Development of an Extraction Method for Estimating Phytoaccessible Fractions of Bromacil and Tebuthiuron
Creators: Maxwell, Jacquelyn L.
Publisher: name: University of Alberta Library
Year: 2022

<p>Bromacil and tebuthiuron are herbicides used from the 1960s to 1990s on industrial sites to control vegetation. Approximately 61,750 sites are considered contaminated when comparing total herbicide concentrations (estimated by 99 percent methanol extraction) against Alberta’s remediation guidelines. Remediation of these sites requires treatment or removal of soil to bring herbicide concentrations below applicable guidelines. As time passes from initial application, a portion of bromacil and tebuthiuron is thought to adsorb to the soil solid phase. The total herbicide concentration in soil is then comprised of the adsorbed (solid phase) and the phytoaccessible (plant accessible) concentrations. Measuring phytoaccessible or soluble herbicide concentrations in soil would avoid unnecessary ex-situ treatment or landfilling. It is likely that adsorption to soil organic matter or mineral particles reduces phytoaccessibility and immobilization technologies could be used to increase the natural soil adsorptive capacity to manage in-situ sites with herbicide contamination.  Calcium chloride solution (0.01 M) is often used to assess soluble (phytoaccessible) concentrations of nutrients and to conduct adsorption/desorption studies of metals and organics. In this work, it was used as an extractant to estimate the phytoaccessible concentrations of bromacil and tebuthiuron in soil. A t-test for non-parametric data with homogeneous variances was used to compare calcium chloride-extractable herbicide concentrations to total concentrations (estimated by 99 percent methanol extraction of aged soil samples, or by spiking soils using a known herbicide concentration).  For this study, 149 x 1 g soil samples were included containing a known concentration of bromacil (84 samples) or tebuthiuron (66 samples). Samples were extracted with 40 mL 0.01 M calcium chloride by shaking and centrifugation. The resulting 10 mL liquid extracts were sent to a commercial laboratory for analysis with High Pressure Liquid Chromatography (HPLC) using Mass Selective Detection. Estimated total concentrations were higher than calcium chloride-extractable concentrations. Median spiked and aged bromacil concentration was 0.300 mg/kg and median calcium chloride-extractable bromacil was 0.190 mg/kg (W = 5131, p &lt; 0.001). Median spiked and aged tebuthiuron concentration was 0.273 mg/kg and median calcium chloride-extractable was 0.140 mg/kg (W = 2694, p = 0.003).   In reviewed literature, organic matter and clay contribute most to bromacil or tebuthiuron adsorption. Categorical analysis of high or low organic matter and high or low clay content against the apparently adsorbed fraction was conducted. A two-by-two Analysis of Variance (ANOVA) assessed if increasing percentage of clay and organic matter increased the apparently adsorbed fraction, which was estimated by subtracting the calcium chloride-extracted concentration from the total estimated concentration. Mean apparently adsorbed bromacil fraction was higher for high organic matter compared to low organic matter soils (F = 8.09, p &lt; 0.01). Clay did not significantly increase apparent bromacil adsorption (F = 2.36, p = 0.133). Similarly, mean apparent adsorbed tebuthiuron fraction was significantly higher for high organic matter compared to low organic matter soils (F = 25.89, p &lt; 0.001), but clay did not significantly increase tebuthiuron adsorption (F = 0.03, p = 0.858).   Calcium chloride-extracted concentrations of bromacil and tebuthiuron were less than the estimated total concentrations likely due to adsorption on organic matter. Where there are large areas of marginal bromacil or tebuthiuron contamination, approaches are needed to reduce risk, meet regulatory requirements, and protect soil health.  Soils with low phytoaccessible concentrations could remain in place and retain nutrients, organic matter, and structure, providing resiliency against current and future challenges to soil ecosystems.</p>