Changes in the sorption-desorption of fungicides over time in an amended sandy clay loam soil under laboratory conditions

The aim of this work was to study the temporal changes in the sorption-desorption of fungicides in a sandy clay loam soil amended with spent mushroom substrate (SMS) under controlled laboratory conditions and the influence that fungicides properties and soil characteristics have on these processes. Soil amendment with SMS is becoming a widespread management practice since it can effectively solve the problems of uncontrolled SMS accumulation and disposal and improve soil quality. However, when simultaneously applied with pesticides, SMS can significantly modify the environmental behaviour of these compounds. Sorption-desorption isotherms of metalaxyl, penconazole, pyrimethanil and iprovalicarb for unamended and amended vineyard soils from La Rioja (Spain) were obtained. Composted SMS (C-SMS) and fresh SMS (F-SMS) from cultivation of different mushrooms were used as amendments at 2 % and 10 % rates. Soil parameters (organic carbon (OC), dissolved organic carbon (DOC), humic acid (HA) and fulvic acid (FA)) and sorption (Kf, nf, Kd, Koc) and desorption (Kfd, nfd, H) parameters of fungicides were determined over 0, 6 and 12 months of soil incubation with SMS under controlled conditions. Addition of amendments to soil increased soil sorption capacity of fungicides. Kd values increased with the hydrophobic character of fungicides (metalaxyl < iprovalicarb < pyrimethanil < penconazole) at both amendment rates. The lower content of DOC and the higher degree of OC humification enhanced sorption of all fungicides by the soil + C-SMS with regard to the soil + F-SMS. In general, sorption of fungicides decreased after 6 and 12 months of soil + SMS incubation, although the humification degree of the remaining OC expressed by HA/FA increased. This might indicate that the OC content was more important for fungicide sorption than the changes in its nature with the incubation time. SMS addition favoured desorption of metalaxyl and iprovalicarb, in general, whereas irreversible sorption of penconazole and pyrimethanil increased. However, the opposite trends were observed when the soil + SMS incubation time increased. The results show an increase in sorption of all fungicides by amending soil with composted or fresh SMS. However, desorption of fungicides increases or decreases depending on the properties of fungicides and soil + SMS. Changes in both processes with the incubation time are more related to the OC content of the amended soil than to the evolution of its nature. These outcomes are of interest for extending SMS application to soil with minimal or no environmental risk when used simultaneously with fungicides.