Earthworm cast as a promising filter bed material and its methanotrophic contribution to methane removal

Abstract

The use of biocovers is a promising strategy toward mitigating CH 4 emission from smaller and/or older landfills. In this study, a filter bed material consisting of a mixture of earthworm cast and rice paddy soil in a biocover was evaluated. Although the CH 4 oxidation rate of the enriched paddy soil was 4.9 μg g-dry soil −1 h −1, it was enhanced to 25.1 μg g-dry soil −1 h −1 by adding an earthworm cast with a 3:7 ratio of earthworm cast:soil (wet weight). CO 2 was found as the final oxidation product of CH 4, and the mole ratio of CO 2 production to CH 4 consumption was 0.27. At a moisture content range of 15–40% and a temperature range of 20–40 °C, the CH 4 oxidation rates of the enriched mixture were more than 57% of the maximum rate obtained at 25% moisture content and 25 °C. By denaturing gradient gel electrophoresis analysis employing primers for the universal bacterial 16S rRNA gene, and terminal-restriction fragment length polymorphism analysis using primers for the pmoA gene, the bacterial and methanotrophic communities in the enriched mixture were mainly originate from paddy soil and earthworm cast, respectively. Both type I (mainly Methylocaldum) and type II methanotrophs (mainly Methylocystis) played important roles in CH 4 oxidation in the enriched mixture.