Binding of [¹⁴C]parathion in soil: A reassessment of pesticide persistence

A steady decrease of extractable [¹⁴C]parathion residues in soils over a 1-month incubation period was accompanied by an increase of unextractable, bound ¹⁴C-labeled residues, resulting finally in total recoveries of extracted plus bound residues of 80 to 87 percent of the applied radiocarbon. Soils containing bound residues were nontoxic to fruit flies. Binding of ¹⁴C-labeled residues was related to the activity of soil microorganisms; soil sterilization resulted in a reduction of binding by 58 to 84 percent. Under flooded (anaerobic) conditions, the binding of compounds labeled with ¹⁴C doubled, and parathion was reduced to aminoparathion. Reinoculation of sterilized flooded soil fully reinstated the binding capacity. [¹⁴C]Aminoparathion was preferentially bound to soil, since its binding within 2 hours was 30 times greater than that of [ ¹⁴C]parathion. Because of the existence of formerly "unseen," unextractable residues, the concept of " persistent" and "nonpersistent" pesticide residues might have to be reconsidered.