Mechanisms of Production of Soil-Bound Residues of [¹⁴C]Parathion by Microorganisms

Unextractable (bound) radiolabeled residues are formed when [¹⁴C]parathion is incubated in soil. The role of microorganisms in producing these bound residues was investigated by incubating [ring-¹⁴C]-parathion in soil-free culture media that had been inoculated with soil microorganisms. The amounts of compounds in culture supernatants, that upon addition to soil became unextractable, increased up to 12 h of microbial culture incubation, when 43% of the added radiocarbon was bound after a 2-h soil incubation period. The increase in soil bound residues was correlated with a decrease in the amount of parathion in the microbial culture and a concomitant increase in the appearance of the major degradation product, aminoparathion. Microbial cells contained only a minor proportion of ¹⁴C-bound residues. Addition of natural microbial growth metabolites to the soil did not affect binding of either parathion or aminoparathion. This latter compound was highly bound to both sterile and nonsterile soils. When [¹⁴C]parathion, [¹⁴C]paraoxon, p-[¹⁴C]nitrophenol, and their respective ¹⁴C-amino compounds were added to soil, the amino compounds were bound to a much greater extent than were the nitro compounds. It appears that the process of parathion degradation has to be separated from that of binding and that the role of the microorganisms in soil binding phenomena consisted in degrading [¹⁴C] parathion to compounds which are more tightly bound to soil than the parent insecticide. Once these compounds were formed, their binding to soil was found to be high.