Fate and effect of imidacloprid on vermicompost-amended soils under dissimilar conditions: Risk for soil functions, structure, and bacterial abundance.

The fate and impact of pesticide on soil depend partly on the agricultural practices, such as prior treatment with pesticide and/or organic amendments. As a means of determining how the previous soil conditions can affect the fate of imidacloprid (IMI) and its effect on soil functions, experiments were made with soil samples, double-amended or not with either vine-shoot (W) or olive cake (O) vermicompost or contaminated or not with IMI. These soil samples, incubated for 3months, were placed in two microcosms (M1 with the pre-amended soils and M2 with the pre-exposed soils), treated with IMI and amended with vermicomposts and then incubated for 3months. The IMI distribution on soil fractions, sorption processes, dissipation kinetics, and biochemical as well as genetic structure and bacterial abundance were determined to assess the fate and impact of IMI on the soil. The addition of W vermicompost to the soil reduced the IMI availability. The dissipation kinetic in soils from M1 and M2 followed, respectively, a single first-order and a double first-order in parallel models. The lowest IMI persistence corresponded to the soil from M2 amended with O-vermicompost with DT50 and DT90 values of 67d and 265d, while in the other soils 90% dissipation required >512d. The vermicomposts-amended contaminated soils increased the dehydrogenase activity by 2- and 4-fold respect the control soils. However, the urease activity decreased due to the IMI influence. The changes in the bacterial community in the contaminated soil amended with O-vermicompost during incubation were correlated with the dissipation rate constant of IMI, suggesting a better tolerance of microorganisms to IMI. Thus, in the soil contaminated with IMI, the amendment with the vermicompost from olive cake can mitigate the impact of this insecticide on soil functions and promote its depuration capability while minimizing environmental risks.