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TECHNICAL REPORT
Organic Compounds in the Environment

Effect of Turfgrass Cover and Irrigation on Soil Mobility and Dissipation of Mefenoxam and Propiconazole

^a Dep. of Horticultural and Crop Science, 2021 Coffey Rd., The Ohio State University, Columbus, OH 43210-1086
^b Dep. of Natural Resources and Environmental Sciences, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801-4798

* Corresponding author (bbranham{at}uiuc.edu)

Received for publication July 14, 2000.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Irrigation effects on pesticide mobility have been studied, but few direct comparisons of pesticide mobility or persistence have been conducted on turfgrass versus bare soil. The interaction of irrigation practices and the presence of turfgrass on the mobility and dissipation of mefenoxam [N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-D-alanine methyl ester] and propiconazole (1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl]methyl]-1H-1,2,4-triazole) was studied. Sampling cylinders (20-cm diam.) were placed in either creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw.] or bare soil. Mefenoxam was applied at 770 g a.i. ha^-1 and propiconazole was applied at 1540 g a.i. ha^-1 on 14 June 1999. Sampling cylinders were removed 2 h after treatment and 4, 8, 16, 32, and 64 days after treatment (DAT) and the cores were sectioned by depth. Dissipation of mefenoxam was rapid, regardless of the amount of surface organic matter or irrigation. The half-life (t_1/2) of mefenoxam was 5 to 6 d in turf and 7 to 8 d in bare soil. Most mefenoxam residues found in soil under turfgrass were in the 0- to 1-cm section at 0, 4, and 8 DAT. Residues were found in the 15- to 30-cm section at 4, 8, 16, 32, and 64 DAT, regardless of turf cover or irrigation. The t_1/2 of propiconazole was 12 to 15 d in turfgrass and 29 d in bare soil. Little movement of propiconazole was observed in either bare soil or turf.

Abbreviations: DAT, days after treatment • t_1/2, half-life

	INTRODUCTION

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PUBLIC awareness of possible environmental contamination resulting from the use of pesticides in turfgrass systems has increased in recent years (Balogh and Anderson, 1992). However, turfgrass is a unique system in that, except at establishment, pesticides are applied directly to the plant material and thatch, a layer of dead and living stems and roots between the green vegetation and soil surface. Movement of turf-applied pesticides into the soil is attenuated by the high organic carbon content of turfgrass thatch (Branham and Wehner, 1985; Branham, 1994; Horst et al., 1996).

Laboratory studies show that turfgrass leaves and thatch strongly sorb organic compounds and thus should have a significant effect on the fate of pesticides applied to turfgrass (Niemczyk et al., 1988; Dell et al., 1994; Lickfeldt and Branham, 1995). Sears and Chapman (1979) showed that even a thin layer of thatch could significantly retard pesticide movement on sandy soils. Retention of pesticides by thatch may result in reduced mobility of pesticides applied to turfgrass (Stahnke et al., 1991; Smith and Bridges, 1996).

Some pesticides dissipate more quickly in thatch compared with soil (Hurto et al., 1979; Gold et al., 1988; Gardner et al., 2000). Horst et al. (1996) found that the half-lives of metalaxyl [N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-DL-alanine methyl ester], pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], chlorpyrifos [O,O-diethyl-O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate], and isazofos (O-5-chloro-1-isopropyl-1H-1,2,4-triazol-3-yl O,O-diethylphosphorothioate) applied to turfgrass were 16, 12, 10, and 7 d, respectively. This compares with previously published soil t_1/2 data of 70, 34, 30, and 90 d, respectively (Balogh and Anderson, 1992).

Mefenoxam is the resolved, biologically active stereoisomer of metalaxyl. Mefenoxam is a fungicide for the control of certain diseases in turfgrass and other crops. A considerable amount of work has been published concerning metalaxyl (Horst et al., 1996; Starrett et al., 1996; Sukul and Spiteller, 2000). Metalaxyl has a water solubility of 8.4 g L^-1 and a K_oc of 29 to 287, indicating the potential for considerable leaching through the turfgrass–soil profile. Metalaxyl has a variable t_1/2, ranging from 7 to 160 d (Balogh and Anderson, 1992). To date, no work has been published on the environmental fate of mefenoxam; however, it would be expected to be very similar to metalaxyl. Yet, mefenoxam has a higher water solubility, 26 g L^-1, than metalaxyl.

