Do compost-based landfill biocover systems designed for methane oxidation emit nitrous oxide in significant amounts?

Landfills constitute a significant source of methane (CH₄), thereby contributing to climate change. CH₄ emissions from old and smaller landfills can be mitigated by compost-based biocover systems designed for optimal microbial CH₄ oxidation. It is well-known that the strong greenhouse gas nitrous oxide (N₂O) is generated during the composting process, which potentially could continue after incorporating compost into the biocover system. Field studies were performed at three full-scale biocover systems established at Danish landfills and included surface screenings, surface flux measurements and gas composition analysis. To assess if N₂O generated in the biofilter-compost material would hamper the climate benefit from CH₄ oxidised in a biofilter, CH₄ removed was compared to N₂O generated, with both calculated in CO₂-eq. Two assessments were performed. The first considered individual measurement locations on the biocover, whereas the second considered the overall performance of the biocover. By comparing CH₄ oxidation rates to the emitted N₂O, both approaches showed that there is no risk that N₂O emission will negatively affect the CH₄ mitigation efficiency of compost-based biocover systems established at landfills. The ratio of N₂O emitted to CH₄ oxidized (both in unit kg CO₂-eq per day) was less than 2.3% for both approaches, and in most cases below 1%.