Impact of preferential methane flow through soil on microbial community composition

• Methane emission hotspots often show a purple discoloration at the soil surface.
• Soil characterized by preferential gas flow harbors a distinct microbial community.
• High methane fluxes are associated with presence of extrapolymeric substances.
• Microbial diversity and evenness in soil decline with increasing influence of landfill gas.
• At emission hotspots, type II methanotrophs dominate over type I.

The anaerobic microbial degradation of waste organic fractions in landfills constitutes one of the principal anthropogenic methane sources. Microbial oxidation of methane in optimized landfill covers or biofilters has been listed as key mitigation technology for the reduction of methane fluxes from landfills that are no longer suitable for energy recovery or flaring. Therefore, it is vital to understand what influences distribution of methane oxidizers and their activity in landfill soils. Here we describe the impact of gas fluxes through preferential pathways (hotspots) in the cover soil of a municipal solid waste landfill in north-western Germany on the soil properties and the microbial communities that colonize the upper soil crust in these environments. Two sites with high surface methane concentrations (>14,000 ppm), two sites with moderate surface methane concentrations (∼400 ppm) and two sites without measurable methane emissions at the surface were investigated. It was found that elevated average soil methane concentrations coincided with increased levels of TOC and TN and the TOC/TN ratio in the topsoil. The increase of the latter posits a change in the composition of the organic matter towards increasing levels of nitrogen-poor components as for example EPS, which were observed in the samples with higher TOC/TN ratios. Elevated average soil methane concentrations were also accompanied by a decrease in the overall bacterial diversity. The community at these sites were dominated by a few lineages such as methanotrophs, particularly of type II, Burkholderiales, Rhodospirillales and Bradyrhizobiaceae. This dominance may have contributed to the purple discoloration at the soil surface at the sites with the highest surface methane concentrations.