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-CO 2 greenhouse gases (GHGs), particularly methane (CH 4 ) and nitrous oxide (N 2 O), remains largely unexplored. Urban trees may channel CH 4 and N 2 O from anaerobic compacted soils, resulting in foliar emission. We provide empirical evidence that urban street tree foliage emits CH 4 and N 2 O 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 CH 4 and N 2 O fluxes across biochar treatments (0, 20, 40 t ha –1 ) and seasons. Foliage of control trees emitted CH 4 (0.03–0.13 nmol·m –2 ·s –1 ) and N 2 O (0.06–0.12 pmol·m –2 ·s –1 ). Biochar shifted foliar CH 4 exchange from weak emission to net uptake and reduced N 2 O 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 CH 4 source (∼30 t yr –1 ) to a net sink (∼60 t yr –1 ) while halving N 2 O emissions. These findings highlight an overlooked GHG pathway and identify biochar as a strategy for mitigating non-CO 2 emissions and accelerating urban decarbonization.