Soil organic matter ageing and its effect on insecticide imidacloprid soil biodegradation in sugar beet crop

Abstract

Imidacloprid insecticide was applied in pelleted seed dressing at sugar beet sowing in March 1993. The field was divided into plots. On some plots, one cow manure treatment (50 tons ha‐1) had been applied either 18 or 6 months before sugar beet sowing. On other plots, two cow manure treatments (2 × 50 tons ha‐1) had been applied: either at 18 and 6 months, or at 18 and 1 months before sugar beet sowing. During the first two crop months period, the imidacloprid soil half‐lives in these plots were 40, 85, 112 and 121 days, respectively. The single cow manure treatment applied 18 months before sugar beet sowing almost had no more influence on the rate of imidacloprid soil biodegradation. The cow manure treatments applied either 6 or 1 months before sowing slowed down the imidacloprid soil biodegradation with similar intensities. To the greater imidacloprid soil concentrations corresponded greater imidacloprid concentrations in the leaves of sugar beet. After the first two months crop period, the rates of imidacloprid soil biodegradation increased significantly. The imidacloprid soil residues became very low, and was similar for all the cow manure treatments.