ISOXABEN SOIL BIODEGRADATION IN PEAR TREE ORCHARD AFTER REPEATED HIGH-DOSE APPLICATION

During the past nine years, each of the plots of a pear tree orchard w ere treated annually with the same herbicide treatment. The following herbicide treatments were compared, each being made by application of a mixture of two or three herbicides: 1a, no herbicide at all, weeds b eing hoed (control 1a); 2, diuron + paraquat 3 + 1 kg/ha; 3, simazine + paraquat 2 + 1 kg/ha; 4, isoxaben + diuron + paraquat 0.5 + 1.6 + 1 kg/ha; and 5, isoxaben + simazine + paraquat 0.5 + 1.25 + 1 kg/ha. In March 1996, one year after the final orchard herbicide treatment, isox aben could not be detected in the soils of any field plots; isoxaben w as incorporated at 0.74 mg/kg in the loamy soils sampled separately in each of the field plots, and the soils were incubated in the laborato ry. Isoxaben soil half-lives were 92 days in the soils treated previou sly with herbicide treatments 1a, 2, or 3 and 42 days in the soils tre ated with herbicide treatments 4 and 5. The repeated isoxaben treatmen ts applied in the past thus enhanced the isoxaben soil biodegradation; diuron, simazine, and paraquat had no influence on this rate enhancem ent. On the other hand, herbicide treatments 4 and 5 were applied in t he orchard in April 1996 on the corresponding plots treated in this ma nner for the last nine years. Isoxaben + paraquat 0.5 + 1 kg/ha was ap plied simultaneously on other plots (control 1b) not treated in the pa st with isoxaben. During the growth season in the orchard, the isoxabe n soil half-lives in the control plots 1b was 101 days, and 41 days in the plots where herbicide treatments 4 or 5 were applied.