<p>Tree planting has the potential to increase carbon sequestration and is used as a common management strategy on former landfill sites to improve their visual appeal and manage issues such as leachates from decomposing organic matter. Tree stems mediate methane (CH<sub>4</sub>) emissions to the atmosphere from anaerobic soils, bypassing bacterial populations that would otherwise break down CH<sub>4</sub> before it is released to the atmosphere. This process has been observed in wetland forests but has yet to be measured in a landfill context. We examined whether trees emitted more CH<sub>4</sub> and carbon dioxide (CO<sub>2</sub>) on a closed UK landfill site relative to a more natural, comparable control site to determine the importance of this natural phenomenon in a managed environment. CH<sub>4</sub> and CO<sub>2</sub> fluxes from tree stem and soil surfaces were measured using flux chambers and an off-axis integrated cavity output spectroscopy analyser. Temporal and seasonal variations in greenhouse gas emissions from landfill tree stems were also investigated, as well as the impact of different landfill management techniques including site closure methods and tree species planted. Analyses showed that tree stem emissions from landfill were larger than from trees in the non-landfill control site. However, there was high variability in the greenhouse gas fluxes from trees on the landfill. Findings from this investigation suggest that conditions associated with landfill construction may increase CH<sub>4</sub> emissions from trees planted on their surface after closure of the site. Trees planted on former landfill sites may therefore result in additional CH<sub>4</sub> emissions to the atmosphere.</p>

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<abstract> <p>Trees growing in natural and managed environments have the capacity to act as conduits for the transport of greenhouse gases produced belowground to the atmosphere. Nitrous oxide (N₂O) emissions have been observed from tree stems in natural ecosystems but have not yet been measured in the context of forested former landfill sites. This research gap was addressed by an investigation quantifying stem and soil N₂O emissions from a closed UK landfill and a comparable natural site. Measurements were made by using flux chambers and gas chromatography over a four-month period. Analyses showed that the average N₂O stem fluxes from the landfill and non-landfill sites were 0.63 ± 0.06 μg m^–2 h^–1 and 0.26 ± 0.05 μg m^–2 h^–1, respectively. The former landfill site showed seasonal patterns in N₂O stem emissions and decreasing N₂O fluxes with increased stem sampling position above the forest floor. Tree stem emissions accounted for 1% of the total landfill N₂O surface flux, which is lower than the contribution of stem fluxes to the total surface flux in dry and flooded boreal forests.</p> </abstract>

Trees have morphological adaptations that allow methane (CH4) generated below ground to bypass oxidation in aerobic surface soils. This natural phenomenon however has not been measured in a landfill context where planted trees may alter the composition and magnitude of CH4 fluxes from the surface. To address this research gap, we measured tree stem and soil greenhouse gas (GHG) emissions (CH4 and CO2) from a closed UK landfill and comparable natural site, using an off-axis integrated cavity output spectroscopy analyser and flux chambers. Analyses showed average CH4 stem fluxes from the landfill and non-landfill sites were 31.8 ± 24.4 µg m–2 h–1 and –0.3 ± 0.2 µg m–2 h–1, respectively. The landfill site showed seasonal patterns in CH4 and CO2 stem emissions, but no significant patterns were observed in CH4 and CO2 fluxes at different stem heights or between tree species. Tree stem emissions accounted for 39% of the total CH4 surface flux (7% of the CO2); a previously unknown contribution that should be included in future carbon assessments.

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