Mathematical prediction of imidacloprid persistence in two Croatian soils with different texture, organic matter content and acidity under laboratory conditions
In the present laboratory study, persistence of imidacloprid (IMI) as a function of initial insecticide concentration and soil properties in two Croatian soils (Krk sandy clay and Istria clay soils) was studied and described mathematically. Upon fitting the obtained experimental data for the higher concentration level (5 mg/kg) to mathematical models, statistical parameters (R 2, scaled root mean squared error and χ 2 error) indicated that the single first-order kinetics model provided the best prediction of IMI degradation in the Krk sandy clay soil, while in the Istria clay soil biphasic degradation was observed. At the lower concentration level (0.5 mg/kg), the biphasic models Gustafson and Holden models as well as the first-order double exponential model fitted the best experimental data in both soils. The disappearance time (DT50) values estimated by the single first-order double exponential model (from 50 to 132 days) proved that IMI can be categorized as a moderately persistent pesticide. In the Krk sandy clay soil, resulting DT50 values tended to increase with an increase of initial IMI concentration, while in the Istria clay soil, IMI persistence did not depend on the concentration. Organic matter of both experimental soils provided an accelerating effect on the degradation rate. The logistic model demonstrated that the effect of microbial activity was not the most important parameter for the biodegradation of IMI in the Istria clay soil, where IMI degradation could be dominated by chemical processes, such as chemical hydrolysis. The results pointed that mathematical modeling could be considered as the most convenient tool for predicting IMI persistence and contributes to the establishment of adequate monitoring of IMI residues in contaminated soil. Furthermore, IMI usage should be strictly controlled, especially in soils with low organic matter content where the risk of soil and groundwater contamination is much higher due to its longer persistence and consequent leaching and/or moving from soil surface prior to its degradation.