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TECHNICAL REPORT
Organic Compounds in the Environment

Laboratory Degradation Studies of Bentazone, Dichlorprop, MCPA, and Propiconazole in Norwegian Soils

The Norwegian Crop Research Institute, Plant Protection, Høgskoleveien 7, N-1432 Ås, Norway

Corresponding author (christian.thorstensen{at}planteforsk.no)

Received for publication June 2, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Laboratory degradation studies were performed in Norwegian soils using two commercial formulations (Tilt and Triagran-P) containing either propiconazole alone or a combination of bentazone, dichlorprop, and MCPA. These soils included a fine sandy loam from Hole and a loam from Kroer, both of which are representative of Norwegian agricultural soils. The third soil was a highly decomposed organic material from the Froland forest. A fourth soil from the Skuterud watershed was used only for propiconazole degradation. After 84 d, less than 0.1% of the initial MCPA concentration remained in all three selected soils. For dichlorprop, the same results were found for the fine sandy loam and the organic-rich soil, but in the loam, 26% of the initial concentration remained. After 84 d, less than 0.1% of the initial concentration of bentazone remained in the organic-rich soil, but in the loam and the fine sandy loam 52 and 69% remained, respectively. Propiconazole was shown to be different from the other pesticides by its persistence. Amounts of initial concentration remaining varied from 40, 70, and 82% in the reference soils after 84 d for the organic-rich soil, fine sandy loam, and loam, respectively. The organic-rich soil showed the highest capacity to decompose all four pesticides. The results from the agricultural soils and the Skuterud watershed showed that the persistence of propiconazole was high. Pesticide degradation was approximated to first-order kinetics. Slow rates of degradation, where more than 50% of the pesticide remained in the soil after the 84-d duration of the experiment, did not fit well with first-order kinetics.

Abbreviations: WHC, water holding capacity

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
MANY processes are involved in the dissipation of pesticides in the environment. Degradation processes are characterized by splitting of the pesticide molecule by chemical, photochemical, or biological processes. Transfer processes are characterized by the pesticide molecule remaining intact. They include sorption to soil constituents, movement of the soil in runoff, movement into the air by volatilization, movement downward in the soil by leaching, and movement upward in the soil through capillary forces.

Phenoxy acids formulated as salts or esters have been extensively used for weed control in agriculture since World War II. They are hydrolyzed to their parent compounds and found in their acidic form in soil (Tadeo et al., 1996). Phenoxy acids are lost from the field mainly by microbial degradation (Upchurch, 1972), although some loss can occur through volatilization, photolysis, and lateral surface movement in water. These are considered minor dissipation pathways (Barnett et al., 1967). Dichlorprop has been reported to degrade completely in soil within 31 d, with a half-life of 6.6 d (Garrison et al., 1996). Degradation occurred with enantiomeric selectivity: the S-(-) isomer degraded faster than the R-(+) isomer, with half-lives of 4.4 and 8.7 d, respectively. Others have reported a degradation of dichlorprop within 14 d (Smith and Hayden, 1981; Thompson et al., 1984; Zipper et al., 1999). Several authors found that MCPA is degraded in soil within 1 to 16 wk, depending upon soil type and environmental conditions, by microbial degradation (Audus, 1964; Smith and Hayden, 1981; Torstensson et al., 1975; Helweg, 1987).

Bentazone is an acidic herbicide often used in combination with phenoxy acids used to control weeds. Degradation experiments of bentazone by Kördel et al. (1991) show half-lives ranging from 30 to 60 d. Half-lives of bentazone from 4 to 21 d at five different field sites in Germany and from 3 to 19 d at six different sites in the United States have also been reported (Huber and Otto, 1994). Other studies of bentazone degradation report half-lives from 1 to 15 wk (Wagner et al., 1996; Bergström and Jarvis, 1993; Otto et al., 1979).

Propiconazole is a systemic foliar fungicide with protective and curative activity against a wide range of fungi and is mainly used in cereals, sugar beets and stone fruits (Tomlin, 1997). In the soil environment, propiconazole is mainly degraded by microorganisms, with hardly any contribution of photolysis and hydrolysis. The main degradation pathways are hydroxylation of the propyl side-chain and dioxolane ring, and finally formation of 1,2,4-triazole (Tomlin, 1997). Most information on persistence is derived from registration documents, with relatively few studies appearing in the open literature. Propiconazole was reported to be very persistent in a laboratory test in clay loam and sandy loam with half-lives of 336 and 277 d, respectively (Bromilow et al., 1999). Half-lives between 40 and 70 d are cited in Tomlin (1997).

