DEGRADATION STUDIES WITH 14C-FENOXAPROP IN PRAIRIE SOILS

Abstract

The persistence of 14C-fenoxaprop acid, the major degradation product of the herbicide 14C-fenoxaprop-ethyl, was studied at different controlled temperature and moisture conditions in three Saskatchewan soils. At 85% of field capacity, degradation approximated to first-order kinetics at soil temperatures of 10, 20, and 30 °C, with half-life values ranging from 42 to 5 d. In a clay at 20 °C, there was no effect of moisture, between 50% and 100% field capacity, on the rate of 14C-fenoxaprop acid breakdown. There was no loss of radioactivity after 65-d in air dry soils. In all soils incubated at 85% field capacity and 20 °C with 14C-fenoxaprop-ethyl, uniformly labeled in the chlorophenyl ring, there was evolution of 14CO2 indicating ring fission. Over a 56-d incubation period, 10–15% of the applied radioactivity was released from the treated soils as 14CO2. Extraction with ammoniated acetonitrile recovered 14–19% of the original radioactivity; between 3 and 5% of the initial 14C was incorporated into soil microbial biomass; and 17–25% into the fulvic acid, 7–12% into the humic acid, and 30–34% into the humin soil fractions. Key words: Herbicide, biomass, fenoxaprop-ethyl, fenoxaprop, persistence