ENHANCED DEGRADATION OF SELECTED SOIL-APPLIED HERBICIDES IN LONG-TERM EXPERIMENTS IN ORCHARDS

摘要

The dissipation of four herbicides mainly acting through the soil has been studied with chemical and biological methods in two field experiments in pear (Pyrus domestica Medik.) orchards with long-term repeated application of the same herbicide(s) on the same plots. In Experiment GS on a loam soil at Gorsem (Province Limburg), chemical analysis revealed that chlortoluron, diuron and isoxaben were degraded significantly faster on plots treated repeatedly since 1987 when compared to plots treated for the first time. In 1996, following application of isoxaben (500 g/ha) for the 10th consecutive year, a soil half-life of 43 days was recorded indicating a significantly faster degradation than in soil treated for the first time (half-life: 101 days). Following their 12th consecutive application in spring of 1998, both urea herbicides were shown to undergo enhanced degradation. However, as shown by their respective half-life times corresponding with repeatedly and first time treated plots respectively, the enhanced degradation phenomenon was moderate for diuron (3000 g/ha) (37 vs. 81 days) but extremely important for chlortoluron (4000 g/ha) (11 vs. 64 days). These findings are particularly important for "integrated fruit production" systems where chlortoluron serves as a replacement for diuron. In Experiment ME on a sandy loam soil at Melle (Province East-Flanders), enhanced degradation of propyzamide (1250 g/ha) was observed in 1998 on plots treated for the 14th time; on these plots, soil half-life time was 10 days only compared to 31 days on plots never treated previously. With greenhouse bioassays, the enhanced degradation of isoxaben and propyzamide could be confirmed in soil treated for the 11th and 14th time respectively. White mustard (Sinapis alba L.) was a highly satisfactory test plant in studies with isoxaben whereas winter wheat (Triticum aestivum L.) was suitable to monitor propyzamide dissipation.