Modeling dicamba sorption and transport through zoysiagrass thatch and soil

The presence of turfgrass thatch complicates the sorption and transport of water soluble pesticides because the surface-applied pesticides must pass t hrough an organic-rich thatch layer prior to entering the soil. The study w as conducted (I) to determine the impact of zoysiagrass thatch (Zoysia japo nica Steud.) on dicamba (3,6-dichloro-2-methoxy benzoic acid) transport thr ough soil columns, and (2) to evaluate the effectiveness of linear equilibr ium (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 isot herms of C-14 dicamba to homogenized samples of zoysiagrass thatch and a Sa ssafras loamy sand soil (fine loamy, mixed mesic, Typic Hapludult) were det ermined. Following the application of bromide to determine transport parame ters, 0.56 kg dicamba ha(-1) was surface applied to undisturbed soil column s containing a surface layer of thatch and columns devoid of thatch and lea chate samples collected for 12 h under steady-state unsaturated conditions. Zoysiagrass thatch (K-f = 0.82) had a three times greater sorption capacit y than the soil (K-f = 0.28) beneath the thatch. Dicamba leaching for colum ns 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 nonequ ilibrium sorption. Indications are that turfgrass thatch may have significa nt effects on dicamba leaching that presently used regulatory models based on LEM approach do not adequately consider.