EFFECT OF CROP RESIDUE MANAGEMENT AND DRAINAGE ON THE PERSISTENCE AND MOVEMENT OF ISOPROTURON IN A STRUCTURED CLAY SOIL OVER THE GROWING-SEASON OF A WINTER BARLEY CROP

The persistence of isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea)] in the cultivated horizon and losses from mole drains in a structured clay soil sown with winter barley and managed by crop residue burning and ash incorporation were compared with those where the crop residues were chopped and plowed in. Desorption coefficients (X(app)) increased over time as isoproturon concentration declined and were greater on the burnt plot than on the unburnt plot on every sampling occasion. By contrast, sorption coefficients (K-d) were more consistent at approximately 5.0 L/kg on both plots. The variability of sorption and desorption was sensitive to temporal variation of rainfall. Sorption nonideality was observed on every sampling occasion with the nonideality index (K-d/K-app), declining with time following isoproturon application. Twenty-seven days following the autumn pre-emergent herbicide application [2.475 kg of active ingredient (ai) on October 9, 1991] KdKapp was approximately 0.5 and after 97 days had fallen to approximately 0.2, where it stabilized until the post-emergent application (1.625 kg of ai/ha on May 4, 1992) three months later. The variability of all parameters was almost as great at the local scale as at the field scale. Total isoproturon losses in drainflow over the growing season were similar at 869 and 943 mg/ha for the burnt plot and unburnt plot, respectively. However, temporal variation in leaching losses was complex, being influenced by a number of factors including dilution effects, soil solution contact times, and possibly particle-assisted transport.