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The dissipation behaviour of Diuron under field conditions was investigated in four studies. After application of Diuron to bare soil at a rate of 8 kg a.s./ha at five locations in, DT50values of 55-236 days were determined (Pogány, 1993). A 3-year field trial in an apple orchard on loess type soil inwith 2, 2.5 and 3 kg Karmex 80/ha (applied always in May) did not show accumulation of biologically effective residues of Diuron (Pätzold & Brümmer, 1997). In a pear orchard on loam soil in, plots were treated with 3 kg a.s./ha (Rouchard et al., 2000). In the plots treated for the first time the soil half-life of Diuron in the 0-10 cm layer was 81 days. In the plots treated annually during the past 12 years, the corresponding soil half-life was 37 days. In the study by Tworkoski et al. (2000) an annual rate of 4.5 kg Diuron/ha was applied in field plots on asilt loam from 1981 to 1995. Residue concentrations decreased exponentially with time. By 125 days after treatment, Diuron residues were close to zero (< 1 % of initial levels, no accumulation took place). The DT50 values were about 40 days and 50 days in 1993 and 1994, respectively. Diuron thus showed a moderately enhanced biodegradation if applications were annually repeated.

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