Persistence and management of enhanced carbetamide biodegradation in soil

The extent of enhanced degradation of the herbicide carbetamide declined over time after herbicide application was discontinued. The kinetics of carbetamide degradation were determined in the same soil for three consecutive years (1994–96) after single annual applications from 1989 to 1992. The DT₅₀ of carbetamide increased from 5.4 d in 1994 to 10.2 d in 1996. However, this was still less than the DT₅₀ in previously untreated soil (23–44 d). A most probable number (MPN) assay demonstrated a link between carbetamide degradation rate and the numbers of micro-organisms capable of carbetamide mineralization. Degradation of six other herbicides was assayed in the carbetamide-pretreated and the previously untreated soils. Propham was the only herbicide which degraded more rapidly in the soil with a history of carbetamide application. Rapid degradation of chlorpropham, a herbicide structurally similar to carbetamide and propham, and propyzamide, a herbicide with similar mode of action and weed control spectrum, was not observed. The results suggest that enhanced biodegradation of carbetamide can be managed by less frequent carbetamide application as a part of a herbicide rotation involving compounds which are structurally dissimilar.