Effect of air-drying on sorption kinetics of the herbicide chlortoluron insoil

Batch techniques were utilized to investigate sorption kinetics of chlortol uron (3-(3-chlor-p-tolyl)-1,1-dimethylurea) in tyro field-moist soils, a si lt loam (Typic Hapludalf) and a loamy sand (Aquic Haplumbrept), containing 1.1 and 0.7% organic C. To investigate effects of soil pretreatment, a part of both soil samples was initially air-dried. Initially sorption-desorptio n isotherms were measured using field-moist soils. Both isotherms exhibited pronounced hysteresis. The fit of a nonlinear kinetic hto stage sorption m odel to the data suggests that hysteresis was caused by nonattainment of eq uilibrium within the U-h agitation periods. This hypothesis is supported by the fact that the estimated kinetic parameters were well suited to predict concentration vs. time experiments in both soils. The prediction of sorpti on-desorption isotherms measured using air dried soils failed. Compared wit h the predicted data, adsorption in air-dried soil was increased while deso rption hysteresis was less pronounced. Further experiments indicated that c hanges of soil organic matter (SOM) caused by air-drying were responsible f or this effect. Measuring W-h partitioning coefficients using the dr-dried soils, regretted to field-moisture for varying periods of time, revealed th at in the course of 80 h (silt loam) and 500 h (loamy sand) after rewetting , the sorption properties of the air-dried soils were again equal to those of the respective field moist soils.