Urban Tree Channeling of Soil Methane and Nitrous Oxide and Its Mitigation Using Biochar

Urban forests are important carbon sinks; however, urban tree exchange of non-CO₂ greenhouse gases (GHGs), particularly methane (CH₄) and nitrous oxide (N₂O), remains largely unexplored. Urban trees may channel CH₄ and N₂O from anaerobic compacted soils, resulting in foliar emission. We provide empirical evidence that urban street tree foliage emits CH₄ and N₂O and demonstrate that soil biochar can mitigate these emissions. Using in situ gas-exchange measurements on Ulmus americana "Valley Forge" and Celtis occidentalis in Toronto, Canada, we compared foliar and soil CH₄ and N₂O fluxes across biochar treatments (0, 20, 40 t ha^-1) and seasons. Foliage of control trees emitted CH₄ (0.03-0.13 nmol·m^-2·s^-1) and N₂O (0.06-0.12 pmol·m^-2·s^-1). Biochar shifted foliar CH₄ exchange from weak emission to net uptake and reduced N₂O emissions by up to 70%; with similar patterns for soil fluxes. Structural equation modeling revealed strong coupling between soil and foliar fluxes, implicating xylem-mediated transport of dissolved gases. City-level scaling suggests that biochar amendment could convert Toronto's urban canopy from a minor CH₄ source (∼30 t yr^-1) to a net sink (∼60 t yr^-1) while halving N₂O emissions. These findings highlight an overlooked GHG pathway and identify biochar as a strategy for mitigating non-CO₂ emissions and accelerating urban decarbonization.