Sorption and degradation of phorate as influenced by soil depth

Abstract

The sorption and degradation of a non-ionic insecticide (phorate) were examined using 14 soil samples collected at different depths from 2 soil types (Tokomaru and Egmont) which differed in their organic matter and clay mineralogy. Organic carbon and biological activity, as measured by substrate-induced respiration (SIR), decreased with soil depth. Sorption of phorate, as measured by the distribution coefficients (Kd), was greater for the Egmont than for the Tokomaru soil and decreased with depth for both soils. The difference in Kd values between the soils and soil depths was related mainly to the difference in the amount of organic carbon. The rate of degradation, as measured by the half-life (t1/2), decreased with soil depth except for the last 2 depths. The initial decrease in the rate of degradation was attributed to the decrease in the biological activity with depth. In the last 2 depths, since the sorption was low, most of the phorate tended to remain in soil solution, resulting in a slight increase in the rate of degradation. These results suggest that the changes in soil organic carbon with depth may be a reasonable indicator of microbial activity and subsequent changes in the rate of pesticide degradation as a function of soil depth.