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a Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen, Denmark
b Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
* Corresponding author (the{at}geus.dk).
Received for publication May 8, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DA, desamino-metribuzin • DADK, desamino-diketo-metribuzin • DK, diketo-metribuzin • DT50, dissipation time for 50% of a pesticide added • DT90, dissipation time for 90% of a pesticide added • Kd, ratio of sorbed pesticide to pesticide in the aqueous phase • LC–MS/MS, liquid chromatography–tandem mass spectrometry
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In the present study the degradation and sorption of metribuzin and major metabolites is studied in further detail. The need for this study of metribuzin degradation is documented by the recent findings of the metabolites DK and DADK in high concentrations in soil water and shallow ground water collected below a field treated with metribuzin (Kjær et al., 2003). The field (Tylstrup, Denmark) is monitored in the Danish Pesticide Leaching Assessment Program. The aim of the program is to test for leaching at the field scale of pesticide or transformation products resulting from regulatory use (metribuzin dose is 245 g a.i. per 10000 m2). At a 1- to 6-m depth, DK was measured in concentrations up to 0.5 µg/L and DADK up to 2.0 µg/L (i.e., 20 times higher than the maximum allowed level according to the Danish Environmental Protection Agency). In contrast, metribuzin was detected in very few samples and DA was absent. Within the past 10 years only four findings of metribuzin in the ground water exceeded the EU threshold limit for pesticides at 0.1 µg/L (Jørgensen, 2002). This clearly demonstrates that metabolites need to be considered in ground water monitoring and in pesticide degradation studies, as pointed out by others (Kolpin et al., 1998; Lawrence et al., 2001).
Metribuzin degradation is often reported to follow first-order kinetics (Webster and Reimer, 1976a; Bowman, 1991; Locke et al., 1994; Lechon et al., 1997; Di et al., 1998; Webb and Aylmore, 2002). First-order kinetics is advantageous for use in modeling as a constant degradation rate and half-life of the pesticide can be estimated. However, deviations from first-order degradation of metribuzin have been reported. Typically, a fast initial dissipation is followed by a gradual decrease in the degradation rate and eventually a very slow long-term degradation. Consequently, alternative kinetic models have been used, such as the power–rate equation developed by Hamaker (1972) employing kinetics having an order other than one (Webster and Reimer, 1976b; Hance and Haynes, 1981; Moorman and Harper, 1989) and a first-order function adjusted for decreasing respiration in the soil samples (Allen and Walker, 1987).
The gradual change in degradation rate may be better described by using two rate constants instead of one (Zimdahl et al., 1994; Kjær et al., 2003). The two-compartment model (Eq. [1]) describes the degradation process as shared between two different compartments, where degradation proceeds at different rates (k1 and k2, [d–1]). In Eq. [1], C is the concentration (mg/kg) and t is time (d). The two constants, a and b, express the quantitative partition between the two compartments, where a + b is approximately equal to C0 (mg/kg):
 | [1] |
The fast degradation in the first compartment occurs when the pesticide is in the soil-water phase and readily available for microorganisms. In the second compartment the pesticide is sorbed to soil particles. Degradation is, therefore, controlled by the rate of desorption–diffusion into the soil-water phase. The partition between the two compartments depends on the pesticide sorption properties and soil characteristics.
In addition to transformation, dissipation of pesticides in the soil may result from binding to soil material and irreversible incorporation into soil components or biological material. The dissipation time for 50% of a pesticide added (DT50) is an important parameter for risk assessment, but it does not distinguish between the different processes, or account for dissipation time of the metabolites. Further, only limited data are available concerning metabolite degradation rates, ratio of sorbed pesticide to pesticide in the aqueous phase (Kd), and water solubility. In general, degradation changes the properties of the pesticide toward smaller, more polar, and often acidic compounds. Thus, metabolites are expected to be more water-soluble and potentially possess a higher risk for leaching.
