Nitrous oxide (N2O) emissions from a forested closed landfill site

Many national governments, organisations and environmental groups have pledged to plant trees in an effort to increase carbon sequestration and mitigate climate change. Tree planting is commonly used as a restoration strategy for former landfill sites, and it is likely that many urban and urban-fringe areas, including closed landfills, will continue to be prioritised for tree planting in the coming years. Trees growing in natural and managed environments have the capacity to act as conduits for the transport of methane (CH₄) produced belowground to the atmosphere. This process has also been observed in natural ecosystems for nitrous oxide (N₂O) and we examined whether trees growing on closed landfills also mediate N₂O emissions to the atmosphere. We investigated whether trees on a closed UK landfill site emitted more N₂O than those on a comparable natural site. Measurements were made from stem and soil surfaces over a four-month period using flux chambers and Gas Chromatography. Results were then scaled up and the contributions of N₂O stem fluxes to the total surface fluxes in different environments were compared. Analyses showed that stem and soil N₂O fluxes from landfill were larger than from trees on the comparable non-landfill site. Tree stem N₂O emissions on the former landfill also showed seasonal patterns and decreased with higher sampling positions above ground level. Findings indicated that tree stem N₂O emissions accounted for less than 1% of the estimated total landfill surface flux, which was comparable to findings from a mesocosm study, but lower than estimates of the total N₂O ecosystem flux in dry and flooded boreal forests (8% and 18%, respectively). Overall, this investigation suggested that trees planted on closed landfill sites may result in additional N₂O emissions to the atmosphere, although the tree stem contribution to the total surface flux on the former landfill was a lower magnitude than that of fluxes previously reported from natural forested ecosystems.