Propiconazole is a triazole fungicide used to control several pathogens. Propiconazole has a water solubility of 110 mg L^-1 and a K_oc of 387 to 1147, indicating the potential for some leaching through the turfgrass–soil profile. Propiconazole is persistent, with a t_1/2 of 109 to 120 d (Wauchope et al., 1991).

Our objective was to investigate the effect of creeping bentgrass cover and irrigation on the mobility and persistence of mefenoxam and propiconazole. We also wished to determine whether the attenuating effect of thatch on pesticide mobility and persistence was uniform across pesticides of different chemical properties.

	MATERIALS AND METHODS

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Field Procedures
Field experiments were conducted in ‘Penneagle’ and ‘Seaside II’ creeping bentgrass turf in 1999 at the University of Illinois Landscape Horticulture Research Center in Urbana, Illinois. The soil was a Flanagan silt loam (fine, smectitic, mesic Aquic Argiudoll; 52 g kg^-1 organic matter, 1.38 g cm^-3 bulk density, and pH 6.5). Very few macropores were detected throughout the study, based on visual examination of soil cores. Creeping bentgrass thatch was 10 to 14 mm thick and had a bulk density of 0.53 g cm^-3, 241 g kg^-1 organic matter, and pH 6.7. Bare soil plots were prepared by stripping the sod from the plot with a sod-cutter.

Sampling cylinders were constructed of 20-cm-diam. Schedule 40 polyvinylchloride (PVC) pipe cut into 30-cm lengths and beveled at one end to ease insertion into the soil. Sampling cylinders were inserted into each plot on 11 June 1999 using a hydraulic press (Alden Enterprises, Okemos, MI) attached to a tractor.

Mefenoxam (Subdue Maxx; Sygenta, Greensboro, NC) or propiconazole (Banner Maxx; Sygenta) was applied on 14 June 1999 at 770 g a.i. ha^-1 and 1540 g a.i. ha^-1, respectively. The pesticides were applied with a backpack sprayer equipped with a TEEJET 8006E (Spraying Systems Co., Wheaton, IL) nozzle at a height of 36 cm, with an effective spray width of 30 cm. The pesticides were applied in 1120 L water ha^-1 at 276 kPa. Irrigation (4 mm) was applied to the plots immediately after treatment.

The experimental area was mowed three times per week at 1.8 cm with a reel mower and clippings were removed. Half of the plots were irrigated with 10 mm of water five times per week (high irrigation schedule). The other plots (low irrigation schedule) were watered as necessary to replace 80% of estimated evapotranspiration (Table 1).

Pesticides were extracted from thawed samples by placing a representative 20-g sample (3–10 g for verdure) in a 500-mL Erlenmeyer flask. Pesticides were extracted from the samples by shaking with ethyl acetate (100 mL) for 4 h (3 h for soil samples) on a platform shaker at 200 rpm. The extracts were vacuum-filtered by passing through a Whatman (Maidstone, UK) G6 glass fiber filter.

Ethyl acetate was removed by rotary evaporation at 40°C. The evaporatory flask was rinsed three times with 5 mL methylene chloride and the rinsate transferred. The methylene chloride was evaporated to 2 mL using a reacti-vap (Pierce, Inc., Rockford, IL). The extract was then passed through a 0.45-µm nylon membrane filter (Gelman Sciences, Ann Arbor, MI) and transferred to an autosampler vial for analysis on a high pressure liquid chromatograph (Beckman Coulter, Fullerton, CA).

