CH4/CO2 ratios indicate highly efficient methane oxidation by a pumice landfill cover-soil

Landfills that generate too little biogas for economic energy recovery can potentially offset methane (CH4) emissions through biological oxidation by methanotrophic bacteria in cover soils. This study reports on the CH4 oxidation efficiency of a 10-year old landfill cap comprising a volcanic pumice soil. Surface CH4 and CO2 fluxes were measured using field chambers during three sampling intervals over winter and summer. Methane fluxes were temporally and spatially variable (−0.36 to 3044 mg CH4 m−2 h−1); but were at least 15 times lower than typical literature CH4 fluxes reported for older landfills in 45 of the 46 chambers tested. Exposure of soil from this landfill cover to variable CH4 fluxes in laboratory microcosms revealed a very strong correlation between CH4 oxidation efficiency and CH4/CO2 ratios, confirming the utility of this relationship for approximating CH4 oxidation efficiency. CH4/CO2 ratios were applied to gas concentrations from the surface flux chambers and indicated a mean CH4 oxidation efficiency of 72%. To examine CH4 oxidation with soil depth, we collected 10 soil depth profiles at random locations across the landfill. Seven profiles exhibited CH4 removal rates of 70–100% at depths <60 cm, supporting the high oxidation rates observed in the chambers. Based on a conservative 70% CH4 oxidation efficiency occurring at the site, this cover soil is clearly offsetting far greater CH4 quantities than the 10% default value currently adopted by the IPCC.

► CH4 fluxes were measured on a 10-year old volcanic soil landfill cap in New Zealand.
► CH4 fluxes were much lower than typical emissions from landfill caps of similar age.
► CH4/CO2 ratios in gas samples from the landfill soil showed strong correlation with CH4 oxidation (R2 = > 0.99).
► This correlation was used to show high (70%) CH4 oxidation by the soil cap.
► The results challenge the IPCC assumption that oxidation rates in landfill cover soils are <10%.