Hydrological Controls of Methane Uptake in Biochar‐Amended Native Vegetation Green Roof Substrates

Biochar is increasingly recognized for its potential to sequester carbon and mitigate greenhouse gas emissions. However, its capacity to enhance methane (CH 4 ) uptake and alter coupled hydrological–biogeochemical processes in urban green roofs remains poorly understood. We quantified CH 4 , CO 2 , and H 2 O vapor fluxes over 2021–2024 in native green roof modules, comparing biochar‐amended and non‐amended organic‐rich substrates. Across all measurement periods, green roofs functioned as consistent but weak CH 4 sinks, with biochar significantly enhancing the uptake. At peak response (Spring 2024), biochar‐amended substrates exhibited more than double the CH 4 uptake of controls (−1.72 ± 0.20 vs. −0.72 ± 0.11 nmol·m −2 ·s −1 ). In contrast, CO 2 fluxes were primarily driven by seasonal variability and showed no consistent treatment effect, whereas biochar increased H 2 O vapor flux during peak growing periods. Vegetation cover increased substantially throughout the study, reflecting both plant establishment and seasonal phenological dynamics, but did not exert an independent influence on greenhouse gas fluxes after seasonal effects were accounted for. Structural equation modelling revealed that biochar enhanced CH 4 uptake in part through its effects on substrate moisture and H 2 O flux, highlighting the central role of hydrological processes in regulating CH 4 uptake in native green roof substrates. Seasonality emerged as a dominant driver of substrate temperature, vegetation cover, moisture, and gas exchange, while direct temperature effects on CH 4 and CO 2 fluxes were weak in native vegetated green roof substrates. Our results demonstrate that biochar‐amended native green roofs can function as persistent atmospheric CH 4 sinks while maintaining hydrological performance, identifying hydrological regulation as a key mechanism enhancing CH 4 uptake and biochar as a multifunctional amendment for strengthening the climate mitigation potential of urban green infrastructure.