ORIGINAL ABSTRACT

Treatment wetlands are increasingly used to treat agricultural wastewater. While plants and substrates contribute to pesticide immobilization and removal, most metabolic degradation is mediated by microorganisms. However, pesticides may negatively affect the microbial communities responsible for their degradation. This study aimed to assess whether biochar, an organic amendment commonly used in treatment wetlands, could mitigate the effects of pesticides on microbial communities. Six treatment wetland mesocosms planted with Scirpus cyperinus were sampled before and after exposure to a pesticide mixture (atrazine, glyphosate, and chlorantraniliprole) applied at three concentration levels. Half of the mesocosms were amended with biochar. Biochar did not impact microbial alpha diversity but altered community composition, particularly by modifying the relative abundance of bacterial taxa involved in nitrogen cycling. Pesticide concentration influenced root-associated fungal communities only. Eight taxa with known pesticide-degrading potential increased in relative abundance following pesticide application. Overall, our results suggest that biochar and pesticide exposure can shape microbial community composition in treatment wetlands, but without clear mitigation of pesticide impacts under the conditions tested. These findings highlight the importance of considering microbial community structure when designing treatment wetlands and underscore the need for temporally resolved studies to better understand microbial responses to pesticide exposure.