Seasonal changes of [phenyl-U-14C]methabenzthiazuron loads in soil solution under practical farming conditions

In a two-year lysimeter experiment with methabenzthiazuron herbicide applied in accordance with good agricultural practice in an orthic luvisol, the dissolved active ingredient fractions in the soil solutions and leaching behaviour were examined with the aid of suction candles. The highest active ingredient concentrations of the first experimental year were determined in the soil solution immediately after pre-emergence application in autumn. The active ingredient concentrations decreased continuously during subsequent sampling. After turning the soil for the succeeding crop, high methabenzthiazuron concentrations were again determined in the soil solutions of the Ap horizon and, for the first time, the major metabolite demethyl methabenzthiazuron also occurred. Below 40 cm soil depth neither methabenzthiazuron nor a metabolite was found in the soil solution. The development of active ingredient concentrations in the soil solution could be divided into three phases for this lysimeter experiment. Methabenzthiazuron was also characterized in the leachates. The mechanisms assumed were, on the one hand, a preferential flow with relatively high methabenzthiazuron concentrations, but small leachate volumes. On the other hand, a mass flow caused an extremely small input of trace amounts of active ingredient into the leachate below the drinking water limit of 0.1 μg L−1.