Assessing the effects of beta-triketone herbicides on the soil bacterial and hppd communities: a lab-to-field experiment.

Maize cultivators often use beta-triketone herbicides to prevent the growth of weeds in their fields. These herbicides target the 4-HPPD enzyme of dicotyledons. This enzyme, encoded by the hppd gene, is widespread among all living organisms including soil bacteria, which are considered as non-target organisms. Within the framework of the pesticide registration process, the ecotoxicological impact of herbicides on soil microorganisms is solely based on carbon and nitrogen mineralization tests. In this study, we used more extensive approaches to assess with a lab-to-field experiment the risk of beta-triketone on the abundance and the diversity of both total and hppd soil bacterial communities. Soil microcosms were exposed, under lab conditions, to 1x or 10x the recommended field dose of sulcotrione or its commercial product, Decano. Whatever the treatment applied, sulcotrione was fully dissipated from soil after forty-two days post-treatment. The abundance and the diversity of both the total and the hppd bacterial communities were not affected by the herbicide treatments all along the experiment. Same measurements were led in real conditions, on three fields cropped with maize: one not exposed to any plant protection products, another one exposed to a series of plant protection products (PPPs) comprising mesotrione and a last one exposed to different PPPs including mesotrione and tembotrione. In this latter, the abundance of the hppd community varied over time. The diversity of the total and the hppd communities evolved over time independently from the treatment received. Only slight but significant transient effects on the abundance of the hppd community in one of the tested soil were observed. Our results showed that beta-triketone herbicides have no risk toward soil bacteria, even by targeting specifically the hppd bacterial community that harbors the targeted enzyme of these herbicides.