Mefenoxam and propiconazole were separated on a 15-cm, 4.6-mm-i.d. column with a bonded 5-µm C-18 phase (Beckman Coulter). Mefenoxam was separated from verdure, thatch, and soil coextractives by injecting 40 µL into a mobile phase of 30:70 (acetonitrile to water). After 8 min the mobile phase was increased to 100% acetonitrile over a 10-min period. Mefenoxam was detected with a UV-Vis detector (Beckman Coulter) at 210 nm with a retention time of 14.3 min. Propiconazole was separated from verdure, thatch, and soil coextractives by injecting 40 µL into a mobile phase of 55:45 (acetonitrile to water). After 5 min the mobile phase was increased to 100% acetonitrile over a 6-min period. Propiconazole was detected with a UV-Vis detector at 210 nm with a retention time of 9.4 min.

Mefenoxam residues were quantified by peak area measurements in comparison with a 10 µg mL^-1 external standard. Propiconazole residues were quantified by peak height measurements in comparison with a 10 µg mL^-1 external standard. The limit of detection for both pesticides was 10 µg kg^-1.

Calibration standards were included after every sixth sample. A control sample fortified at 1 mg kg^-1 and a method blank were included with each batch of 22 samples. Mefenoxam recovery from soil, verdure, and thatch samples averaged 91% with a coefficient of variation of 6%. Propiconazole recovery from soil, verdure, and thatch samples averaged 90% with a coefficient of variation of 5%. The mass of mefenoxam or propiconazole remaining in each soil profile was estimated from the concentration of pesticide present in a soil core section and the mass of the core section.

The experiment was designed as a split-split-plot with irrigation as the whole plot, bare soil vs. turf cover as the sub-plot, and soil depth section as the sub-sub-plot. On each sampling date, data were subjected to analysis of variance (SAS Institute, 1990). Half-life values were estimated by regressing the log of pesticide residues remaining versus time.

	RESULTS

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Dissipation of mefenoxam was rapid, regardless of the presence of turfgrass cover or amount of irrigation applied (Fig. 1). The calculated t_1/2 in turfgrass was 6 ± 1 d (R² = 0.99, P = 0.001) under high irrigation and 5 ± 1 d (R² = 0.99, P = 0.002) under low irrigation. The calculated t_1/2 in bare soil was 8 ± 4 d (R² = 0.95, P = 0.022) under high irrigation and 7 ± 2 d (R² = 0.99, P = 0.006) under low irrigation.

View larger version (22K): [in this window] [in a new window]   Fig. 1. Total mefenoxam residue in verdure, thatch, and soil as a function of sampling time.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Distribution of mefanxoam residues among verdure, thatch, and different soil depths over time in 1999. Horizontal T lines represent standard error of the means.

View larger version (21K): [in this window] [in a new window]   Fig. 3. Total propiconazole residue in verdure, thatch, and soil as a function of sampling time in 1999.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Distribution of propiconazole residues among verdure, thatch, and different soil depths over time in 1999. Horizontal T lines represent standard error of the means.

	DISCUSSION

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Previous work has shown that soil mobility of certain pesticides applied to turfgrass is not affected by irrigation or precipitation events (Niemczyk and Krueger, 1987; Cisar and Snyder, 1996). However, Niemczyk and Krause (1994) found that the mobility of some preemergence herbicides was correlated with major rainfall events.

Because of its high water solubility and low soil K_oc, irrigation and rainfall are expected to have a significant effect on leaching of mefenoxam. Metalaxyl leached through 40-cm-deep lysimeters that received 100 mm of precipitation (Odanaka et al., 1994). Horst et al. (1996) found at least 28% of applied metalaxyl in soil under Kentucky bluegrass (Poa pratensis L.) moving through the entire 60-cm soil column. Metalaxyl movement through 25-cm-deep soil columns was 0, 9, 73, and 83% of applied material after being subjected to 50, 100, 150, and 200 mm, respectively, of simulated rainfall (Sharom and Edgington, 1986).

