Changes in sorption/bioavailability of imidacloprid metabolites in soil with incubation time

Changes in sorption/bioavailability of two metabolites, imidacloprid-urea { 1-[(6-chloro-3-pyridinyl) methyl]-2-imidazolidinone} and imidacloprid-guani dine {1-[(6-chloro-3-pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine} of the insecticide imidacloprid {1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-i midazolidinimine} with aging in different soils were determined, Soil moist ure was adjusted to -33 kPa and C-14- and analytical-grade imidacloprid-ure a and imidacloprid-guanidine were added to the soil at a rate of 1.0 mg kg( -1). Spiked soils were incubated at 25 degreesC for 8 weeks. Replicate soil samples were periodically extracted successively with 0.01 N CaCl2, aceton itrile, and 1 N HCl. Imidacloprid-urea sorption, as indicated by sorption c oefficient values, was highest in the soil with highest organic C content, and increased by an average factor of 2.6 in three soils during the 8-week incubation period. Imidacloprid-guanidine sorption increased by a factor of 2.3 in the same soils. The increase in sorption was the result of a de cre ase in the metabolite extractable with CaCl2 (solution phase); the amount o f metabolite extractable with acetonitrile and HCl (sorbed phase) did not s ignificantly change with incubation time. It appears the increase in sorpti on was because the rate of degradation in solution and on labile sires was faster than the rate of desorption from the soil particles. It may have als o been due to metabolite diffusion to less accessible or stronger binding s ites with time. Regardless of the mechanism, these results are further evid ence that increases in sorption during pesticide aging should be taken into account during characterization of the sorption process for mathematical m odels of pesticide degradation and transport.