New insights into how chalcopyrite mitigates nitrous oxide emissions from denitrification

Green infrastructure, particularly bioretention systems, is an effective nature-based system for enhancing nitrogen removal from stormwater. However, a critical yet often overlooked challenge is the emission of nitrous oxide (N₂O) during denitrification in bioretention systems, which results from incomplete denitrification due to the typically low carbon-to-nitrogen ratio of stormwater. In this study, three experimental groups (chalcopyrite, pyrite, and control without mineral addition) were established to evaluate denitrification performance and elucidate the mechanisms underlying N₂O reduction. Compared with the control groups, the chalcopyrite groups reduced N₂O flux and achieved a nitrate removal efficiency of 71.3%, representing a 19.0% increase. Notably, the chalcopyrite groups resulted in an 11-fold increase in the abundance of the nosZ gene, which encodes nitrous oxide reductase, relative to the control groups. Moreover, chalcopyrite significantly enhanced electron transport system activity, thereby facilitating electron transfer among the four denitrification enzymes (nitrate, nitrite, nitric oxide, and nitrous oxide reductases). Collectively, these results indicate that chalcopyrite promotes autotrophic denitrification driven by the oxidation of sulfur and iron ions, while the released copper ions further contribute to N₂O reduction. These findings provide new insights into the potential of chalcopyrite as an effective amendment in nature-based systems, enabling concurrent enhancement of nitrogen removal and mitigation of N₂O emissions.