To obtain reliable fate information for pesticides in the Nordic countries, the selection of representative soils is important. Soils from the Nordic region differ from soils in other regions due to the indirect influence of climatic factors on degradation through the genesis and qualitive content of clay minerals and organic matter. Because of the weathering period after the last glaciation in Scandinavia, not more than 10000 yr ago, the content of the clay minerals differs from those in southern Europe (Eklo and Lode, 1994). There is also more organic matter in Nordic soils (Greve et al., 1998) due to the climate (high rainfall, low drainage, and colder climate). It is also important to compare findings in Nordic soils with results from other soils (i.e., EUROSOILS, which are selected from mid- and southern Europe, and which have been used for interlaboratory degradation and sorption studies). The need for being able to compare results from the Nordic countries with other countries is becoming more and more essential, since evaluation of many chemicals is now coordinated within the European Union (EU), or the Organisation for Economic Co-Operation and Development (OECD). Thus, laboratory tests according to test guidelines are used for pesticide registration. The selected soils will serve as reference soils in ecotoxicological tests and recovery experiments of pesticides in the Norwegian environment.

Pesticide dissipation in soils has been studied extensively. Usually, pure active ingredients are used in laboratory dissipation experiments, while for field experiments formulated pesticides are commonly used. It is usually considered that results on sorption and degradation with the active ingredient can be extrapolated to the formulated pesticide. However, the physical state of the pesticide can be different. It is therefore important to use representative agricultural soils and commercial formulations of pesticides because this is more relevant for agriculture.

The objective of this work was partly to study the degradation of a formulation containing a mixture of bentazone, dichlorprop, and MCPA and a formulation containing propiconazole alone in three reference soils from Norway, and partly to study the degradation of propiconazole in soil from the Skuterud watershed, which is a part of the Agricultural and Environmental Monitoring Program of Pesticides in Norway.

The results will be used in a future work, with mathematical models, to predict the fate of pesticides as site-specific data for degradation, which is important for input parameters.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil
Two of the soils, the fine sandy loam from Hole and the loam from Kroer, were selected based on information from the Norwegian Institute of Land Inventory's Digital Soil Database, which currently covers about 2500 km² of agricultural land. These are representative agricultural soils in Norway. A forest soil of highly decomposed organic matter from Froland was selected as a third soil due to its high organic content. The selected soils serve as reference soils in ecotoxicological tests of pesticides in the Norwegian environment. A fourth soil from the Skuterud watershed was used in addition to the three other soils in degradation studies of propiconazole. The Skuterud watershed is a part of the Agricultural and Environmental Monitoring Program of Pesticides in Norway. In the experimental field from the Skuterud watershed, three different sites were taken and within each site three horizons (layers) were used. All locations lie in the southern part of Norway. Physical and chemical properties are listed in Table 1. The Froland soil was a forest soil, all other soils were from agricultural landscapes. The soils were dried to 30 to 50% of water holding capacity (WHC) and passed through a 2-mm sieve. All soils were stored in the dark at 4°C.

Degradation Experiment
Two weeks before the start of the degradation experiment an amount of each soil was placed in the dark at 20°C and at 60% WHC, in a container covered with perforated aluminium foil to allow gas exchange. Approximately 1000 g of the highly decomposed organic matter soil from Froland was sterilized by irradiation from a ⁶⁰Co source with a dose of 32 kGy (three 8-h exposures) to serve as a reference for microbial degradation before pesticides were added. A mixture of bentazone, dichlorprop, and MCPA (Triagran-P) was dissolved in 50 mL distilled water and added to soil at 60% WHC (equivalent to 1000 g dry soil). The initial concentrations were: 14 mg bentazone/kg dry soil, 10 mg dichlorprop/kg dry soil, and 12.2 mg MCPA/kg dry soil. Propiconazole (Tilt 62.5) was dissolved in 50 mL distilled water and added to soil at 60% WHC (equivalent to 1000 g dry soil) to give an initial concentration of 5 mg propiconazole/kg dry soil. Water was added to bring the soils to a moisture content of 70% WHC. The soils were covered with clean polyethylene, allowed to equilibrate for 1 h at room temperature, and thoroughly mixed. Wet soil samples, equivalent to 70% WHC, were transferred into glass containers covered with perforated aluminium foils. The soils were stored in the dark at 20°C. Moisture content was maintained at the initial level, equivalent to 70% WHC. From the reference soils, four samples of 30 g and duplicate samples from the Skuterud watershed were taken at each sampling 0, 1, 2, 3, 4, 5, 6, 8, and 12 wk after treatment and stored at -20°C until analysis. For analysis, 25 g was extracted.

