Mechanisms of woodchip-pyrite bioretention systems for nitrogen removal and N₂O emission control

• The woodchip-pyrite bioretention achieved high nitrogen removal efficiency. • The woodchip-pyrite bioretention exhibited significantly reduced N₂O emissions. • Woodchips accelerate pyrite decomposition. • The addition of woodchip increased microbial electron transfer system activity. • The addition of woodchip increased denitrifying enzyme activity. Bioretention, as one of the flexible and economical green infrastructures, is widely used for urban stormwater management. Current modified strategies for bioretention have largely focused on improving nitrogen removal efficiency, while studies addressing their potential for N 2 O mitigation remain limited. In this study, the performance of woodchip-pyrite bioretention (W-PB) was compared with that of sand bioretention (SB) and pyrite bioretention (PB). The denitrification performance and N 2 O emissions of the three systems were quantified, and the underlying N 2 O mitigation mechanism was elucidated from the perspective of microbial metabolic functions. During a 120-day monitoring period, the average removal efficiency of nitrate in W-PB was 81.4 % ± 11.2 %. The average N 2 O emission flux in W-PB was 13.21 ± 3.06 μgN 2 O N/(m 2 ·h), which represents reductions of 41.4 % and 28.5 % compared to the SB and PB, respectively. Scanning electron microscope results indicated that the addition of woodchips enhanced the decomposition of pyrite, thereby increasing the electron donors for denitrification. This enhancement was further reflected by the increased activity of the electron transfer system and the elevated activities of NIR and NOS enzymes in the W-PB, suggesting a more complete denitrification process. As a result, the denitrification potential in the submerged zone reached 0.3735 ± 0.0022 ng N/(g·h), which was 3.35 and 2.06 times higher than that in SB and PB, respectively. Additionally, high-throughput sequencing results show an increased microbial abundance and diversity in W-PB. The results provide theoretical and technical support for the low-carbon-oriented optimization and engineering application of bioretention systems in urban stormwater nitrogen removal. Graphical Abstract of Mechanisms of Woodchip-Pyrite Bioretention Systems for Nitrogen Removal and N₂O Emission Control.