Foliar Contributions to Methane and Nitrous Oxide Exchange in Urban Green Roof Systems

ABSTRACT Urban green roofs are increasingly deployed for stormwater and microclimate regulation, yet their contribution to non‐CO 2 greenhouse‐gas exchange remains poorly understood, particularly for vegetation. We quantified foliar methane (CH 4 ) and nitrous oxide (N 2 O) fluxes in a factorial field experiment involving two vegetation types and biochar amendments at the University of Toronto (Toronto, Canada). Daytime foliar fluxes of CH 4 , CO 2 , and H 2 O were measured with dynamic leaf chambers coupled to off‐axis integrated cavity output spectroscopy, and foliar N 2 O with optical feedback cavity‐enhanced absorption spectroscopy, across four seasonal campaigns (fall 2023 to fall 2024). Across plant types and seasons, biochar (20 t ha −1 ; 5.4% v/v) increased foliar CH 4 uptake approximately threefold relative to controls (−0.492 ± 0.046 vs. −0.167 ± 0.022 nmol·m −2 ·s −1 ; p < 0.001) and reduced foliar N 2 O emissions (1.11 ± 0.043 vs. 1.50 ± 0.039 pmol·m −2 ·s −1 ; p < 0.001), while foliar CO 2 flux was unaffected ( p = 0.29). Native vegetation exhibited stronger CH 4 uptake than stonecrop under both control and biochar treatments and higher CO 2 uptake and H 2 O flux overall. Soil–foliar coupling differed by gas: foliar N 2 O was strongly associated with soil N 2 O flux ( R 2 = 0.252–0.427; p < 0.001), whereas foliar CH 4 relationships with leaf H 2 O and soil CH 4 fluxes were weaker and vegetation dependent. A single nocturnal campaign in stonecrop showed no CO 2 uptake and minimal transpiration, with no treatment effects on nighttime CH 4 or N 2 O fluxes. City‐scale extrapolation suggests that biochar use could increase net foliar non‐CO 2 climate mitigation by ~270% relative to control conditions, driven by both enhanced CH 4 uptake and reduced N 2 O emissions.