soils with low organic matter content, but the sorption kinetics data were described with various models.36 However, the OSMT model has been reported to describe sorption kinetics of hydrophobic organic chemicals in soils and organoclays.28,35 The OSMT parameter α value was highest in the upper part of the E horizon (15−45 cm), which has the lowest organic matter content. The parameter α had lower values in the other sampling depths (A, E2, and Bh). Also, β values (eq 3) were

Here, we propose that the overall ε was constant during the experiment, affecting the estimate of k in the zero-order process (eqs 8 and 9). Assuming that ε was constant, the correct IMD degradation rate value (k) would be given by eq 9. Consequently, So (So = So*/ε) 4917

DOI: 10.1021/acs.jafc.5b00532 J. Agric. Food Chem. 2015, 63, 4915−4921

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Journal of Agricultural and Food Chemistry

Table 3. IMD Sorption Coefficients (KD and KOC) for Different Depths (cm) and Diagnostic Horizons in IFSa horizon

depth (cm)

A E1 E1 E2 Bh

0−15 15−30 30−45 45−60 60−75

KD ± SEb (mL g−1)c 1.66 0.31 0.23 0.08 1.59

± ± ± ± ±

0.04 a 0.01 b 0.01 c 0.001 d 0.03 a

KOC

log KOC

208 163 230 13 63

2.3 2.2 2.4 1.1 1.8

a Soil samples are from IFS with secondary forest regrowth. bSE = standard error. cDifferent letters indicate significant difference between KD values.

In essence, sorption equilibrium data confirmed that IMD was a weakly sorbed chemical in IFS (Table 3). However, the KOC values for 45−60 cm (E2) and 60−75 cm (Bh) were much lower than the layers above (A and E1) and were not consistent with the KOC concept. Initially, we hypothesized that the spodic layer would show higher IMD sorption. In contrast, the surface horizon samples (0−15 cm) showed higher KOC value than the Bh horizon. It is possible that the nature and functional groups of the organo-mineral complexes in the Bh horizon could have reduced IMD sorption on this layer. Previous studies have suggested that IMD sorption in soils is predominantly explained by the soil organic carbon content,11 which is a characteristic for hydrophobic organic chemicals partitioning in soils.37 IMD studies on soil sorption have found similar values for IMD KOC in soils with low organic matter content. Liu et al.38 found values averaging 220 in soils with low organic matter content (0.25−0.71%). IMD Degradation. The extraction efficiency (ε) data obtained from the autoclaved−frozen soil samples did not show a significant relationship with time based on the linear regression (p > 0.31; Table 4). The data confirmed the assumption that our degradation experiments had an extraction efficiency (ε) constant as a function of time, as was previously reported for IMD.15 However, the co-solvents used in our study (80:20 acetonitrile/water) yielded ε values very close to 100% during the first 8 months of the degradation experiment, especially in the A and E horizons (Table 4). Therefore, if ε is 100%, then k = k* and So = S*o (eqs 7−10). Data in Figures 3 and 4 show the linear relationship between extracted concentrations and time (p < 0.0001) based on a zero-order degradation rate. The t tests on the regression slopes showed that k values were significantly different from zero (p < 0.0001). The degradation rates were also significantly different from each other, with p < 0.0001 (Table 5 and Figures 3 and 4). IMD degraded at different rates in the A horizon samples (0−15 cm) compared to the other depths. The E1 horizon samples (15−30 and 30−45 cm) showed very similar degradation rates as well as the highest t1/2 values. The E2 layer (45−60 cm) and the Bh layer (60−75 cm) showed no significant difference in degradation rates and had the lowest t1/2 values. IMD half-life (t1/2) ranged from 0.98 to 2.60 years across all sampling depths. The insecticide was very persistent at the rate used, and degradation was not a function of the concentration (i.e., a zero-order process). Baskaran et.al.15 data were somewhat similar to ours, showing that IMD had t1/2 values of 2.71 years in soils and 2.95−3.37 years in construction bedding materials (sand). This is the first study evaluating IMD sorption and degradation in sandy soils of Florida. The data revealed that

Figure 1. IMD sorption kinetics with an initial spiking concentration (Co) of 2.5 μg mL−1 in IFS samples from A, E, and Bh soil horizons. Symbols indicate observed relative concentrations as a function of time. Solid lines represent the OSMT kinetic model.

close to 1 for all depths. This is in agreement with the OSMT model assumption that most sorption sites were kinetic. In essence, IMD showed that sorption equilibrium was reached much sooner than 24 h, which is the standard protocol for sorption equilibrium studies, confirming the validity of our batch equilibrium experiments and the obtained distribution coefficients (KD). IMD Sorption Equilibria. The sorption isotherms for IMD are shown in Figure 2. In Table 3, KD values are compared

