Bioavailability of nonextractable (bound) pesticide residues to earthworms

There is an ongoing debate regarding whether nonextractable (:bound) pesticide residues in soils are occluded OF may remain bioavailable in the long term in the environment. This study investigated the release of C-14-labeled residues, which were previously nonextractable after exhaustive extraction with organic solvents in soils, and their uptake by earthworms (Aporrectodea longa). After a 100-day incubation of soils treated with C-14-labeled atrazine, isoproturon, and dicamba and exhaustive Soxhlet extractions with methanol and dichloromethane, nonextracted C-14-labeled residues remaining in the soils were 18, 70, and 67%, respectively. Adding clean soil in the ratio of 7:1 increased the volumes of these extracted soils. After earthworms had lived in these previously extracted soils for 28 days, 0.02-0.2% of previously bound C-14 activity was absorbed into the earthworm tissue. Uptake by earthworms was found to be 2-10 times higher in soils containing freshly introduced C-14-labeled pesticides as compared to soils containing nonextractable C-14-labeled residues. The differential bioavailability observed between freshly introduced C-14-labeled pesticides and those previously nonextractable may be related to the ease of transfer of the C-14 activity into the solution phase. By the end of the 28-day incubation period, 3, 23, and 24% of previously nonextractable C-14-labeled isoproturon, dicamba, and atrazine residues, respectively, were extracted by solvents or mineralized to (CO2)-C-14. The amounts of C-14 activity released were not significantly different in the presence or in the absence of earthworms in soils containing previously nonextractable residues. However, the formation of bound residues was 2, 2, and 4 times lower for freshly introduced C-14-labeled isoproturon, dicamba, and atrazine, respectively, suggesting that the presence of earthworms retarded bound residue formation.