FATE, SORPTION, AND LEACHING OF THE HERBI CIDE DIURON AFTER ANNUAL APPLICATION IN AN ORCHARD SOIL (ORTHIC LUVISOL)

A three years field trial was conducted in an apple orchard on a loess site (Orthic Luvisol) with annual applications of the herbicide diuro n. An accumulation of biologically effective residues of the herbicide was not observed, although a carry-over of herbicide residues in a ra nge of 46-77 mu g/kg dry soil in the 0-5 cm layer occurred between dif ferent years. After. application in May, the residues were rapidly deg raded during the following summer months. Disappearance of diuron resi dues from the 0-5 cm layer conformed to first-order kinetics during th e first six months after application. The DT50-values ranged from 14 t o 30 days. As the degradation rate decreased with time, the level of r emaining residues on a long-term scale was best represented by two com bined exponential functions for the fast and-slow degradation rate. Th e distribution coefficient between adsorbed and dissolved residues inc reased with time, indicating decreasing mobility of herbicide residues . The distribution coefficients and the-extractable residue contents w ere strongly negatively correlated. A few days after a diuron applicat ion in May 1994, about 15% of the applied amounts could be analyzed in subsoil samples from 30-60 cm depth (6%) and 60-90 cm depth (9%), whi le high concentrations of diuron remained in the 0-5 cm soil layer. Th is vies the result of a heavy rainfall (20 mm) and a rapid movement of seepage water into the subsoil due-to preferential transport in soil macropores like earthworm channels and shrinkage cracks, which frequen tly occurred in this untilled loess soil. Different transport patterns were observed during the winter months in 1992/93 and 1993/94, when o nly low amounts of the diuron applied in spring were leached down to 3 0-60 cm depth. In winter time, the remaining low concentrations of ext ractable residues were strongly adsorbed and therefore remained in the upper soil layers.