Soil organic matter can delay—but not eliminate—leaching of neonicotinoid insecticides

Soil organic matter (SOM) retains and attenuates many contaminants; however, its interactions with neonicotinoid insecticides under field conditions remain poorly understood. The goal of this study was to determine if SOM influences the persistence or leaching of two neonicotinoid insecticides: thiamethoxam (TMX) and its transformation-product clothianidin (CLO). Thiamethoxam-coated soybean [Glycine max (L.) Merr.] was planted into a clay soil containing different soil organic carbon (SOC) concentrations. Leachate and soil samples were collected for 10 wk after planting and were analyzed for insecticide concentrations using liquid chromatography–tandem mass spectrometry. Single and multiple linear regressions were performed between SOC, leaching volumes, and measured insecticide concentrations, focusing on rainfall events near the beginning, middle, and end of the study. Correlations were also tested between SOC and cumulative mass of leached insecticides. Neither SOC nor per-event leachate volumes explained variability in TMX leaching or residual CLO concentrations in soils; however, by the conclusion of the study residual thiamethoxam concentrations in soil were negatively correlated with cumulative volume of leached water. Initially, the concentration and total mass of leached CLO were significantly and negatively correlated with SOC content; however, this effect faded with time. Leachate dynamics also affected CLO transport, with positive correlations between leachate volume and CLO concentration during the latter events. This analysis demonstrates that SOM can reduce peak loading of neonicotinoids but may not alter cumulative leaching over the entire growing season.