Sarah J. Welch

Evidence for the accelerated degradation of isoproturon in soils

The herbicide isoproturon was degraded rapidly in a sandy loam soil under laboratory conditions (incubation temperature, 15°C ; soil moisture potential, -33 kPa). Degradation was inhibited following treatment of the soil with the antibiotic chloramphenicol, but unaffected by treatment with cycloheximide, thus indicating an involvement of soil bacteria. Rapid degradation was not observed with other phenylurea herbicides, such as diuron, linuron, monuron or metoxuron incubated in the same soil under the same experimental conditions. Three successive applications of isoproturon to ten soils differing in their physicochemical properties and previous cropping history induced rapid degradation of the herbicide in most of them under laboratory conditions. There were, however, no apparent differences in ease of induction of rapid degradation between soils which had been treated with isoproturon for the last five years in the field and those with no pre-treatment history. A mixed bacterial culture able to degrade isoproturon in liquid culture was isolated from a soil in which the herbicide degraded rapidly.

Measurement and prediction of isoproturon movement and persistence in three soils

Abstract The degradation of isoproturon in three soils at different moistures was measured in the laboratory at a range of temperatures. Measurement of herbicide desorption from the soil at intervals during the laboratory incubation demonstrated an apparent increase in the strength of adsorption with time. Appropriate constants derived from the laboratory data were used with records of air temperature, rainfall and evaporation in a computer model to predict the vertical distributions of isoproturon in three field experiments. One experiment was conducted in 1986 on a sandy clay loam and two in 1987, one on a similar soil and the second on a clay soil. When compared with measured residues in 1986, the model gave good predictions of the pattern and extent of distribution down the profile but amounts of isoproturon in the individual layers were overestimated. Movement down the profile in 1987 was less accurately predicted at either site than at the single site in 1986. Further refinements are required before such a system could be used to give reliable advice on the need to re-treat fields in which weed control from earlier applications of isoproturon appeared to be poor or slow.

Induction and Transfer of Enhanced Biodegradation of the Herbicide Napropamide in Soils

In laboratory incubations, the times to 50% loss (DT 50 ) of a first application of napropamide were approximately 25, 45 and 75 days in soil incubated at 25, 15 and 5°C respectively. When treated for a second time, the DT 50 values were 4, 7 and 15 days at the same temperatures, irrespective of the temperature of the first incubation. This indicates that enhanced degradation of napropamide in soil can be both induced and expressed at low temperature. A mixed microbial culture able to degrade the herbicide to a single degradation product, identified by HPLC retention time as naphthoxypropionic acid, was obtained from a soil capable of rapid degradation. Addition of a sub-sample of this mixed culture to a previously untreated soil introduced rapid degrading ability. When small amounts of soil capable of rapid degradation were added to previously untreated soil, in both the laboratory and the field, the degradation rate of napropamide increased compared with that in unamended soils.