Biochar greatly enhances methane oxidation in urban green roof substrate

Green roofs are increasingly implemented in cities to manage stormwater and urban cooling, yet their potential role in greenhouse gas (GHG) mitigation remains unclear. Methane, a potent GHG, is only weakly oxidized by roof substrates, and ways to enhance this sink remain unexplored despite its significance at the city-scale. We report a long-term (2020–2024) field study on biochar effects on CH₄ fluxes in extensive Sedum green roofs, where biochar-amended modules (~5% v/v; 20 t ha⁻¹) maintained consistently higher CH₄ uptake, peaking in spring 2023 at nearly fivefold greater than controls (−1.91 ± 0.25 vs. −0.40 ± 0.10 nmol·m⁻²·s⁻¹). Notably, biochar did not increase CO₂ emissions, indicating enhanced CH₄ uptake without compromising carbon efficiency. Enhanced CH ₄ uptake correlated with higher substrate moisture and H₂O flux, suggesting a moisture–evaporation co-regulated diffusivity mechanism facilitating microbial CH₄ oxidation, highlighting biochar as a scalable means to enhance climate-mitigation capacity of urban green infrastructure.