The degradation experiment was performed in accordance with two intercomparison studies conducted at our laboratory. The parent compounds in the soil extracts were determined by gas chromatography (GC)–electron capture detection (ECD) or high performance liquid chromatography (HPLC)–ultraviolet (UV) and the pesticide concentration fitted to a first-order kinetic according to the equation:

where C is the mass of compound in the soil (mg/kg) at a given time, t is time (d), C₀ is the original mass of compound added to the soil (mg/kg), and k is the apparent degradation rate coefficient including sorption effects (Nkedi-Kizza and Brown, 1998).

Analytical Procedure
Phenoxy Acids and Bentazone
A portion of the 25-g wet soil sample (70% WHC) was extracted with 100 mL of 0.01 M NaOH using an orbital shaker for 1 h at 100 rpm. After centrifugation at 7600 x g for 10 min, the supernatant was decanted and the extraction repeated. The combined supernatants were acidified with 6 M HCl to pH 2.5 ± 0.5 and centrifuged again at 7600 x g for 10 min. The supernatant was concentrated on a solid-phase cartridge (ENV+ 500-mg, 6-mL styrene divinylbenzene cartridge; IST, Hengoed, Mid Glamorgan, UK). The extraction cartridge was rinsed by passing 5 mL of methanol through the cartridge followed by 10 mL of water adjusted to pH 2.5 ± 0.5 with 6 M HCl. Sample loading was performed at a flow rate of 30 mL/min under vacuum. After extraction, the cartridges were dried with a gentle stream of nitrogen. Elution was performed by gravity using 2 mL of methanol with 5% NH₃. The eluate was evaporated to dryness and redissolved in 1 mL 20 mM phosphate buffer (pH 3). A 50-µL aliquot was analyzed by high performance liquid chromatography (HPLC) equipped with a UV detector set at 200 nm. A C₈ analytical column (stainless steel, 25 cm x 4.6 mm i.d.) packed with Spherisorb 5-µm particles (Phase Separations Ltd., Deeside, UK) was used for HPLC analysis. The mobile phase was 20 mM phosphate buffer (pH 3)–acetonitrile (65:35 v/v) at a flow rate of 1 mL/min. The limit of quantification was 0.1 µg/g.

Propiconazole
Twenty-five grams of wet soil (70% WHC) was extracted with 25 or 100 mL (100 mL in the organic-rich soil) ethyl acetate–acetone (1:1 v/v) for 1 h and a part of the organic solvent was transferred to tubes. The soil extracts were analyzed by gas chromatography (GC)–electron capture detection (ECD) (Carlo Erba, Milan, Italy) that was fitted with a HP-1 (Little Falls, DE) capillary column (25 m x 0.32 mm). Helium carrier gas velocity was set at 1.7 mL/min. The detector make-up gas, methane and argon (5:95), was set at 23 mL/min. Initial oven temperature was set at 60°C and held for 5 min. The oven temperature was then ramped to 160°C at 20°C/min and then to 270°C at 4°C/min and a final 10-min hold. A 2-µL sample size was injected in the splitless mode (purge of 90 s). The limit of quantification was 0.1 µg/g.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Percent remaining (C x 100/C₀) versus time for all experiments are shown in Fig. 1–3. The data are fitted to first-order kinetics and the calculated half-lives for all four pesticides in the reference soils are given in Table 2. Half-life values were in the range of 8 to 133 d for bentazone, 4 to 38 d for dichlorprop, 4 to 16 d for MCPA, and 20 to 210 d for propiconazole, with the shortest degradation time in the organic-rich soil from Froland. Half-lives of propiconazole in soil from the Skuterud watershed were in the range 120 to 1199 d. An acceptable first-order kinetic is defined as r² > 0.7 (Organisation for Economic Co-Operation and Development, 1999). The straight-line relationships obtained for bentazone, dichlorprop, and MCPA indicate an acceptable fit to first-order kinetics for the degradation of all three of these compounds in the reference soils. Propiconazole could only be fitted reasonably well to first-order kinetics in the Froland soil. A first-order kinetic did not fit for propiconazole in the Hole fine sandy loam, the Kroer loam, and in soil from the Skuterud watershed, where the half-life exceeds 84 d. The half-life of bentazone was longer than 84 d, but first-order kinetics could be fitted well. Generally, for half-lives exceeding the duration of the experiment, first-order kinetics did not give a good fit. First-order kinetics was used for comparative purposes and is often chosen due to its simplicity and is used in the major part of degradation studies. It does not necessarily give the best model for describing the degradation. A review of more than 400 degradation experiments revealed that only about 35% showed real first-order kinetic behavior during experimental duration (Timme et al., 1986).

