HYDROGEN ISOTOPIC COMPOSITION OF BACTERIAL METHANE - CO2/H-2 REDUCTION AND ACETATE FERMENTATION

In order to discern the delta D-water-delta D-CH4 relationship between CO2/H-2 reduction and acetate fermentation in freshwater environments where CH4 is produced from both CO2/H-2 and acetate, incubation experiments of a paddy soil were made, using water with four different delta D values. From the observed delta D-water-delta D-CH4 relationships and a fractional contribution of acetate estimated from delta(13)C of CH4, delta D-water-delta D-CH4 relationships were obtained for CO2/H-2 reduction and for acetate fermentation, respectively. The relationships were expressed as follows; delta D-CH4(CO2) = (0.683 +/- 0.020)delta D-water - (317 +/- 20) delta D-CH4(Ac) = (0.437 +/- 0.045)delta D-water - (302 +/- 15). The obtained difference in delta D between freshwater and CH4 from CO2/H-2 (317 parts per thousand) was much larger than that reported in marine sediments (180 parts per thousand). Accordingly, no significant difference in delta D of CH4 between CO2/H-2 reduction and acetate fermentation is expected when the delta D of water is close to SMOW, and delta D of CH4 from CO2/H-2 is lower than that from acetate when the delta D of water is low. The large fractionation obtained for CO2/H-2 reduction may be caused by a high H-2 concentration in the freshwater soil which is rich in easily decomposable organic matters. The estimated slope (0.437) of the delta D-water-delta D-CH4 relationship for acetate fermentation indicates that some of the hydrogen atoms in the methyl group of acetate exchanged with water during the acetate production. On the basis of the experimental results, the range of delta D-delta(13)C for CH4 from CO2/H-2 reduction and that for acetate fermentation were calculated for the freshwater environments. Although only a negative correlation between delta D and delta(13)C of CH4 has been expected, our results indicate the possibility of a positive correlation between them in freshwater areas.