The aim of this study is to provide more detailed knowledge about the pathway, rate, and mechanism of degradation for metribuzin as well as the metabolites. Also, sorption and desorption properties are described. In the present work degradation and sorption experiments were performed with soil from the Tylstrup field to benefit from the opportunity to compare the experimental results with field-scale monitoring data.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Methanol was high performance liquid chromatography (HPLC) grade from Romil (Cambridge, UK). Water passed through a Millipore (Billerica, MA) system was used for soil extraction, sample dilution, and spike solutions, whereas HPLC-grade water from Rathburn (Walkerburn, Scotland) was used for the HPLC mobile phase. Acetic acid (pro analysis, 100%) was purchased from Merck (Darmstadt, Germany). Ottawa Sand Standard (20–30 mesh) purchased from Fischer Scientific (Hampton, NH) was used as inert filling material in soil extractions. R2 Agar was purchased from Difco (Sparks, MD).
Soil
Soil from the Tylstrup field (Kjær et al., 2003) was collected at two depths, 5 to 20 cm (topsoil) and 3 to 4 m (subsoil, just above ground water level) to cover the variation in microbial activity and sorption capacity. Each fraction was thoroughly mixed, passed through a 5-mm sieve, and stored at 5°C in darkness. Soil properties are shown in Table 1.
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-radiation (LR Plast, Glostrup, Denmark). The bags were stored in darkness at 5°C and kept closed until use.
Sample Preparation for Degradation Experiments
The herbicide formulation Sencor WG was used for the degradation experiments to imitate field degradation. Analytical-grade metribuzin was added to a small number of samples for comparison. Degradation of the metabolites DA, DK, and DADK was studied independently in the four types of soil using the metabolite as spike compound. The initial concentration of the spike compound in the soil samples was about 0.5 mg/kg. This corresponds to the level in the upper 10 cm of the soil if the maximum dose of metribuzin is sprayed in one treatment. Even if metabolite concentration in the subsoil may be lower, a uniform level was chosen for all samples and compounds to evaluate a worst-case scenario. The initial concentrations in the soil samples were as follows: metribuzin (Sencor) = 0.53 mg/kg; metribuzin (analytical standard) = 0.43 mg/kg; DA = 0.67 and 0.62 mg/kg for topsoil and subsoil, respectively; DK = 0.46 and 0.30 mg/kg for topsoil and subsoil, respectively; and DADK = 0.67 mg/kg.
Soil corresponding to 25 g dry wt. was transferred to a 250-mL blue cap flask. Aqueous spike solution (1 mL) was added to the soil surface as evenly as possible. For the sterile samples, the flasks were autoclaved before use and weighing and spiking was performed in a flow-bench with sterilized tools. Finally, the flasks were incubated at 10 ± 1°C, simulating the average Danish soil temperature at 1 m below the soil surface. Once a month the flasks were opened for 10 min to maintain aerobic conditions.
Sterile spike solutions were prepared to prevent contamination of the sterile samples. A small amount of solid compound was dissolved directly in 2 or 5 mL of autoclaved MilliQ water (Millipore). This solution was filtered through a 0.2-µm sterile nylon filter into a blue cap flask containing the remaining volume of autoclaved water. The exact concentrations were determined afterward by liquid chromatography–tandem mass spectrometry (LC–MS/MS). To dissolve the metabolites DK, DA, and DADK, 250 µL of acetone was added to the primary solutions. The final concentration of acetone in the spike solution was 0.5% (v/v), which was not considered to influence the soil bacteria.
After 7 mo a number of sterile soil samples and two nonsterile samples were tested for microbial activity. From each sample 10 to 15 g soil was thoroughly shaken with 100 mL autoclaved 0.9% NaCl solution. An aliquot of the aqueous phase (100 µL) was spread on R2 Agar plates and incubated for 90 d at 20°C. No bacterial growth was observed at the plates derived from the sterile soil samples within this period.
Sample Preparation for Sorption Experiments
Sorption of metribuzin, DA, DK, and DADK (in mixtures and separately) was measured by batch experiments. Mixture samples were prepared with initial concentrations of each compound at 50, 75, 100, 150, and 250 µg/L in the aqueous phase, except for DK being at 75 to 375 µg/L. In addition, control samples of each compound alone were prepared at the highest concentration, 250 µg/L. Soil for the sorption experiments was air-dried and passed through a 2-mm sieve. Soil (5 g) was transferred to a 12-mL screw cap test tube. Four milliliters of "artificial ground water" were added, made up from tap water and MilliQ water in equal proportions to obtain an ion strength similar to average Danish ground water. After equilibration for 24 h, 1-mL standard solution was added to obtain the respective concentrations. The samples were equilibrated at 10°C in the dark for 96 h. Before analysis, the samples were centrifuged for 30 min at 1500 x g and an aliquot of the supernatant solution was filtered through a 0.20-µm filter and analyzed by LC–MS/MS.
