Transfer of prosulfocarb and boscalid residues from maize leaves to soil and their effects on soil microorganisms

Plant materials that have been in contact with pesticides can be incorporated into the soil, posing a potential risk to non-target soil organisms and, hence, soil functions. This study investigated effects of two pesticides applied to maize leaves on the soil microbial community, activity and function. The herbicide prosulfocarb (PSC) and the fungicide boscalid (BSC) were applied alone or in combination to fresh or aged maize leaves, which were incorporated into soil. During a 56-day incubation we quantified pesticide residues in soil and maize leaves as well as maize-derived C incorporation into different microbial fractions (CO₂, extractable organic carbon, microbial biomass and main microbial groups). Prosulfocarb residues on maize and in soil decreased to below 5 % after 56 days. However, BSC residues were transferred from maize into the soil, as indicated by an increase in BSC residues in soil of around 15 %. Prosulfocarb initially inhibited the synthesis of soil bacterial phospholipids by 25–45 %, which was accompanied by a decrease in the incorporation of maize-derived C into microbial biomass by 68–70 %. Following this, microorganisms shifted their nutrient acquisition strategy towards carbon and phosphorus, which led to increased utilization of easily available maize-derived C. Boscalid transiently inhibited the growth of soil fungi, reduced soil respiration, and mineralization of maize. In the future, pesticide accumulation through transfer from plant material into soils and the mode of action dependent effects on soil microorganisms need to be considered for risk assessment.