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

Abstract

Anaerobic bacterial degradation of landfill waste produces a globally significant source of the greenhouse gas methane. Stable isotopic measurements of methane [δI3C(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.