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 hyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) at the fi eld scale and to relate sorption partition coefficients (K-d) to lands cape position and soil survey map units. Atrazine sorption was measure d on 241 surface samples from a 6.25-ha field using batch-equilibratio n methods. Field-scale variability in atrazine sorption coefficients w as described using spherical semivariograms. Less than 20% of the tota l semivariance in atrazine Kd values was found at lag distances <10 m, indicating there was relatively little variability at this scale, hlu ltiple regression analyses using pooled data revealed that atrazine so rption 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 are as (potholes). Soils from foot slope and bark slope landscape position s were intermediate in atrazine sorption. The magnitude of atrazine so rption by soils in different landscape positions was also related to v ariations in soil organic C content, pH, and clay content. The greater sorption of atrazine by soils in the pothole depressions may reduce t ransport of this herbicide to tile lines that commonly drain these soi ls. Field-scale maps of atrazine sorption based on landscape position more effectively predicted the actual distribution of the K-d values d etermined by kriging, than did maps based on soil series.