Leaf microbial communities possess a large panel of enzymes permitting the breakdown of leaf polymers as well as the transformation of organic xenobiotic compounds present in stream waters. This study aims to assess the potential of leaf microbial communities, exhibiting different exposure histories to pesticides (upstream versus downstream), to biotransform three maize herbicides (mesotrione, S-metolachlor, and nicosulfuron) in single and cocktail molecule exposures. The results showed a high dissipation of nicosulfuron (sulfonylurea herbicide) (from 29.1 ± 10.8% to 66 ± 16.2%, day 40) in both single and cocktail exposures, respectively, but not of mesotrione and S-metolachlor. The formation of nicosulfuron metabolites such as ASDM (2-(aminosulfonyl)-N,N-dimethyl-3-pyridinecarboxamide) and ADMP (2-amino-4,6-dimethoxypyrimidine) and the weak sorption (<0.4%) on the leaf matrix confirmed the transformation of this molecule by leaf microorganisms. In addition, the downstream communities showed a greater ability to transform nicosulfuron than the upstream communities suggesting that the exposure history to pesticides is an important parameter and can enhance the biotransformation potential of leaf microorganisms. After 40-day single exposure to nicosulfuron, the downstream communities were also those experiencing the greatest shifts in fungal and bacterial community diversity suggesting a potential adaptation of microorganisms to this herbicide. Our study emphasizes the importance of leaf microbial communities for herbicide biotransformation in polluted stream ecosystems where fungi could play a crucial role.