Desorption experiments were performed using topsoil samples. The supernatant was removed and the remaining water volume was calculated by weighing. For desorption experiments 4 mL of fresh "ground water" was added to each sample. The samples were equilibrated for 96 h at 10°C and analyzed as the sorption experiments.
Soil Extraction
Extraction and analysis followed the method described in Henriksen et al. (2002) with minor adjustments. Soil samples were extracted with methanol and water (75:25 v/v), using an Accelerated Solvent Extraction 200 from Dionex (Sunnyvale, CA). The samples were equilibrated with the solvent for 10 min at 60°C and 10.3 MPa, followed by flushing with fresh solvent and purging with nitrogen.
An aliquot of the soil extract was filtered through a 0.45-µm nylon filter. This aliquot was further diluted three to six times with methanol and water (75:25, v/v) to obtain a concentration of the original compound of about 50 µg/L. The undiluted extract was used for analysis of the metabolites produced in the samples.
Liquid Chromatography–Tandem Mass Spectrometry Analysis
Analysis was performed with a Waters (Milford, MA) 2690 high performance liquid chromatography system, connected to a Quattro Ultima triple-quadrupole mass spectrometer from Micromass (Manchester, UK). Electrospray ionization (ESI) was used in the positive mode for metribuzin and DA, and in the negative mode for DK and DADK. Analytes were separated with an XTerra RP18 column from Waters (2.1 x 100 mm, 3.5-µm particle size) using a mobile phase of methanol and 0.1% acetic acid (50:50, v/v) with flow rate at 180 µL/min. Capillary voltage was 2.80 to 3.20 kV, cone voltage 50 to 80 V, and collision energy for tandem mass spectrometry 18 to 20 eV, depending on the analyte. The injected sample volume was 10 µL. Analytes were quantified by selected reaction monitoring, measuring a single characteristic fragment ion of each analyte. The respective ion traces were as follows: metribuzin, m/z 215 
187; DA, m/z 200 172; DK, m/z 183 139; and DADK, m/z 169 97.
Data Analysis
In each series of sample analysis the content of metribuzin and metabolites in the soil samples was adjusted for variations in extraction efficiency by including freshly spiked recovery control samples. Despite this procedure, a rather high day-to-day variation of recovery was observed in the subsoil samples, which exceeded the variation among the triplicates. In the topsoil the random day-to-day variation was much less pronounced.
Curve fittings for degradation kinetics and Freundlich isotherms were performed in SigmaPlot (SPSS, 1999). For fittings to the two-compartment model, constraints for acceptable fits were chosen for the variables a and b (Eq. [1]) to be within 0 and 130, as the ideal sum of a + b is 100(%).
To quantify how sorption of the individual compound was influenced by the other compounds present in the mix samples, a ratio was calculated for the control samples and the mix samples (Kd,mix/Kd,control) having equal initial concentrations of about 250 µg/L. Differences between metabolite formation in nonsterile and sterile soil were tested by two-way ANOVA for repeated measurements performed in Excel 5.0 (Microsoft, 2003).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Metribuzin Degradation
In the topsoil, fast initial dissipation of metribuzin was observed, with less than 50% of the applied amount of metribuzin remaining after 30 d. After this, the rate gradually decreased and 5% could be recovered one year after incubation. When comparing experiments using the formulation Sencor WG with the pure analytical standard, no difference in metribuzin dissipation could be observed. The experimental data were better described using the two-compartment model rather than the first-order model. The two-compartment fit is shown in Fig. 1a
. However, a possible lag phase was not described by the two-compartment model, resulting in an overestimate of the constants a and b (Eq. [1]; a + b should be close to C0, corresponding to 100%). The best-fit equations for degradation of metribuzin and the metabolites, as well as dissipation time for 50 and 90% of a pesticide added (DT50 and DT90, respectively) calculated using these equations, are listed in Table 2.
