Effects of biochar and wood pellets amendments added to landfill cover soil on microbial methane oxidation: A laboratory column study

• Mixed soil-biochar layers performed better than thin biochar layers in column tests.
• Thin biochar layers affected moisture distribution and infiltration in soil columns.
• A highly active methanotrophic population was observed within the biochar layer.
• Soils with uncharred wood pellets achieved similar oxidation rates as biochar.
• Oxidation rates were primarily governed by soil moisture and soil type.

Alternate landfill covers designed to enhance microbial methane (CH4) oxidation and reduce the negative impacts of landfill gas emissions on global climate have recently been proposed and investigated. In this study, the use of biochar as a soil amendment is examined in order to assess the feasibility and effectiveness for enhanced CH4 removal in landfill covers when incorporated under high compaction conditions and relatively low soil moisture. Four different cover configurations were tested in large soil columns for ∼510 days and potential CH4 oxidation rates were determined following long-term incubation in small batch assays. Cover designs tested include: a thin biochar layer at 15–18 cm; 2% mixed soil-biochar layer at 20–40 cm; 2% mixed soil-uncharred wood pellets at 20–40 cm; and soil obtained from intermediate cover at an active landfill site. The placement of a thin biochar layer in the cover significantly impacted moisture distribution and infiltration, which in turn affected CH4 oxidation potential with depth. An increase in CH4 removal rates was observed among all columns over the 500 day incubation period, with steady-state CH4 removal efficiencies ranging from ∼60 to 90% in the final stages of incubation (inlet load ∼80 g CH4 m−2 d−1). The thin biochar layer had the lowest average removal efficiency as a result of reduced moisture availability below the biochar layer. The addition of 2% biochar to soil yielded similar CH4 oxidation rates in terminal assays as the 2% uncharred wood pellet amendment. CH4 oxidation rates in terminal assays were positively correlated with soil moisture, which was affected by the materials’ water holding capacity. The high water holding capacity of biochar led to higher oxidation rates within the thin biochar layer, supporting the initial hypothesis that biochar may confer more favorable physical conditions for methanotrophy. Ultimate performance was apparently affected by soil type and CH4 exposure history, with the highest oxidation rates observed in the unamended field soil with higher initial methanotrophic activity.