THE IMPACT OF COAL-BED METHANE SEEPAGE ON A SOIL MICROBIAL COMMUNITY

Methane generated in the subsurface is a major source of atmospheric methane, but its release is mitigated by methane-oxidizing bacteria (methanotrophs) residing in soils. Coal-bed methane (CBM) seeps from a subcrop of the Fruitland Formation near the Pine River in Southwest Colorado. The impact that this seepage has on the overlying soil microbial community was studied at incremental depths to 1.8 meters. Methane concentrations in soil gas were as high as 93% by volume in deeper soils and decreased non-linearly with decreasing depth indicating consumption by methanotrophs. Structural profiles of microbial cell membrane phospholipid fatty acids (PLFAs) extracted from soils were used to estimate the relative abundance of methanotrophs and non-methanotrophs (presumably heterotrophs). Concentrations of the biomarker PLFAs for type I (16:1ω8cis) and type II (18:1ω8cis) aerobic methanotrophs were as high as 13 and 18 nmoles (g dry soil)^-1, respectively, confirming the role of methanotrophs in the oxidation of CBM. Depth profiles of methanotroph PLFA concentrations varied over different sampling dates indicating dynamic populations. The PLFA-based cell numbers of heterotrophs in surface soils (0 to 10 cm) were similar at both CBM-impacted and control sites, but heterotrophs decreased much more rapidly with depth at the CBM site. This is likely due to low O₂ concentrations in the deeper CBM soils that resulted from high rates of methane-oxidation. Natural abundance ¹³C and ¹⁴C contents of PLFAs were measured to estimate the relative assimilation of both CBM- and plant-derived carbon by soil microbes. δ¹³C_PLFA values of the CBM soils (-25.1 to -66.9%) were substantially more negative than those for the control soil (-14.5 to -32.5%) indicating that CBM is an important carbon source for both methanotrophs and heterotrophs. Δ¹⁴C_PLFA values for the CBM soils (-351 to -936%) confirmed the ¹³C results and were used to estimate the contribution of CBM as an indirect carbon source for heterotrophs. CBM contributed between 32 and 91% of the carbon assimilated by heterotrophs. Results from this study indicate that CBM seepage and resulting methanotrophic activity supply an additional carbon source to the heterotrophic community but ultimately limit its size by substantially reducing oxygen concentrations.