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In Fig. 1, the topsoil dissipation of metribuzin and formation of metabolites are shown for the sterile and nonsterile soil. It is notable that the steep decrease in metribuzin concentration is similar for both curves in Fig. 1a. In the period from Days 16 to 33, the content of metribuzin decreased from 90 to 40% of the initial amount in the nonsterile soil and from 120 to 80% in the sterile soil (Fig. 1a). Within the same time the formation of DA, DK, and DADK in the nonsterile topsoil (Fig. 1b) corresponded to only 12% of the initial amount of metribuzin, calculated on a molar basis, and only 3% in the sterile soil (Fig. 1c). Thus, comparing the fast dissipation of metribuzin within this period with the formation of metabolites in the nonsterile and sterile soil, it is clear that the dissipation cannot result from degradation only. Even if other unknown metabolites might have been formed from DADK within this time, this is unlikely to explain the total loss. It is more likely that part of the decrease in metribuzin is caused by strong sorption to the soil organic matter, possibly incorporated, as it could not be released by the rather harsh extraction conditions applied. During the following time interval (Days 33–137, Fig. 1a and 1b), better agreement was observed between metabolite formation and metribuzin dissipation in the nonsterile soil, indicating that degradation had become the dominating process. Even if only 5% metribuzin was remaining after one year of incubation, degradation and release of metabolites may still occur. On a long time scale the formation of bound metribuzin residues may not be a totally irreversible process. For instance, metribuzin that is covalently bonded to humic acids can be released as de-aminated metribuzin by hydrolysis as demonstrated by Landgraf et al. (2001). Also, slow diffusion into micropores and organic matter may be reversed if the gradient changes. Thus, metabolites formed at the end of the experiment may derive from metribuzin released from the soil by desorption. At the Tylstrup field, leaching of DK and (primary) DADK from the root zone was still observed three years after metribuzin application. This clearly indicates the presence of a slow long-term desorption and degradation (Kjær et al., 2003).
Subsoil degradation of metribuzin occurred very slowly, as illustrated in Fig. 2 . About 80% of the applied metribuzin was still present after one year. This is consistent with the formation of metabolites, predominantly DA and DK, which reached a level of 10% of applied metribuzin. The data did not fit very well to any kinetic equation. Using a first-order fit, a half-life exceeding 500 d was calculated. There was no indication of strong sorption of metribuzin in the subsoil or systematic differences between sterile and nonsterile degradation.
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Metabolite Formation
In the nonsterile topsoil, the metabolite DA reached a level of 6 to 7% of applied metribuzin within a short time interval after spiking with metribuzin (Fig. 1b). The DA formation was immediately followed by formation of DADK. A high peak of DK was observed at Days 60 to 80 apparently followed by a further increase of DADK, indicating a rapid transformation of DK as well as DA. Even if a relatively high content of DK was measured at Day 375, the rather constant presence of DA during the whole incubation period, concurrent with the fast disappearance of DK, indicate that the slow long-term degradation may occur primary via the DA pathway. Also in the sterile topsoil, DA was found during the whole incubation period, but in a lower concentration (Fig. 1c), and especially the subsequent formation of DADK was notably lower. Matrix interference in the sterile topsoil was too high to detect DK at a level realistic for metabolite formation. Thus, no results for DK are presented in Fig. 1c even though DK may have been present in the samples at levels below 0.03 mg/kg (approximately 10% of applied metribuzin).
Formation of DA in soil may occur by photochemical reactions at the soil surface and by biotic and abiotic processes in the soil (Hatzios and Penner, 1988; Schilling et al., 1985; Webster and Reimer, 1976b). Recently, a mechanism was suggested by Landgraf et al. (2001) for abiotic degradation of metribuzin to DA by interaction with the humic substances in the soil. Using infrared spectroscopy the changes in chemical bonding and functional groups were studied by adding metribuzin to humic acid. An increase in amide bonds and a decrease in alcohols were observed. They suggested reactions between the metribuzin amino group and carboxylic acid and alcoholic functional groups in the humic substances, followed by the release of DA. The steady formation of DA in the sterile topsoil in the present work supports this theory.
Degradation of Metabolites
Topsoil and subsoil degradation of each metabolite are shown in Fig. 3
. In the topsoil only 8 to 16% of the respective metabolites applied were remaining after 125 d. Similar to metribuzin, the loss was probably partly caused by other processes than degradation as shown by results for the sterile soil samples.
