Imidacloprid sorption and transport in cropland, grass buffer and riparian buffer soils
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Knowledge of neonicotinoid sorption and transport in soil is crucial to understanding environmental risk associated with the most widely used class of insecticides. To determine mobility and transport of the neonicotinoid imidacloprid (ICD), batch sorption and column leaching experiments were performed using soils collected from cropland (crop), grass vegetative buffer strips (VBS), and riparian VBS. Single-point, solid-to-solution partition coefficients (Kd) were determined by reacting soil collected from the vegetation treatments at six sites with radiolabeled (14C) ICD. For the column experiments, soils from the three vegetation treatments collected at one site were packed into individual glass columns and water flow was characterized by applying Br - as a nonreactive tracer. A single pulse of 14C-ICD was applied, and ICD leaching was monitored for up to 45 days. Bromide and ICD breakthrough curves for each column were simulated using CXTFIT, a convection/dispersion-based transport equilibrium model, and HYDRUS-1D, a multisite sorption chemical non-equilibrium transport model. Sorption results indicated that ICD sorbs more strongly to soil from riparian VBS (Kd= 22.6 L kg-1) than crop (Kd= 11.3 L kg-1; p = 0.04) and soil organic carbon (OC) was the strongest predictor of ICD sorption (p less than 0.0001). The column transport study found peak concentrations of ICD at 5.83, 10.84 and 23.8 pore volumes for crop, grass VBS, and riparian VBS, respectively. HYDRUS-1D results indicated that the two-site, one-rate linear reversible model best-described results of the breakthrough curves, indicating the complexity of ICD sorption and demonstrating its mobility in soil. Greater sorption and longer retention by the grass and riparian VBS soil compared to the crop soil suggests that VBS may be a viable means to mitigate ICD loss from agroecosystems.