Drivers of Spatial Variation in Methane Emission and The Role of Vegetation in Methane Oxidation from Capped Landfills in Southern Ontario

Abstract

Landfills are a major source of methane (CH₄), accounting for 24% of Canada’s total anthropogenic CH₄ emissions. This study investigates the environmental and meteorological factors driving temporal and spatial CH₄ flux variation and the role of vegetation in controlling CH₄ oxidation in landfill cover soils. This study was conducted across two capped landfill sections: the Original Landfill Area (OLA) (an older site without active gas collection system) and the Northern Expansion Area (NEA) (a relatively younger capped site with active gas collection) at a landfill in Southern Ontario. Both field data monitoring across the seasons and controlled incubation were carried out to critically assess CH₄ oxidation behavior in landfill cover soil by integrating vegetation treatments, environmental controls, and site specific controls. Field study revealed that the highest rate of CH₄ oxidation was observed with dry (<10%) and warmer (>25 °C) soil conditions between late summer and early fall, coinciding with reduced rainfall and higher air temperatures. CH₄ uptake was suppressed in wetter and colder conditions in winter and spring, correlating to cold and wetter soil conditions. Meteorological factors such as precipitation and air temperature significantly affected flux variations while wind speed and atmospheric pressure had a negligible impact. Presence of vegetation in landfill covers improved soil characteristics like bulk density, porosity, organic matter content, nitrate, phosphate, potassium and ammonium concentration, though it did not consistently alter the CH₄ fluxes. CH₄ fluxes were higher from the OLA’s cover soils with slightly elevated fluxes in vegetated soil, while comparably lower fluxes were observed from NEA’s cover soils, possibly due to its active gas collection system. Occasional CH₄ hotspots were observed, markedly at NEA even with active gas collection, suggesting localized structural failures in landfill cap or potential subsurface gas leaks. Quantified CH₄ oxidation rates on a per gram of cover soil basis indicated higher oxidation capacity in NEA, likely influenced by hotspots, providing favorable substrate supply to sustain efficient microbial CH₄ oxidation in the landfill cover.

The incubation results demonstrated that optimal CH₄ oxidation occurred at 20–40% soil moisture and 25 °C under vegetated and non-vegetated soil covers while CH₄ consumption markedly declined at cool and hot thermal conditions (5 °C and 35 °C), and in soil with higher moisture content (60–80%), reflecting microbial sensitivity to O₂ restrictions and thermal stress. There was no significant vegetation effect, even under controlled incubation conditions, suggesting its influence may be indirect or more context specific (determined by soil structure and microclimate differences).

These findings emphasize that vegetation alone cannot stand as a mitigation strategy, but it formed favorable soil characteristics for CH₄ oxidation. A complex interplay between cap maturity, soil physical and chemical conditions, gas transport, CH₄ and O₂ availability, microbial community activity, and weather conditions determined CH₄ dynamics in capped landfill covers across seasons. Adaptive landfill covers should be designed for proper moisture control through drainage to control excessive water retention, incorporating soil amendments rich with organic matter to enhance soil properties for gas diffusivity and microbial habitat to maximize CH₄ oxidation capacity of cover soil. Thus, passive CH₄ mitigation approaches should be combined with engineered infrastructure facilities such as gas extraction systems for better CH₄ mitigation in capped landfill covers. Results from this study will also contribute to national greenhouse gas (GHG) inventories for reporting and accounting and to the development of control strategies or policies towards Canada’s CH₄ emission reduction targets.