Influence of surface and subsurface soil properties on atrazine sorption and degradation

Studies to predict pesticide fate often lack measurements of model input parameters. Using independent data sets and understanding how soil properties affect herbicide retention and degradation may result in more accurate prediction of herbicide fate. We conducted laboratory studies to determine the influence of soil properties on atrazine adsorption and degradation. These data will be used in a separate study involving a pesticide fate model. Atrazine adsorption and desorption isotherms were constructed for six soil depths of a Hastings silty clay loam (fine, montmorillonitic, mesic Udic Argiustoll) using batch equilibration. The Freundlich adsorption constants (log Kf) ranged from 0.38 (60 to 90 cm) to 2.91 (0 to 30 cm). Adsorption was higher in the low pH, high organic matter-containing surface soil compared to the lower soil depths. Multiple regression of the adsorption constants against selected soil properties indicated that organic matter content was the best single predictor of atrazine adsorption (R(2) = 0.98) followed by soil pH (R(2) = 0.82). Combining organic matter and cation exchange capacity in the model produced the lowest C(p) statistic (2.33) and highest R(2) value (0.99). We observed hysteresis in atrazine adsorption-desorption isotherms by higher adsorption slopes (1/n)ads compared to desorption slopes (1/n)des. Soils that adsorbed more atrazine also desorbed less atrazine. Desorption correlated negatively with organic matter content and positively with soil pH. Atrazine degradation after 84 d of incubation generally decreased with increasing depth. The first-order degradation rate was highest 0 to 30 cm deep (0.0187 day(-1)) and lowest 270 to 300 cm deep (0.0031 day(-1)). Atrazine degradation was faster in soil treated annually for 12 yr than in soil with no previous atrazine history (p = 0.01).