Starrett et al. (1996) found that depth and frequency of application of irrigation water can affect the movement of mobile pesticides such as mefenoxam. The authors found 7.7% of the applied metalaxyl in the leachate from soil columns containing Kentucky bluegrass that received heavy irrigation (four, 25.4-mm applications). In contrast, 0.2% was measured in leachate from the soil columns receiving light irrigation (16, 6.4-mm applications). In our study, irrigation regime was varied by frequency of application, not amount at application. It is likely that an irrigation regime consisting of fewer, larger amounts would have resulted in increased leaching of mefenoxam, as shown by Starrett et al. (1996).

Turfgrass cover did not affect mobility of mefenoxam. In a previous study, metalaxyl recovered in leachate from plots with varying amounts of turfgrass cover was 36.4, 26.7, 14.1, and 16.5% on plots with turfgrass densities of 0, 33, 66, or 100%, respectively (Petrovic et al., 1996). However, based on our soil distribution data, the amount of mefenoxam leaching below the deepest sampling depth, if measured, would have been similar on plots with turfgrass or bare soil. The main reason for the high rate of leaching in the study by Petrovic et al. was the unrealistically high irrigation rate of 19.5 mm d^-1.

The t_1/2 for mefenoxam was 5 to 8 d, which is in close agreement with previous studies in turf (Horst et al., 1996). Mefenoxam residues in turf were significantly less than in bare soil at both 4 and 8 DAT (Table 2), indicating more rapid microbial metabolism of mefenoxam in verdure and thatch than in soil. Sharom and Edgington (1982) reported that metalaxyl was stable in sterilized soils but degraded in unsterilized soils, indicating that degradation by microorganisms is an important factor governing the fate of mefenoxam. Since mefenoxam is not strongly bound by thatch, the higher microbial activity in thatch may be circumvented by more frequent irrigation. The t_1/2 was 1 d longer in soil and turf under more frequent irrigation; however, this difference was not statistically significant.

The t_1/2 of propiconazole in this study was between 10 and 30 d, which is much lower than what was previously reported in Wauchope et al. (1991). Bai and Liu (1987) observed a t_1/2 of about 5 d when propiconazole was applied to wheat (Triticum aestivum L.). Similar results were found by Garland et al. (1999), who observed half-lives of 8 to 10 d when propiconazole was applied to leaves of a peppermint (Mentha x piperita L. nothosubsp. piperita) crop.

Limited downward movement of propiconazole was observed, which agrees with previously published studies (Kim and Suh, 1998; Liu and Weber, 1986). Kim and Suh (1998) found 4.4% of applied propiconazole in leachate from a sandy loam soil after 16 wk. The majority (97%) of the material remained in the upper 20 cm of the profile. Similar results were reported by Liu and Weber (1986). When applied to wheat, more residues were found on straw and leaves than were found in soil (Bai and Liu, 1987).

Other avenues of pesticide fate include runoff, volatilization, and photodegradation (Frederick et al., 1996). Our study was conducted on plots with little slope, which minimized the possibility for runoff losses. Volatilization can be a significant mode of loss of mefenoxam (Petrovic et al., 1996). However, volatilization of propiconazole is of minor significance (Balogh and Anderson, 1992).

A study conducted by Petrovic et al. (1996) measured pesticide residues in leachate on plots with various amounts of turfgrass cover. The present study is one of the first to directly examine the mobility and dissipation of pesticides in turfgrass compared with bare soil, with climate and soil type held constant.

Post-treatment irrigation practices may not be as important in determining the soil mobility of pesticides as is soil moisture at application time or large precipitation events within 14 d of application. The results of this study indicate that the effect of turfgrass cover or irrigation practices may be more important in determining leaching and dissipation of moderately mobile pesticides such as propiconazole. However, pesticides with high water solubilities and low soil K_oc values, such as mefenoxam, are prone to leaching regardless of turf cover or precipitation.

	ACKNOWLEDGMENTS

 
Financial support for this research was provided by the United States Golf Association, Far Hills, NJ.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Part of a thesis by the senior author in partial fulfillment of the requirements for the Ph.D. degree at the University of Illinois. Mention of product and equipment trade names is for identification purposes only and does not imply a warranty or endorsement to the exclusion of other products that may be similar.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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