In vitro methane removal by volcanic pumice soil biofilter columns over one year.

Soil methane (CH4) biofilters, containing CH4-oxidizing bacteria (methanotrophs), are a promising technology for mitigating greenhouse gas emissions. However, little is known about long-term biofilter performance. In this study, volcanic pumice topsoils (0-10 cm) and subsoils (10-50 cm) were tested for their ability to oxidize a range of CH4 fluxes over 1 yr. The soils were sampled from an 8-yr-old and a 2-yr-old grassed landfill cover and from a nearby undisturbed pasture away from the influence of CH4 generated by the decomposing refuse. Methane was passed through the soils in laboratory chambers with fluxes ranging from 0.5 g to 24 g CH4 m(-3) h(-1). All topsoils efficiently oxidized CH4. The undisturbed pasture topsoil exhibited the highest removal efficiency (24 g CH4 m(-3) h(-1)), indicating rapid activation of the methanotroph population to the high CH4 fluxes. The subsoils were less efficient at oxidizing CH4 than the topsoils, achieving a maximum rate oxidation rate of 7 g CH4 m(-3) h(-1). The topsoils exhibited higher porosities; moisture contents; surface areas; and total C, N, and available-P concentrations than the subsoils, suggesting that these characteristics strongly influence growth and activity of the CH4 oxidizing bacteria. Soil pH values and available-P levels gradually declined during the trial, indicating a need to monitor chemical parameters closely so that adjustments can be made when necessary. However, other key soil physicochemical parameters (moisture, total C, total N) increased over the course of the trial. This study showed that the selected topsoils were capable of continually sustaining high CH4 removal rates over 1 yr, which is encouraging for the development of biofilters as a low-maintenance greenhouse gas mitigation technology.