Persistence and biodegradation of diazinon and imidacloprid in soil

Organophosphorus and chloronicotinyl pesticides are effective for controlling various destructive insects. We investigated the persistence of diazinon and imidacloprid in soil under laboratory conditions. The study was carried out on silty loam, sandy loam and sandy soils. Experiments were also conducted to examine the effect of soil microorganisms on the degradation of these pesticides in the silty loam soil. Pesticide residues in the samples collected at different times after application were measured by high performance liquid chromatography (HPLC). A first-order kinetic model described the degradation rate of pesticides. Pesticides degradation proceeded at a slower rate in sandy and sandy loam soil than that in silty loam soil. The half-lives (t(1/2)) were 4.15 and 9.9 weeks in silty loam soil; 7.53 and 12.16 weeks in sandy loam soil and 10.34 and 12.6 weeks in sandy soil for diazinon and imidacloprid, respectively. Soil microorganisms had a major role in the acceleration of pesticides degradation. Corresponding degradation half-lives were 22.36 and 3.63 weeks for diazinon while, they were 28.88 and 6.93 weeks for imidacloprid in sterilized and non-sterilized soil, respectively. At least two degradation products of diazinon and only one of imidacloprid were observed in non-sterilized soil.