Atrazine Sorption at the Field Scale in Relation to Soils and Landscape Position

Understanding the spatial variation of herbicide sorption in soils is important in determining the potential for leaching at the field scale. Our objectives were to determine the spatial variability of atrazine sorption (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine) at the field scale and to relate sorption partition coefficients (Kd) to landscape position and soil survey map units. Atrazine sorption was measured on 241 surface samples from a 6.25-ha field using batch-equilibration methods. Field-scale variability in atrazine sorption coefficients was described using spherical semivariograms. Less than 20% of the total semivariance in atrazine Kd values was found at lag distances <10 m, indicating there was relatively little variability at this scale. Multiple regression analyses using pooled data revealed that atrazine sorption was influenced by soil organic C, pH, and, to a lesser extent, soil clay. We also evaluated the relationship of atrazine sorption to landscape position and soil series. Less atrazine was sorbed by soils from upland shoulder slopes than by soil in level and depressional areas (potholes). Soils from foot slope and back slope landscape positions were intermediate in atrazine sorption. The magnitude of atrazine sorption by soils in different landscape positions was also related to variations in soil organic C content, pH, and clay content. The greater sorption of atrazine by soils in the pothole depressions may reduce transport of this herbicide to tile lines that commonly drain these soils. Field-scale maps of atrazine sorption based on landscape position more effectively predicted the actual distribution of the Kd values determined by kriging, than did maps based on soil series.