View larger version (12K): [in this window] [in a new window]   Fig. 1. Percent remaining of bentazone as a function of incubation time in sterilized and nonsterilized organic-rich soil from Froland. Error bars present standard deviation, n = 4.

View larger version (14K): [in this window] [in a new window]   Fig. 3. Percent remaining of propiconazole as a function of incubation time at Site 1 from three different layers in soil from the Skuterud watershed. Error bars present standard deviation, n = 2.

View larger version (19K): [in this window] [in a new window]   Fig. 2. Percent remaining as a function of incubation time of (a) bentazone, (b) dichlorprop, (c) MCPA, and (d) propiconazole in the Hole loam, the Kroer fine sandy loam, and the organic-rich soil from Froland. Error bars present standard deviation, n = 4.

The sorption properties of the reference soils have been previously characterized for the four pesticides (Thorstensen et al., 2000). The results from the sorption studies with the same pesticides as in the references soils have shown the following decreasing order of sorption: Froland organic-rich soil > Kroer loam > Hole fine sandy loam. The sorption was high for all pesticides in Froland. A faster degradation was observed in the organic-rich soil compared with the agricultural soils. Sorption of organic pesticides and the biological activity might increase with an increase in the organic matter content (Kanazawa et al., 1988; Johnson and Sims, 1993; McCormick and Hiltbold, 1966). These factors, however, can have opposite effects on pesticide degradation. Sorption has often been shown to decrease pesticide degradation rates by reducing their availability to microbial attack (Ogram et al., 1985; Smith et al., 1992). An increase in soil microbial activity is likely to increase the rate of pesticide degradation. The influence of microbial activity on pesticide degradation has often been inferred indirectly from the effect of other factors such as temperature, moisture, and nutrient supply on degradation (Veeh et al., 1986; Alvey and Crowley, 1995).

In general, a faster degradation of dichlorprop, MCPA, and propiconazole occurred in the Hole fine sandy loam compared with the Kroer loam. The remaining concentrations after 84 d were <5% for MCPA in these soils. However, for dichlorprop, 25% remained in the Kroer loam, while in the Hole fine sandy loam <5% remained. Propiconazole also degraded faster in the Hole fine sandy loam compared with the Kroer loam and after 84 d, 70 and 77% remained, respectively. A slower degradation rate occurred for bentazone in the Hole fine sandy loam compared with the Kroer loam, with 69 and 52% remaining, respectively, after 84 d.

A faster degradation of dichlorprop, MCPA, and propiconazole in the Hole fine sandy loam compared with the Kroer loam may be explained by a higher pH and lower organic content in the loam. Increased degradation at higher pH has been indicated by others (Hance, 1979; Torstensson et al., 1975). The fastest degradation occurred in the organic-rich soil from Froland. Although the pH in this soil is very low (2.9), the fast degradation in this soil could be explained by a high biological activity, where fungi plays an important role. It is also possible that formation of bound residue related to soil with a high content of organic matter content could be interpreted as degradation. Therefore, only the most organic-rich soil was sterilized. The microbial decomposition process in organic-rich soil can be seen in Fig. 1. After 30 d, 91 and 42% of bentazone remained in sterilized and unsterilized soil from Froland, respectively. There is a difference of 35% in degradation of bentazone after 28 d in the organic-rich nonsterile soil in Fig. 1 and 2, which is probably caused by a reduced biological activity due to soil freezing. This indicates that freezing and storage of the soil should be avoided.

