Assessment of the environmental fate of pesticides and their ecotoxicological impact on soil microorganisms : the case of chlorpyrifos, isoproturon and tebuconazole

Pesticides are mainly applied to cultivable soils in agriculture to protect crops from various pests. Only a tiny part of sprayed pesticides ends up in their target organisms, while the rest persists on plants or in soil from where it can disperse to surrounding environments and harm non-target organisms. To minimize environmental damage caused by pesticides, their environmental fate and potential impact on non-target organisms is evaluated in an environmental risk assessment procedure by EU-authorities before their authorization and introduction to the market. The pesticide approval process is criticized for compromising a non-precautionary principle, as numerous formerly used pesticides are now banned due to the late emergence of environmental issues after years of their usage. Especially the formation of pesticide transformation products and the ecotoxicological impact of pesticide residues on soil microorganisms supporting numerous ecosystem functions are not sufficiently evaluated during the environmental risk assessment process. Within this context, my PhD aimed to support pesticide environmental risk assessment by the application and development of innovative methods of analytical chemistry and molecular biology to study the environmental dissipation (degradation / sorption / transformation) of pesticides in soil and their ecotoxicological impact on soil microorganisms. The three pesticides chlorpyrifos (CHL), isoproturon (IPU) or tebuconazole (TCZ) were chosen to establish lower tier and higher tier pesticide exposure scenarios in lab-to-field experimental designs. A research highlight of my PhD is the development of a combined approach of suspect screening and molecular typology to detect and classify TCZ transformation products in soil and to estimate their environmental parameters. Moreover, findings of my PhD partly confirmed risk assessment studies, as the lower tier pesticide exposure scenario suggested a low but significant impact of CHL and TCZ (and no impact of IPU) on the soil bacterial composition, estimated by next-generation sequencing of 16S rDNA amplicons. However, in the higher tier pesticide exposure scenario, CHL and TCZ were found to induce significant changes in the microbial activity (evaluated via pesticide mineralization as a proxy for soil buffering functions) and the bacterial diversity (evaluated via next-generation sequencing of 16S rDNA amplicons). The results of my PhD have the potential to enrich future post-authorization risk assessments of CHL (due in 2020), IPU (due in 2017) and TCZ (due in 2023) with additional findings. In addition, the research of my PhD opens a new door for the enhancement of pesticide environmental risk assessment, especially concerning the methods used to assess the ecotoxicological impact of pesticides on soil microorganisms. French summary