Modeling Dicamba Sorption and Transport Through Zoysiagrass Thatch and Soil

Abstract

The presence of turfgrass thatch complicates the sorption and transport of water soluble pesticides because the surface-applied pesticides must pass through an organic-rich thatch layer prior to entering the soil. The study was conducted (1) to determine the impact of zoysiagrass thatch (Zoy-sia japonica Steud.) on dicamba (3,6-dichloro-2-methoxy benzoic acid) transport through soil columns, and (2) to evaluate the effectiveness of linear equilibrium (LEM), two site nonequilibrium (2SNE) and one site nonequilibrium (1SNE) models to predict dicamba transport through columns containing a surface layer of thatch and columns devoid of thatch. The equilibrium sorption isotherms of 14C dicamba to homogenized samples of zoysiagrass thatch and a Sassafras loamy sand soil (fine loamy, mixed mesic, Typic Hapludult) were determined. Following the application of bromide to determine transport parameters, 0.56 kg dicamba ha−1 was surface applied to undisturbed soil columns containing a surface layer of thatch and columns devoid of thatch and leachate samples collected for 12 h under steady-state unsaturated conditions. Zoysiagrass thatch (Kf = 0.82) had a three times greater sorption capacity than the soil (Kf = 0.28) beneath the thatch. Dicamba leaching for columns with thatch layers was ca. 21% less than soil columns devoid of thatch. When dicamba breakthrough curves were fitted to the different forms of the convective dispersive equation, the 2SNE model simulated dicamba transport better than LEM and 1SNE models, indicating the presence of two-site nonequilibrium sorption. Indications are that turfgrass thatch may have significant effects on dicamba leaching that presently used regulatory models based on LEM approach do not adequately consider.