Dimoxystrobin Sorption and Degradation in Sandy Loam Soil: Impact of Different Landscape Positions

Pesticide sorption and degradation may be affected by soil spatial variability ranging from field to pore scale. The study objectives were to (i) determine whether soil sampling by landscape position (shoulder, side slope, and foot slope) accounts for the range in soil properties that may affect pesticide sorption and degradation, (ii) measure the sorption and degradation of the fungicide dimoxystrobin ((DMS) (E)-2-(methoxyimino)-N-methyl-2-[alpha-(2,5-xylyloxy)-o-tolyl]acetamide) at these positions, and (iii) determine the effect of disrupting natural soil pore geometry (homogenizing) on DMS degradation. A 50 x 90-m section of grassed hillside on Ruston soil (fine-loamy, siliceous, thermic Typic Paleudult) was sampled on a 10-m grid to 7.5-cm depth, and enzymatic activity, organic C, pH, and texture were measured. Shoulder position soil had more clay and lower pH than soil downslope. Sorption and degradation of DMS in surface 7.5 cm homogenized or intact core samples from these landscape positions were determined. Soil treated with C-14-labeled DMS at 0.28 kg ha(-1) was incubated at 23 degrees C up to 120 days, then extracted with methanol, and analyzed for DMS (high performance liquid chromatography) and non-DMS C-14. Sorption was nearly linear and greatest on shoulder soil, with a distribution coefficient of 5.4 L kg(-1) compared with 3.8 L kg(-1). Degradation of DMS was slow (similar to 65% recovery at 120 days) and unaffected by either homogenizing soil or landscape position. However, unextractable and mineralized C-14 was greater in homogenized than that in intact soil and generally increased downslope, perhaps reflecting more favorable conditions for biodegradation or decreasing sorption. Degradation of DMS followed nonlinear kinetics.