Biochar enhances methane uptake in engineered green roof substrate

Biochar is increasingly applied to soils to enhance carbon sequestration and mitigate greenhouse gas emissions, yet its role in regulating methane (CH₄), a potent greenhouse gas, in engineered substrates remains poorly understood. In particular, there is a lack of field data on how biochar amendments influence CH₄ fluxes from engineered substrates in urban environments, where substrate composition, shallow profiles, and intensive management fundamentally differ from natural soils. Here, we present a long-term (2020–2024) field study examining the effects of biochar amendment on CH₄ fluxes from engineered substrates used in extensive green roofs dominated by stonecrop species ( Phedimus kamschaticus and Sedum spp.). Biochar-amended modules (~ 5% v/v; 20 t ha⁻¹) consistently exhibited greater CH ₄ uptake than unamended controls across seasons, with peak uptake rates in spring 2023 nearly fivefold higher (− 1.91 ± 0.25 vs. −0.40 ± 0.10 nmol⋅m⁻²⋅s⁻¹). Importantly, biochar addition did not increase carbon dioxide (CO₂) emissions, indicating enhanced CH ₄ uptake without increased carbon losses. Variability in CH ₄ uptake was strongly associated with substrate moisture and water vapour flux, suggesting that biochar modifies gas diffusivity and oxygen availability through moisture-mediated physical controls that favour microbial CH ₄ oxidation. These results demonstrate that biochar can substantially enhance CH ₄ uptake in engineered green roof substrates and extend the application of biochar-based mitigation strategies to engineered systems beyond conventional soils. The mechanistic insights provided here are broadly relevant to understanding CH ₄ cycling in biochar-amended engineered substrates.