A very slow degradation of propiconazole occurred in the soil from the Skuterud watershed, Fig. 3. Only the degradation curve at Site 1 is shown, because all sites revealed similar degradation. A faster degradation could be observed in the upper layer than below, which is not unexpectable because the top layers have in general a higher biological activity. After 84 d more than 96% of the initial concentration remained in the subsoils. Fifty percent degradation was not achieved for propiconazole from any of the sites. A first-order kinetic did not fit for propiconazole in soil from the Skuterud watershed and the extrapolated individual half-lives are therefore not given. The degradation of propiconazole in the upper layer of soil from the Skuterud watershed is comparable with the results from agricultural soils and confirms the persistence of propiconazole.

The degradation rate of dichlorprop, MCPA, and propiconazole decreased with an increase in soil organic carbon, which is attributed to the increase in sorption. Increasing sorption results in a low concentration of the pesticides in solution that is available for microbial degradation. In contrast with these results, when the organic content was 37%, both sorption and degradation rate increased. The enhanced degradation at 37% organic carbon may be related to increased biological activity caused by fungi. A degradation study of 2,4-D by Bolan and Baskaran (1996) showed an increase in t_1/2 with increasing K_d, but t_1/2 decreased when the K_d values reached a limit.

In Norwegian agriculture, mixtures of herbicides are commonly used (e.g., phenoxy acids in combination with bentazone or ioxynil). There is little information available concerning the behavior of combinations of pesticides. Many interactions are possible, including those related to changes in the size or species composition of the soil microbial population, to effects on specific enzymic reactions, or to physicochemical effects such as competition for sorption sites. Technical standards of, for example, phenoxy acids, are in their acidic form, while the commercial products often are salts or esters.

A study of dichlorprop by Garrison et al. (1996) showed that the degradation of dichlorprop occurred with enantiomeric selectivity and the S-(-) isomer degraded faster than the R-(+) isomer, with half-lives of 4.4 and 8.7 d, respectively. This indicates that studies with formulations contaning one of the isomers is more relevant than those using racemic technical mixtures. By using representative commercial formulations we probably provide results more relevant to agricultural practice, compared with other studies using technical standards. Charnay et al. (2000) reported that additives used to formulate commercial pesticides influenced soil degradation of triticonazole.

The half-lives of bentazone, dichlorprop, and MCPA in this study are similar to other studies. However, there is such a wide range of values found in the literature that it is unlikely that values outside this range would be found. This is the first degradation study in Norwegian reference soils and is of great importance for the authorities. Also, within EU the results are important, because there is a need to compare results from the Nordic countries with other countries, since evaluation of many chemicals is now coordinated within EU or OECD. The results are applicable also for areas with a colder climate and young alluvial soils.

Neither dichlorprop nor MCPA is considered to be an environmental pollutant, owing to their rapid degradation in soil. Long half-lives of propiconazole are reported and are consistent with our results (Bromilow et al., 1999). Both bentazone and propiconazole are persistent and may therefore be potential pollutants to the environment.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A fast degradation occurred in the highly decomposed organic soil from Froland, with half-lives of 8 d for bentazone, 4 d for dichlorprop, 4 d for MCPA, and 20 d for propiconazole. The enhanced degradation in the organic-rich soil may be related to increased biological activity due to the high organic content of this forest soil.

For the Hole fine sandy loam and for the Kroer loam, half-lives were 10 and 38 d for dichlorprop and 7 and 16 d for MCPA, respectively. In the Kroer loam and in the Hole fine sandy loam half-lives of bentazone were longer than the duration of the experiment (84 d). Half-lives of propiconazole exceeded the duration of the experiment in all of the soils with the exception of the highly decomposed organic soil from Froland. It can be concluded that propiconazole and bentazone are potential pollutants to the environment due to their persistence.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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