Sorption and degradation of phorate as influenced by soil depth

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 (K-d): was greater for the Egmont than for the Tokomaru soil and decreased with depth for both soils. The difference in K-d 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 (t(1/2)), 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.