Plants mitigate microbial response to pesticides in riparian environments

Rivers serve as a vector for pollutant transport, as well as a dispersal agent for invasive plant species. Through flooding events, these stressors can be distributed to areas such as the riparian zone, which may then alter soil microbial community activity and function. To test this, a pot experiment was conducted with pesticide application and plant community as treatment factors. Plant community treatments included control (no plants), native riparian vegetation, and riparian vegetation with the invasive species Impatiens glandulifera. Pots were flooded with either tap water or a pesticide mixture for a period of 7 days. Root-zone porewater was then collected at the base of each plant. Microbial activity and function were assessed by incubating soil with collected porewater and conducting a MicroResp assay, in which different carbon substrates were added, and CO₂ evolution (a proxy for microbial activity) was measured. Results showed that porewater from pesticide-exposed pots (regardless of plant community) led to higher microbial respiration compared to porewater from control pots. Conversely, porewater from pots without pesticide exposure resulted in lower respiration, suggesting that plants may mitigate pesticide-induced changes in microbial activity under flooded conditions. Catabolic diversity, however, remained consistent across treatments, indicating that microbial function was unaffected. Total organic carbon analysis of porewater revealed that the quantity remained constant across treatments, suggesting that composition, rather than concentration, influenced microbial activity. Future work will include untargeted analyses of porewater to characterize its chemical composition and targeted analyses for the presence of root exudates to better understand their role in shaping microbial responses.