Effect of long-term field application of pendimethalin: enhanced degradation in soil

The effect of long-term application of pendimethalin in a maize-wheat rotat ion on herbicide persistence was investigated. Pendimethalin was applied at 1.5 kg AI ha (1) separately as one or two annual applications for five con secutive years in the same plots. Residues of pendimethalin were determined by gas chromatography. Harvest-time residues of the herbicide decreased gr adually over the years and at the end of five years less than 3% of applied pendimethalin was recovered from soil as against 18% in the first year. Re sidues were found distributed in the soil profile up to 90 cm depth at the end of the experiment with peak distribution of 0.03 mu gg (1) in the surfa ce layer of soil treated with 10 herbicide applications. The minimum distri bution was, however, in the deepest soil (75-90 cm) profile. Some of the me tabolites of pendimethalin ie dealkylated pendimethalin derivative, partial ly reduced derivative and cyclized product were also traced in surface and sub-surface soils up to 90cm. A study of the rate of degradation of pendime thalin in held-treated soils under laboratory conditions revealed faster de gradation compared to control soils. Only the surface soil (0-15cm) showed this enhanced degradation of the herbicide, which could be due to the adapt ability of the aerobic micro-organisms to degrade pendimethalin. Microbes c apable of degrading herbicide were isolated, identified and pendimethalin d egradation was confirmed in nutrient broth. (C) 2000 Society of Chemical In dustry.