Binding of "persistent" and "nonpersistent'' 14C-labeled insecticides in an agricultural soil.

Abstract

The extractability and formation of bound 14C-labeled residues in an agricultural loam soil were investigated with the “nonpersistent” insecticides [ 14C]methylparathion and [ 14C]fonofos (Dyfonate) and with the “persistent” insecticides [ 14C]dieldrin and P,P‘-[’~C]DDT. With [14C]methylparathion only 7% of the applied radiocarbon was extractable 28 days after soil treatment, while 14C-bound residues amounted to 43% of the applied dose. With [‘4C]fonofos, however, still 47% of the applied dose was extractable and 35% of the applied radiocarbon was bound. Only a fraction of the radiocarbon extracted from [14C]methylparathion treated soil was associated with the parent compound, while extractable ‘%-labeled residues from the other insecticide-treated soils were primarily due to the presence of the parent compounds. Smaller amounts of soil-bound residues had been formed with the “persistent” insecticides, amounting after 28 days to only 6.5% of the applied [14C]dieldrin and to 25% of the applied p,p’-[14C]DDT, while 95 and 72%, respectively, were still recovered by organic solvent extraction. They differed from the organophosphorus compounds in their relatively low binding properties and their high extractability from soils. Contrary to results with [14C]parathion, the mechanism of binding of [ 14C]fonofos was not dependent on the presence of soil microorganisms. At higher application rates of the insecticides, relatively less radiocarbon was bound, possibly due to saturation of binding sites. Bound residues were found to be either nontoxic to fruit flies or of drastically reduced insecticidal activity. The significance of the formation of insecticide bound residues in soils in reassessing persistence of pesticides is discussed.