Nitrous oxide mitigation in biochar-pyrite-based bioretention systems via zonal microenvironment regulation

Bioretention systems (BRS) are widely used in urban stormwater management for runoff control and water quality improvement. However, mitigating nitrous oxide (N₂O) while enhancing nitrogen removal remains a key challenge for BRS. In this study, biochar-pyrite-based BRS (BP-BRS and BPB-BRS) were constructed and compared with a pyrite-amended BRS (P-BRS) and a control (CK-BRS). Under varying antecedent drying durations, rainfall intensities, and influent nitrate loadings, BPB-BRS achieved total nitrogen removal efficiencies of 59.2%-85.7% and reduced N₂O emissions by 39.3%-84.6% compared with the other configurations. These improvements were associated with the enrichment of sulfur-oxidizing bacteria (up to 35.2%) and electroactive bacteria (up to 12.7%), a 5.66-fold increase in electron transport system activity, and a higher nosZ/(nirS + nirK) ratio (1.24 vs. 0.61-0.84), indicating enhanced potential for terminal N₂O reduction. Biochar exerted zone-specific functions. In the vadose zone, it enhanced NH₄⁺-N retention and nitrification across wet-dry cycles, limiting transient NO₂^--N accumulation and nitrification-associated N₂O formation. In the submerged zone, biochar-pyrite coupling sustained electron supply from pyrite oxidation, while biochar served as an electron shuttle and temporary electron reservoir and, together with electroactive bacteria, facilitated electron transfer during denitrification. This integrated electron generation-transfer-consumption framework promoted terminal N₂O reduction, thereby limiting N₂O accumulation. Overall, BPB-BRS achieved simultaneous nitrogen removal and N₂O mitigation, supporting low-carbon bioretention design.