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 meas urements of methane [delta(13)C(CH4) and delta D(CH4)] can often finge rprint different sources of methane (natural vs, anrhropogenic) and he lp identify the bacterial processes involved in methane production, La ndfill microbial communities are complex and diverse, and hence so too is the biogeochemistry of methane formation. To investigate the influ ence of (I) the methane formation pathway (acetoclastic methanogenesis and CO2 reduction), and (2) delta D of water on the stable isotopic c omposition of landfill methane Two model butyrate-degrading landfill s ystems were established. The systems were inoculated with domestic ref use from a landfill and incubated in the laboratory for 92 days. Both systems were identical except delta D of water initially added to syst em 2 was 118% heavier than system 1. Between days 39 and 72 the system s were resupplemented with butyrate. Production of CH4 and CO2 and cha nges in volatile fatty acid concentration confirmed that active methan ogenic populations had been established CH4 became C-13 enriched in bo th incubations with time. Interpreting changes in acetate, butyrate, a nd propionate concentration during incubation is complicated but these observations and other information suggest that the dominant methanog enic substrate changed from CO2/H-2 to acetate as the: experiment prog ressed. This is also consistent with the observed C-13 enrichment of C H4, as C-13 discrimination during methane production from acetate is l ess than from CO2. In contrast, delta D(CH4) remained relatively const ant, suggesting that this measurement may not provide a reliable basis for distinguishing between CH4 from CO2 reduction and acetoclastic me thanogenesis, as has previously been suggested.