Graduate Thesis Or Dissertation
 

Interaction of ethofumesate with dry soil

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  • Previous studies have shown loss of ethofumesate activity when the herbicide was applied to dry soil (2% w/w moisture content) in both field and greenhouse studies, even when rainfall or irrigation was received within a few days. Laboratory studies were conducted to determine the mechanism for this activity loss. Dry soils (Woodburn silt loam, 2% moisture) were treated with 20 ppmw radiolabeled ethofumesate and were either wetted immediately to 35% moisture or remained dry for 4 days before wetting. After equilibration, the soil samples were centrifuged to extract the soil solution for analysis. Radiolabeled ethofumesate and degradation products in the soil solution were separated by thin-layer and column chromatography and assayed by liquid scintillation. Ethofumesate and metabolites also were extracted with methanol 4 days after herbicide application from both dry and wetted soils. After extraction, the soil samples were oxidized to determine the quantity of radioactivity remaining in the soil. Ethofumesate degradation, as affected by soil moisture, pH, temperature, and the length of time soils remained dry after herbicide application, was studied in a Woodburn soil. Several soil moisture levels were maintained between 0.7 and 6.7% (w/w) by placing soil samples in desiccators at various relative humidities. To ascertain the effect of pH, the soil reaction was adjusted to 3.9, 4.9, 7.0, 8.0, and 9.0 prior to drying and treatment. The influence of soil temperature on degradation was determined by incubating dry soil samples at temperatures from 20 to 50 C and constant relative humidity. The time period between ethofumesate application and soil wetting was varied between 0 and 8 days to study the rate of ethofumesate degradation. To determine the effect of soil type, the herbicide was applied to air-dried Woodburn, Dayton, Madras, and Agency soils. All soil samples were treated with 20 ppmw ethofumesate. Ethofumesate applied to soil that remained dry for 4 days degraded in significant quantities to two metabolites, while little ethofumesate degraded in soils wetted immediately. The major metabolite accounted for more than 80% of the degradation products and was identified by gas chromatography and mass spectroscopy to be an oxidation product, 2, 3- dihydro- 3,3- dimethyl- 2- oxo -5- benzofuranyl methanesulfonate. The second metabolite was not identified. The percentage of applied ethofumesate that was tightly adsorbed, not extracted with methanol, was at least 5% greater for applications to soils that remained dry than for soils that were wetted immediately.. Soil moisture levels substantially influenced metabolite formation. Increasing moisture contents to greater than 3% (w/w) reduced degradation to negligible levels. Rapid ethofumesate degradation occurred at soil moisture levels between 1 and 3%. Metabolites were detected 1 day after application to soils at 2% moisture content and as much as 18% of the ethofumesate was degraded in 4 days. Degradation was four to five times greater in soils of pH 4.9 or 7.0 than in more acid or more alkaline soils and ethofumesate metabolite formation increased 6-fold as soil temperature increased from 20 to 50 C. Of the soil characteristics correlated with ethofumesate degradation, percent clay and the level of soluble salts were best correlated with degradation (r = .89 and -.95, respectively). Ethofumesate adsorption studies were conducted in four soils comparing a batch equilibrium technique with a centrifuge extraction technique. Madras and Agency soils and two Woodburn soils were treated with 1 to 20 ppmw ethofumesate. The batch technique had a water-to-soil ratio of 5:1 while the ratio for the centrifuge technique was 0.35:1. The quantity of ethofumesate adsorbed with the centrifuge method was approximately twice that adsorbed with the batch method for a given amount of applied ethofumesate. For a given equilibrium concentration, the batch technique adsorbed more ethofumesate; i.e., the Freundlich K values were higher for the batch method. At least 24 hours were required for equilibrium to be established with the batch technique while the equilibrium concentration was established in a few hours with the centrifuge technique. Ethofumesate degrades rapidly in dry soils to two metabolites. The herbicide also is tightly adsorbed in greater amounts in dry soil than in wet soil. Ethofumesate degradation and increased adsorption results in the activity loss of ethofumesate on dry soil. Soil moisture content is critical in determining the extent of degradation. Moisture levels below 1% or above 3% reduce degradation substantially. Metabolite formation is optimal for soilt of moderately acid to neutral reaction (4.9 to 7.0) and the degradation rate also depends on temperature, increasing linearly as temperature increases. Metabolite formation is influenced by the chemical and physical properties of the soil; clay content and soluble salts are most highly correlated with degradation.
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