Sorption of a hydrophilic pesticide: Effects of soil water content and matric potential Tyson E. Ochsner *1, Brandon M. Stephens 2, William C. Koskinen 1, and Rai S. Kookana 3 1 USDA-ARS, Soil and Water Management Research Unit, St. Paul, MN Dep. Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN CSIRO Land and Water, PM B2, Glen Osmond, SA 5064, Australia

Introduction The leaching risk of a pesticide in soil is characterized primarily by the sorption coefficient (K d ). Lower K d values indicate greater potential for leaching. The effects of soil water content and matric potential on K d are not well understood. This is a serious gap because pesticides are often applied on or near the soil surface where the water content varies dramatically.

Pesticide -- Dicamba Dicamba is a widely used broadleaf herbicide sold under the trade names Banvel and Clarity. During 2002, approximately 11% of US corn was treated with dicamba at a rate of 0.22 kg active ingredient ha -1 yielding a total of 512 Mg of active ingredient applied. Fig. 1. Molecular structure of dicamba, a hydrophilic, weakly- sorbing herbicide with pKa of 1.9.

Soils Sorption of dicamba was measured in three soils (Table 1), each at two initial water contents. Soils were moistened to the desired initial water content using a solution containing 14 C-labeled dicamba; the resulting dicamba concentration was ~1  g g -1.

Unsaturated transient flow experiments After equilibration, the soil was packed into a column with 20 sections (0.9 cm thick). The column was positioned horizontally and connected to a Mariotte bottle (Fig. 2). The bottle supplied 5 mM CaCl 2 solution (without dicamba) to the column inlet at a constant pressure. Infiltration terminated when the wetting front reached ¾ of the way through the column. Fig. 2. Soil column and Mariotte bottle set-up used for horizontal infiltration experiments.

Water and pesticide distributions Infiltrating water displaced the antecedent solution, creating a plane of separation (vertical line, Fig. 3). For positions beyond this plane, the total pesticide content per unit mass of dry soil was regressed against the solution volume per unit mass of dry soil. Fig. 3. Distributions of water and dicamba after horizontal infiltration.

For positions beyond this plane, the total pesticide content per unit mass of dry soil was regressed against the solution volume per unit mass of dry soil (Fig. 4). The intercept divided by the slope gave the K d. Fig. 4. Dicamba concentration versus solution volume. Determination of K d

Sorption coefficients

Regression analysis No significant relationship existed between K d and matric potential. We found a strong linear relationship between K d and the product of soil water content and organic C content (r 2 = 0.86, Table 3).

Conclusions The number of dicamba sorption sites increases with soil organic C content, while the accessibility of these sites increases with soil water content. This may be caused by the decreasing hydrophobicity of soil organic matter with increasing water content. The effects of water content on pesticide sorption require further research and may ultimately have implications for the methods used to determine sorption and for managing pesticide application.