Snorkel-biochar coupling enhances organic matter and nitrogen pollutant removal while mitigating greenhouse gas...

Bioretention systems treating urban stormwater face a trade-off between organic matter/nitrogen pollutant removal and greenhouse gas (GHG) emissions. A snorkel-biochar composite bioretention system (SN.BC) was developed to address this trade-off. Over 182 days, SN.BC maintained stable treatment under varying antecedent dry days (ADDs) and pollutant loadings. From Day 16 onward during the initial 112-day operation, the mean removal efficiencies of COD, TN, NO₃^--N, and NH₄⁺-N were 95.0 ± 0.4%, 86.5 ± 0.5%, 97.9 ± 1.2%, and 93.0 ± 0.3%, respectively. GHG emissions were assessed under varying ADDs and pollutant loadings. Across these conditions, the mean global warming potential calculated from 8-h post-drainage emissions for SN.BC was 59.4% lower than for Control. Integrated evidence suggested that the snorkel may have facilitated interlayer electron transfer and altered spatial redox stratification, whereas biochar may have provided microbial habitats and local electron-buffering capacity. These changes were associated with greater functional potential for nitrification and denitrification and lower methanogenic potential. The enrichment of ammonia-oxidizing and nitrite-oxidizing microorganisms in the upper layer, together with denitrification-associated taxa in the middle and bottom layers, was consistent with spatial differentiation of nitrogen transformation potential. Moreover, the co-occurrence of genes related to CH₄ oxidation and NO₃^-/NO₂^- reduction, together with enrichment of the archaeal genus Candidatus Methanoperedens, suggested that denitrification-dependent anaerobic methane oxidation-associated metabolism may have contributed to concurrent NO₃^-/NO₂^- removal and CH₄ mitigation. This snorkel-biochar strategy offers a promising route for coupling organic matter and nitrogen removal with GHG mitigation through spatial redox regulation.