SEASONAL-CHANGES OF [PHENYL-U-C-14]METHABENZTHIAZURON LOADS IN SOIL SOLUTION UNDER PRACTICAL FARMING CONDITIONS

In a two-year lysimeter experiment with methabenzthiazuron herbicide a pplied in accordance with good agricultural practice in an orthic luvi sol, the dissolved active ingredient fractions in the soil solutions a nd leaching behaviour were examined with the aid of suction candles. T he highest active ingredient concentrations of the first experimental year were determined in the soil solution immediately after pre-emerge nce application in autumn. The active ingredient concentrations decrea sed continuously during subsequent sampling. After turning the soil fo r the succeeding crop, high methabenzthiazuron concentrations were aga in determined in the soil solutions of the A(p) horizon and, for the f irst time, the major metabolite demethyl methabenzthiazuron also occur red. Below 40 cm soil depth neither methabenzthiazuron nor a metabolit e was found in the soil solution. The development of active ingredient concentrations in the soil solution could be divided into three phase s for this lysimeter experiment. Methabenzthiazuron was also character ized in the leachates. The mechanisms assumed were, on the one hand, a preferential flow with relatively high methabenzthiazuron concentrati ons, but small leachate volumes. On the other hand, a mass flow caused an extremely small input of trace amounts of active ingredient into t he leachate below the drinking water limit of 0.1 mu g L-1. (C) 1998 E lsevier Science Ltd. All rights reserved.