CARBON ISOTOPIC COMPOSITION OF BACTERIAL METHANE IN A SOIL INCUBATION EXPERIMENT - CONTRIBUTIONS OF ACETATE AND CO2/H2

Anaerobic incubations of paddy soil collected from Konosu, Japan, were carried out for 10 weeks to clarify the general principles that govern the variation of carbon isotopic composition of bacterial methane from freshwater areas. The concentrations and isotopic compositions of CH4, CO2, and acetate produced in the incubated system were measured. The deltaC-13 value of biogenic CH4 was highly variable ranging from -60 to -33 parts per thousand, corresponding to changes in its formation pathways: acetate fermentation and CO2/H-2 reduction. Acetate accumulated only during the first week. After it became depleted, acetate production completely limited CH4 production. The deltaC-13 value of the methyl carbon of acetate, which was accumulated with addition of inhibitor for methanogenesis, ranged from -43 to -30 parts per thousand. The deltaC-13 value of methane from acetate was estimated to be -43 to -30 parts per thousand, after acetate depletion. The deltaC-13 value of CH4 from CO2/H-2 reduction was estimated to be -77 to -60 parts per thousand. An enrichment in C-13 of CO2 localized around the site of methanogenesis was suggested. CO2 produced in situ during methanogenesis (e.g., CO2 from carboxyl group of acetate) was possibly utilized as well. Using the above deltaC-13 values of two endmembers of CH4, the change of acetate contribution for CH4 production was calculated: less than 12% until week 1, 65 to 100% at weeks 1 to 3, 16 to 28% at weeks 3 to 5, and 35 to 40% after week 5. Variability of CH4 deltaC-13 resulted from the difference in contribution of each biological process. Also, the deltaC-13 value of CH4 was a useful indicator for assessing the contribution of each process to methane production in sulfate depleted freshwater areas.