The carbon and hydrogen stable isotope composition of bacteriogenic methane: A laboratory study using a landfill inoculum

Anaerobic bacterial degradation of landfill waste produces a globally significant source of the greenhouse gas methane. Stable isotopic measurements of methane [δ^I3C(CH₄) and δD(CH₄)] can often fingerprint different sources of methane (natural vs. anthro‐pogenic) and help identify the bacterial processes involved in methane production. Landfill microbial communities are complex and diverse, and hence so too is the biogeochem‐istry of methane formation. To investigate the influence of (l) the methane formation pathway (acetoclastic methanogenesis and CO₂ reduction), and (2) SD of water on the stable isotopic composition of landfill methane, two model butyrate‐degrading landfill systems were established. The systems were inoculated with domestic refuse from a landfill and incubated in the laboratory for 92 days. Both systems were identical except δD of water initially added to system 2 was 118% heavier than system 1. Between days 39 and 72 the systems were resupplemented with butyrate. Production of CH₄ and CO₂ and changes in volatile fatty acid concentration confirmed that active methanogenic populations had been established. CH₄ became ¹³C enriched in both incubations with time. Interpreting changes in acetate, butyrate, and propionate concentration during incubation is complicated, but these observations and other information suggest that the dominant methanogenic substrate changed front CO₂/H₂ to acetate as the experiment progressed. This is also consistent with the observed ¹³C enrichment of CH₄, as ¹³C discrimination during methane production from acetate is less than from CO₂. In contrast, δD(CH₄) remained relatively constant, suggesting that this measurement may not provide a reliable basis for distinguishing between CH₄ from CO₂ reduction and acetoclastic methanogenesis, as has previously been suggested.