Studies of time-dependent sorption of linuron and isoproturon in soils

Changes in sorption over time were measured for the substituted urea herbicides linuron (3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea) and isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea) in three different soils. Studies were made at two incubation temperatures (5 degrees C to restrict degradation and 20 degrees C to promote degradation), with soil moisture contents equivalent to field capacity. Measurements were made of the changes over time in both methanol extractable herbicide residues and in herbicide concentrations in the soil solution. Sorption distribution coefficients of linuron and isoproturon increased with residence time in the soil, and the increases were more pronounced at 20 degrees C than at 5 degrees C, suggesting an interaction between sorption and degradation. With linuron, comparative studies were done with the herbicide applied to soil as either a commercial formulation, or as pure, analytical grade compound. No significant differences were observed between the two treatments. In further experiments with isoproturon, incubations were made at 5 OC with initial concentrations of 2, 4, 6, 8 and 10 mg isoproturon kg(-1) soil and total residues and soil solution concentrations were measured after 4 days. Sorption measurements of isoproturon were also made using a standard batch equilibrium method. Initial in-situ isotherms were similar to standard batch equilibrium isotherms, although equilibrium was reached faster in the shaken than in the static systems. These data suggest that the apparent increase in sorption distribution coefficient with time is linked in part to preferential degradation in the soil solution, in part to diffusion to less accessible sorption sites, and is in part an artefact caused by marked curvature in some of the sorption isotherms. (C) 1999 Elsevier Science Ltd. All rights reserved.