β-triketone herbicides are among the most used herbicides in corn crop to control broadleaf weeds. These herbicides inhibit the 4-hydroxyphenylpyruvate dioxygenase (4-HPPD) and lead to bleaching and death of weeds. This enzyme is not only found in plants but in all living organisms, including microorganisms where it takes part to the tyrosine degradation pathway. Thus, microorganisms classified as “non-target organisms” by current EU regulation for pesticide authorization, might be impacted by β-triketones, with possible domino effect on microbial functions supporting soil ecosystem services (Thiour-Mauprivez et al. 2019). Since microorganisms have been proposed by EFSA as key-drivers to be monitored to better protect soil ecosystem services, we tested the hypothesis that hppd bacterial community can constitute a biomarker of exposure to β-triketones. Within this context, we developed a toolbox to monitor the abundance, the diversity and the activity of the hppd bacterial community. Abundance and diversity of hppd bacterial community were tested in a lab-to-field experimental design following the tiered-approach recommended by EFSA to conduct pesticide environmental risk assessment (ERA). Under lab conditions, soil microcosms not exposed (control) or exposed to x1 or x10 times the agronomical dose of sulcotrione (active ingredient) or Decano® (one of the commercial formulation of sulcotrione) were studied. Under field conditions, samples were collected in corn crop exposed to β-triketones or not (control). Analytical chemistry was applied to all samples to study the dissipation of β-triketone, search for residues and estimate the scenario of exposure of soil microorganisms. Nucleic acids (DNA/RNA) were extracted from soil samples. Home-made degenerated primers, specific to the hppd gene of soil bacteria, allowed us to measure the abundance (quantitative PCR), the composition (α-diversity) and the diversity (β-diversity) (NGS) of the hppd bacterial community (Thiour-Mauprivez et al. 2020). Finally, the inhibition of the activity of the 4-HPPD was assessed on pure bacterial strains under wet lab conditions to measure EC50 of β-triketones. Our poster will be presented to the audience with the aim to identify the better proxy of the hppd bacterial community that could be used as a biomarker to reflect the exposure of soil microbial community to β-triketone residues. As a perspective my work might be extended to other pesticides targeting other enzymes that are also present in so call non-target organisms such as sulfonylureas inhibiting acetohydroxy acid synthase (AHAS).