Methane emissions from forested closed landfill sites: Variations between tree species and landfill management practices

Trees in natural and managed environments can act as conduits for the transportation of methane (CH₄) from below ground to the atmosphere, bypassing oxidation in aerobic surface soils. Tree stem emissions from landfill sites exhibit large temporal and spatial variability in temperate environments and can account for approximately 40% of the total surface CH₄ flux. Emission variability was further investigated in this study by measuring CH₄ and CO₂ fluxes from landfill sites with different management strategies and varying tree species over a 7-month period. Stem and soil measurements were obtained using flux chambers and an off-axis integrated cavity output spectroscopy analyser. Analysis showed average stem and soil CH₄ emissions varied significantly (p < 0.01) between landfills with different management practices. On average, tree stem CH₄ fluxes from sites with no clay cap but gas extraction, clay cap and gas extraction, and no clay cap and no gas extraction were 1.4 ± 0.4 μg m⁻² h⁻¹, 47.2 ± 19.0 μg m⁻² h⁻¹, and 111.9 ± 165.1 μg m⁻² h⁻¹, respectively. There was no difference in stem CH₄ fluxes between species at each site, suggesting environmental conditions (waterlogging) and site age had a greater influence on both stem and soil fluxes. These results highlight the importance of management practices, and the resultant environmental conditions, in determining CH₄ emissions from historic landfill sites.