Above- and below-ground methane fluxes and methanotrophic activity in a landfill-cover soil

Abstract

Landfills are a major anthropogenic source of the greenhouse gas methane (CH4). However, much of the CH4 produced during the anaerobic degradation of organic waste is consumed by methanotrophic microorganisms during passage through the landfill-cover soil. On a section of a closed landfill near Liestal, Switzerland, we performed experiments to compare CH4 fluxes obtained by different methods at or above the cover-soil surface with below-ground fluxes, and to link methanotrophic activity to estimates of CH4 ingress (loading) from the waste body at selected locations. Fluxes of CH4 into or out of the cover soil were quantified by eddy-covariance and static flux-chamber measurements. In addition, CH4 concentrations at the soil surface were monitored using a field-portable FID detector. Near-surface CH4 fluxes and CH4 loading were estimated from soil–gas concentration profiles in conjunction with radon measurements, and gas push–pull tests (GPPTs) were performed to quantify rates of microbial CH4 oxidation. Eddy-covariance measurements yielded by far the largest and probably most representative estimates of overall CH4 emissions from the test section (daily mean up to ∼91,500μmolm−2d−1), whereas flux-chamber measurements and CH4 concentration profiles indicated that at the majority of locations the cover soil was a net sink for atmospheric CH4 (uptake up to −380μmolm−2d−1) during the experimental period. Methane concentration profiles also indicated strong variability in CH4 loading over short distances in the cover soil, while potential methanotrophic activity derived from GPPTs was high (vmax∼13mmolL−1(soil air)h−1) at a location with substantial CH4 loading. Our results provide a basis to assess spatial and temporal variability of CH4 dynamics in the complex terrain of a landfill-cover soil.