Nitrogen removal in wastewater treatment plant effluent by biochar-based constructed wetlands under low temperature...

Low-temperature conditions often impaired nitrogen polishing performance in constructed wetlands (CWs) and might simultaneously promote the accumulation of dissolved and gaseous nitrous oxide (N₂O) in wastewater treatment plant effluent. In this study, biochar-amended plant-coupled CWs were evaluated under winter conditions (4-13°C) to improve nitrogen removal from secondary effluent, while also examining their effects on dissolved N₂O (d-N₂O) and gaseous N₂O (g-N₂O) dynamics. Moderate biochar addition (10 g/L) achieved the highest performance, reducing d-N₂O and g-N₂O by up to 26% while improving total nitrogen removal by 50%. Biochar optimized rhizosphere redox conditions, enriched denitrifiers, and increased the nosZ/(norB + norC) ratio, promoting complete N₂O reduction. Simultaneously, enhanced photosynthetic capacity and antioxidant activity strengthened plant-mediated N₂O assimilation. The porous structure and electron exchange capacity of biochar further facilitated microbial electron transfer and sustained denitrification. These findings demonstrated that biochar-driven plant-microbe synergy effectively alleviates d-N₂O accumulation in CWs during winter, offering a low-carbon strategy for climate-resilient wastewater polishing.