Simulative analysis of vegetation on CH₄ emission from landfill cover soils: Combined effects of root-water uptake, root radial oxygen loss, and plant-mediated CH₄ transport

Vegetation on landfill cover soil plays a complicated role in methane (CH₄) transport, oxidation, and emissions. This study proposes an improved theoretical model that couples the effects of root-water uptake, radial oxygen loss (ROL), plant-mediated CH₄ transport, and microbial CH₄ oxidation for capturing water-gas-heat flow and CH₄ oxidation in landfill unsaturated cover soil. Parametric studies were conducted to investigate the combined effect of transpiration rate (Tp), ROL rate, root architecture, and root depth on CH₄ oxidation. The simulation results indicate that the effect of root-water uptake on CH₄ oxidation depends on the initial water content. When the water content was <15%, 10% more CH₄ was emitted to the atmosphere from vegetation-covered areas than from bare areas. More than 23% CH₄ was oxidized when the initial water content was 35% owing to improved aeration. Plants with a parabolic root architecture are more efficient in CH₄ oxidation than other root architectures under wet conditions. However, under dry conditions, roots with a parabolic architecture could release more CH₄ to the atmosphere. The CH₄ oxidation was significantly enhanced when the ROL rate (oxygen released per unit root surface area per unit time) was higher than 1.0 × 10⁻⁴ mol/m²·s. Plants are the dominant pathway for CH₄ release, and more than 31% of input CH₄ was emitted to the atmosphere from plants directly when the initial water content was 35%. This implies that high water-demand plants with parabolic root architectures in dry climate require frequently irrigation to improve the CH₄ oxidation efficiency. In wet climate, plant some species with high water-demand, high ROL capacity, and parabolic root architecture would be feasible strategies for landfill CH₄ mitigation.