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Diketo-metribuzin dissipated rather fast in both the sterile and nonsterile topsoil, and it is notable that comparable levels were reached in the two soils after 90 d (Fig. 3a). However, variation in the content of DK in the nonsterile soil between the first measurements makes the results less decisive. Considering the few data points in the DK curves, degradation of DK can also be evaluated indirectly by considering formation of the metabolite DADK in these samples. In the nonsterile samples, DADK reached a level of 14% of applied DK within 14 d, and it was still 9% in the last measurement, indicating a steady degradation of DK. The level of DADK was almost the same in the sterile samples, and at the end of the experiment it exceeded the nonsterile samples (Fig. 3a). The relatively high degree of abiotic degradation of DK compared with DA (Fig. 3b) may be due to the same reaction with the humic substances as described for metribuzin (Landgraf et al., 2001). The amino group is still present in DK and the mechanism causing de-amination may be similar. Differing from the present results, DK was found in a field experiment to be the most persistent of the three metabolites and the only metabolite present in the soil the following season after spraying (Webster and Reimer, 1976a). Subsoil degradation was very slow for all the metabolites (Fig. 3d–f) and fitting such data to a model is delicate as the dissipation time calculated exceeds the incubation time. Taking this into consideration the two-compartment fit demonstrated the best correlation. The DT50 values calculated were 900 d or more (Table 2).
Abiotic Degradation
The importance of microbial and abiotic transformation in the steps of the degradation pathway was studied by comparing the primary metabolite formation from metribuzin (DA and DK) in the sterile and nonsterile soil samples, and by comparing formation of DADK from the samples treated with DA and DK, respectively. Significant difference (p < 0.05) was found in all cases by two-way analysis of variance (ANOVA), both in the topsoil and the subsoil. Further, to estimate the relative contribution from biotic and abiotic degradation the quantitative differences were calculated. For example, in the samples originally treated with DA, the microbial degradation is estimated by subtracting the content of DADK (the triplicate means) in the sterile soil samples from the content of DADK in the nonsterile soil, at each point of measurement. This is illustrated in Fig. 4
, showing the difference curves for the formation of DADK in nonsterile and sterile topsoil by transformation of DA and DK, respectively. It was assumed that the more positive the difference, the more dominating was the microbial degradation. However, this can only be a rough estimate since metabolite accumulation is the result of metabolite formation and metabolite degradation processes. As seen in Fig. 4, the difference decreases over time, probably do to a higher degree of metabolite accumulation in the sterile soil, which is why estimation of biotic and abiotic degradation should primarily be based on the first part of the curves. Accordant to that, it appears in Fig. 4 that the difference between biotic and abiotic degradation is bigger for the step DA DADK than for DK DADK, indicating that the transformation of DA is more microbially dependent than that of DK.
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DA, whereas the second step DA DADK was predominantly microbially dependent as illustrated in Fig. 4. In the DK pathway in the topsoil, the first step MBZ DK could not be evaluated, since DK was not detected in the sterile topsoil spiked with metribuzin. The second step, de-amination of DK to form DADK, had a considerable contribution from abiotic degradation (Fig. 4).
In the subsoil, the differences were considerably smaller due to the low degree of degradation in general. The most distinct difference was seen for the step DK DADK, which had a relatively high contribution from microbial degradation compared with abiotic degradation. Overall, more degradation took place in the sterile topsoil than in subsoil (sterile as well as nonsterile). In particular the de-amination process proceeded much faster in the topsoil, supporting the hypothesis that humic substances are involved in the process of abiotic degradation.
Sorption and Desorption
Freundlich isotherms were determined for sorption and desorption of the four compounds in the topsoil (Fig. 5)
. In the subsoil isotherms could be determined for DA and DADK only, since the subsoil sorption of metribuzin and DK was offset by the variation within the triplicates. The Freundlich constants for the sorption and desorption isotherms are summarized in Table 3. The sorption isotherms indicated saturation (n < 1) in all cases except for a linear isotherm of DADK in the topsoil.
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Considering sorption of organic pollutants the soil content of organic matter is often considered of highest importance. For metribuzin, Kd varied from 31.7 L/kg in a soil containing 60% organic matter to 0.56 L/kg in a very sandy loam (Sharom and Stephenson, 1976). In accordance, in the present study on metribuzin a Kd of 0.94 L/kg was measured in the low organic sandy loam (in the topsoil control sample).
