ORIGINAL ABSTRACT

Modern agriculture relies heavily on pesticide use to meet the demands of food quality and quantity. Therefore, pesticides are often applied in mixtures, leading to a diverse cocktail of chemicals and their metabolites in soils, which can affect non-target organisms such as soil microorganisms. Pesticides are tested for their single effects, but studies on their interactive effects are scarce. This study aimed to determine the effects of up to three simultaneously applied pesticides on the soil microbial community and on their special function in pesticide degradation. Agricultural soil without previous pesticide application was exposed to different mixtures of the herbicide glyphosate (GLP), the phenoxy herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) and the fungicide difenoconazole (DFC) for up to 56 days. Isotopic and molecular methods were used to investigate effects of the mixtures on the microbial community and to follow the mineralization and utilization of GLP. An initial increase in the metabolic quotient by up to 35 % in the presence of MCPA indicated a stress reaction of the microbial community. The presence of multiple pesticides reduced both gram positive bacterial fatty acid methyl esters (FAMEs) by 13 % and the abundance of microorganisms with the genetic potential for GLP degradation via the AMPA (aminomethylphosphonic acid) pathway. Both the number of pesticides and the identities of individual pesticides played major roles. Surprisingly, an increase in 13C-labelled GLP mineralization of up to 40 % was observed while carbon use efficiency (CUE) decreased. Interactions between multiple pesticides might alter the behavior of individual pesticides and be reflected in the microbial community. Our results highlight the importance of investigating not only single pesticides, but also pesticide mixtures and their interactions.