Figure 2. IMD sorption isotherms in samples from five soil depths (0−15, 15−30, 30−45, 45−60, and 60−75 cm) comprising IFS horizons A, E, and Bh (Table 3). Symbols represent equilibrium concentrations in solution (Ce) and soil sorbed (Se).

between different depths and horizons. The KD values for the 0−15 cm (A horizon) and 60−75 cm (Bh horizon) samples were very similar (1.66 ± 0.042 and 1.59 ± 0.032 mL g−1, respectively). This was confirmed by Tukey’s HSD test showing no significant differences between the two (q = 1.7; p > 0.50). The soil samples from the E horizon (15−30, 30−45, and 45−60 cm) were statistically different from each other (q = 36.8; p < 0.001). The sorption coefficients were lowest in the E horizon samples (0.08−0.31), as was expected because this soil horizon has the lowest organic matter content in IFS. 4918

DOI: 10.1021/acs.jafc.5b00532 J. Agric. Food Chem. 2015, 63, 4915−4921

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Journal of Agricultural and Food Chemistry

Table 4. Extraction Efficiency ε (%) as a Function of Time for IMD in IFS Incubated for 514 Days (A and E1) and 434 Days (E2 and Bh) extraction time (days)

a

horizon

depth (cm)

A E1 E1 E2 Bh

0−15 15−30 30−45 45−60 60−75

ε ε ε ε ε

(%) (%) (%) (%) (%)

9

52

85

120

168

241

100 100 100 100 100

100 100 97 a a

a a a 100 100

100 88 109 a a

a a a 109 115

100 101 110 96 89

Not measured.

Figure 4. IMD degradation as a function of time in samples from soil depths of 45−60 and 60−75 cm (IFS horizons E2 and Bh). IMD extracted concentrations St* (symbols) and linear regression fits (solid lines).

Table 5. IMD Degradation Rates (k) in IFS Incubated for 514 Days (A and E1) and 434 Days (E2 and Bh) horizon

depth (cm)

ka ± SEb (μg g−1 day−1)c

A E1 E1 E2 Bh

0−15 15−30 30−45 45−60 60−75

0.011 0.008 0.006 0.014 0.016

Figure 3. IMD degradation as a function of time in samples from soil depths of 0−15, 15−30, and 30−45 cm (IFS horizons A and E1). IMD extracted concentrations S*t (symbols) and linear regression fits (solid lines).

± ± ± ± ±

0.001 0.001 0.001 0.002 0.002

a b b c c

Sod ± SE (μg g−1)

t1/2e (day)

t1/2 (year)

± ± ± ± ±

511 686 948 440 359

1.40 1.88 2.60 1.21 0.98

11.24 10.98 11.38 12.31 11.50

0.33 0.16 0.25 0.55 0.54

a

k = degradation rate. bSE = standard error. cDifferent letters indicate significant difference between soil depths. dSo = initial concentration. e t1/2 = half-life.

IMD undergoing sorption kinetics is a weakly sorbed chemical with KOC values of 163−230 in the A and E horizons and is very persistent, with t1/2 of 0.98−2.60 years. The main concern in Florida Spodosols after IMD soil-drench applications is the shallow and fluctuating water table. This study showed that IMD sorption (and, therefore, retardation) would be lowest in the E horizon. Consequently, once the chemical moves beyond the first 15 cm of soil (A horizon), the potential for leaching beyond the citrus root zone increases considerably. This was an expected outcome for IFS that has a very low organic matter content, especially in the eluviated horizon (E), which is generally stripped of colloidal coatings (clay and organic matter) that could help to retain important plant nutrients20 and possibly organic solutes, such as IMD.

Our data on IMD sorption and degradation confirmed a high potential for IMD leaching beyond the citrus root zone and that it is necessary to establish field studies to evaluate different IMD soil-drench application rates, to monitor irrigation and rainfall events after soil drenching, and to estimate how much leaching is occurring under such conditions. Furthermore, IMD fate and transport modeling in sandy soils of Florida will have to consider the plant uptake component, not only to avoid loss by leaching but also to ensure ACP control across seasons. In the event that IMD is actually leaching, one strategy would be to increase the residence time in the citrus root zone with 4919

DOI: 10.1021/acs.jafc.5b00532 J. Agric. Food Chem. 2015, 63, 4915−4921

Article

Journal of Agricultural and Food Chemistry

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amendments of pyrolyzed organic matter (biochar) or organic composts.39 These amendments as agricultural practices need further research and validation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: kizza@ufl.edu. Funding

The study was funded by a United States Department of Agriculture (USDA)−Tropical and Subtropical Agriculture Research (TSTAR) grant. Notes

The authors declare no competing financial interest.

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REFERENCES

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