Assessment of farm soil, biochar, compost and weathered pine mulch to mitigate methane emissions

Previous studies have demonstrated the effective utility of volcanic pumice soil to mitigate both high and low levels of methane (CH₄) emissions through the activity of both γ-proteobacterial (type I) and α-proteobacterial (type II) aerobic methanotrophs. However, the limited availability of volcanic pumice soil necessitates the assessment of other farm soils and potentially suitable, economical and widely available biofilter materials. The potential biofilter materials, viz. farm soil (isolated from a dairy farm effluent pond bank area), pine biochar, garden waste compost and weathered pine bark mulch, were inoculated with a small amount of volcanic pumice soil. Simultaneously, a similar set-up of potential biofilter materials without inoculum was studied to understand the effect of the inoculum on the ability of these materials to oxidise CH₄ and their effect on methanotroph growth and activity. These materials were incubated at 25 °C with periodic feeding of CH₄, and flasks were aerated with air (O₂) to support methanotroph growth and activity by maintaining aerobic conditions. The efficiency of CH₄ removal was monitored over 6 months. All materials supported the growth and activity of methanotrophs. However, the efficiency of CH₄ removal by all the materials tested fluctuated between no or low removal (0-40 %) and high removal phases (>90 %), indicating biological disturbances rather than physico-chemical changes. Among all the treatments, CH₄ removal was consistently high (>80 %) in the inoculated farm soil and inoculated biochar, and these were more resilient to changes in the methanotroph community. The CH₄ removal from inoculated farm soil and inoculated biochar was further enhanced (up to 99 %) by the addition of a nutrient solution. Our results showed that (i) farm soil and biochar can be used as a biofilter material by inoculating with an active methanotroph community, (ii) an abundant population of α-proteobacterial methanotrophs is essential for effective and stable CH₄ removal and (iii) addition of nutrients enhances the growth and activity of methanotrophs in the biofilter materials. Further studies are underway to assess the feasibility of these materials at small plot and field scales.