Zhe Weng, Michael T Rose, Ehsan Tavakkoli, Lukas Van Zwieten, Gavin Styles, William Bennett, Enzo Lombi

Glyphosate represents one quarter of global herbicide sales, with growing interest in both its fate in soils and potential to cause non-target phytotoxicity to plants. However, assessing glyphosate bioavailability to plants from soil residues remains challenging. Here we demonstrate that the diffusive gradient in thin-films technique (DGT) can effectively measure available glyphosate across boundary conditions typical of the soil environment: pH 4-9, P concentrations of 20-300 μg P L-1 and NaHCO3 concentrations of 10-1800 mg L-1. In this study, four soils with different glyphosate sorption properties were dosed with up to 16 mg kg-1 of glyphosate and phytotoxicity to wheat and lupin was measured against the DGT-glyphosate concentrations. An improved dose response curve was obtained for root elongation of wheat and lupin across soil types when DGT-glyphosate was used instead of alkaline-extractable (i.e., total extractable) glyphosate. Total extractable glyphosate concentrations of 2.6 and 5.0 mg glyphosate kg-1 in the sandy Tenosol, equivalent to 2.9 and 6.5 μg L-1 DGT-extractable glyphosate, reduced the root length of lupins (but not wheat) by 32-36% compared with the untreated control. DGT is therefore a promising method for assessing phytotoxic levels of glyphosate across different soils.