Diffusive Gradients in Thin-Films (DGT) as a Predictor of Herbicide Residue Bioavailability and Phytotoxicity across Contrasting Soil Types

Abstract Herbicides that remain in soil long after their application can be toxic to organisms and crops sown in subsequent cycles. However, quantifying the bioavailability and potential toxicity of herbicide residues across different soil types remains problematic. In this study, we evaluated the diffusive gradients in thin-films (DGT) technique for measuring potentially bioavailable imazamox (IMX) residues in six different soil types from southern Australia. Soils were spiked with IMX (1 to 360 μg IMX kg–1) and incubated for 28 days. Following incubation, a root elongation test was conducted to assess IMX residue toxicity to wheat (Triticum aestivum cv. Calibre), and herbicide residue concentrations were quantified using DGT (CDGT) and a solvent extraction technique. Solvent-extractable IMX concentrations were highest in the Sodosol, followed by Tenosol, Vertosol, Chromosol, Dermosol, and Calcarosol for IMX-spiked concentrations between 5 and 180 μg kg–1. Furthermore, dose-response analysis between wheat root elongation, CDGT, and solvent-extractable IMX demonstrated that DGT was a more consistent predictor of IMX residue toxicity for all soil types tested. Overall, this study demonstrates that the DGT technique is an effective tool for establishing soil toxicity thresholds across diverse soil types and for elucidating relationships between bioavailable herbicide residues and phytotoxicity.