Differential effects of environmentally relevant concentrations of ibuprofen on denitrification and nitrous oxide emissions in river sediments

The increasing presence of ibuprofen in aquatic ecosystems poses significant challenges to their biogeochemical functions, including nitrogen transformations. In this study, we employed ¹⁵N-labeling techniques to investigate the effects of environmentally relevant concentrations of ibuprofen (0–10,000 ng L⁻¹) on denitrification and the associated nitrous oxide (N₂O) emissions in river sediments over a 60-day period. The results revealed a hump-shaped response in denitrification rates to ibuprofen addition across a range of nitrate concentrations (1–60 mg N L⁻¹), with rates peaking near 200 ng L⁻¹ ibuprofen, followed by inhibition at certain higher concentrations, leading to a reduction of up to 25.8 % compared to the treatment without ibuprofen. Kinetic analysis showed that the maximum denitrification rate followed the same hump-shaped trend, despite a decrease in nitrate affinity with increasing ibuprofen concentrations. The abundance of denitrifying bacteria mirrored the pattern observed in denitrification rates across different ibuprofen concentrations. However, increasing ibuprofen concentrations consistently accelerated N₂O production rates. Microbial analysis suggests that the increase in N₂O production genes was faster than for reduction genes, while the decrease was slower with increasing ibuprofen concentrations. This study highlights the hump-shaped response of denitrification rates and the consistent increase in N₂O emissions induced by ibuprofen, offering insights for developing environmental management strategies to mitigate ibuprofen and nitrogen pollution, as well as reducing N₂O emissions.