Considering topsoil, the metabolite DADK had the lowest sorption followed by DK (comparing calculated Kd values at equal concentration). This is in agreement with the findings resulting from field applications of the pesticide as these two transformation products were monitored in the ground water and pore water at the Tylstrup field (Kjær et al., 2003). Also, similar results were seen in a lysimeter experiment where the mobility followed the order DADK > DK > metribuzin > DA in Plainfield sand (mixed, mesic Typic Udipsamments) (Bowman, 1991). In contrast, DK was found to be the strongest sorbed species in a batch experiments with Dundee loam (fine-silty, mixed, active, thermic Typic Endoaqualfs) (Locke et al., 1994). This may result from the presence of different types of sorption sites in the more clayey soil. Also, the soil pH of the Dundee loam soil was higher (6.43). In another study on metribuzin, a negative correlation between sorption and soil pH was demonstrated. At pH 6.7 the sorption was about twice as the sorption at pH 4.6 (Ladlie et al., 1976). However, the sensitivity to pH may not be equal for metribuzin and the metabolites. Consequently, variations in soil pH may result in a change of the mutual order of sorption.
In the subsoil the sorption of DA and DADK was lower and isotherms for metribuzin and DK could not be calculated. This may be due to a much lower sorption capacity in the subsoil. Considering DA and DADK, it is notable that the order of sorption differs from the topsoil, with strongest sorption in the subsoil of the two de-aminated metabolites DA and DADK. A likely cause is that other mechanisms may be involved in the subsoil sorption in which the amino group affects the equilibrium toward the solution phase.
Desorption was studied in the topsoil only, due to the low sorption in subsoil. As seen in Fig. 5, the desorption isotherms are steeper than the sorption isotherms and displaced to the left, indicating hysteresis for metribuzin, DA, and DK (except for the last data point for DK, see below). Interestingly, the desorption isotherm for DADK is less steep, with the last two data points dropping off toward zero (not included in the isotherm). This is probably caused by strong interactions among the compounds in the mix samples during the desorption process. Comparing Kdes at the highest concentration for the mix samples and the separate control samples, significant divergences were observed except for metribuzin. It appears from the ratios in Table 3 that Kdes for DA was almost twice as high in the mix samples. The opposite was observed for DK and DADK. Also, hysteresis was observed for DADK in the separate samples only. In conclusion, the process of desorption appears to be more influenced by the total concentration of compounds in solution than the sorption process. The mechanism might be that in the mix samples DA gradually takes over the unoccupied sites left by desorbing DK and DADK. Consequently, the equilibrium continuously displaces in favor of more DA sorbed to the soil in exchange for DADK and DK.
This type of measurement may be more descriptive of the conditions relevant for field conditions. Hence, during metribuzin transformation in the environment it can be anticipated that the active compound as well as transformation products are present simultaneously in the soil. This aspect should be considered when employing results from single-compound experiments.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abiotic degradation of metribuzin and the metabolites was notably faster in the topsoil than in the subsoil, implying that the humic substances in the soil are directly involved in or facilitate transformation, especially the de-amination process.
Sorption in the topsoil followed the order DA > metribuzin > DK > DADK. Desorption followed the same order, but the differences were more pronounced. Important differences between measurements made in single-component and mixture experiments were observed. When present in mixture, the metabolite DA was desorbed to a less extent at the expense of DK and DADK and an exchange mechanism was suggested. Sorption to the subsoil was lower, and only measurable for the de-aminated metabolites DA and DADK.
From the present study, it may be expected that the majority of metribuzin will dissipate during the growth season due to degradation, strong sorption, and bound residue formation. Both metabolites DA and DK are formed in the degradation process followed by DADK. Slow degradation continues for a long period, primarily controlled by the desorption process. Due to differences in sorption and especially desorption properties, metribuzin and DA will preferentially be retarded in the topsoil whereas DK and DADK are considerably more present in the solution phase and thus exposed to leaching. As DK and DADK move through the subsoil layers, DADK will be most retarded, causing DK to reach the ground water in a higher extent and before DADK. This scenario is in good agreement with monitoring results from a field-scale monitoring program. Also, the present work demonstrates the need for knowledge of the primary metabolites with respect to degradation, sorption, and desorption properties when assessing the risk of pesticide leaching, and the multicompound effects on such processes are documented.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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