Adsorption of pesticides by sewage sludge, grape marc, spent mushroom substrate and by amended soils

A comparative adsorption study of linuron, diazinon and myclobutanil, representing three classes of pesticides, by sewage sludge (SS), grape marc (GM) and spent mushroom substrate (SMS) and soils amended with these residues was carried out. The study assayed the effect of the nature of the residue, soil characteristics, pesticide properties and residue-soil incubation time on adsorption. Soil-residues were incubated outdoors for one month and 12 months. Adsorption was not related to pesticide hydrophobicity. For unamended soils, distribution coefficients (Kd) ranged between 1.77-6.60mLg(-1) for linuron, 0.54-5.52mLg(-1) for diazinon and 1.35-4.52mLg(-1) for myclobutanil, and increased significantly for amended soils: up to 4.8 times for linuron, 6.9 times for diazinon and 5.3 times for myclobutanil. Kd values revealed the highest adsorption of linuron and diazinon by GM and of myclobutanil by SMS. These coefficients significantly changed after 12 months of incubation as compared to 1 month. The adsorption of linuron and diazinon decreased and this was attributed to the decreasing of the organic carbon (OC) content over time. Influence of OC evolving to more stable forms by humification was not evident. However, the Kd values of myclobutanil increased with respect to those for 1-month incubated soils. These results indicated the need to consider different factors of soil/pesticide/residue in order to avoid environmental risks by the simultaneous use of pesticides and organic residues as amendments in agricultural practices.