Fractionation of stable carbon isotopes during microbial propionate consumption in anoxic rice paddy soils

Abstract. Propionate is an important intermediate during the breakdown of organic matter in anoxic flooded paddy soils. Since there are only a few experiments on carbon isotope fractionation and the magnitude of the isotopic enrichment factors (ε) involved, we measured propionate conversion to acetate, CH4 and CO2 in anoxic paddy soils. Propionate consumption was measured using samples of paddy soil from Vercelli (Italy) and the International Rice Research Institute (IRRI, the Philippines) suspended in a phosphate buffer (pH 7.0) both in the absence and presence of sulfate (gypsum) and of methyl fluoride (CH3F), an inhibitor of aceticlastic methanogenesis. Under methanogenic conditions, propionate was eventually degraded to CH4, with acetate being a transient intermediate. Butyrate was also a minor intermediate. Methane was mainly produced by aceticlastic methanogenesis. Propionate consumption was inhibited by CH3F. Butyrate and CH4 were 13C-depleted relative to propionate, whereas acetate and CO2 were 13C-enriched. The isotopic enrichment factors (εprop) of propionate consumption, determined by Mariotti plots, were in a range of −8 ‰ to −3.5 ‰. Under sulfidogenic conditions, acetate was also transiently accumulated, but CH4 production was negligible. Application of CH3F hardly affected propionate degradation and acetate accumulation. The initially produced CO2 was 13C-depleted, whereas the acetate was 13C-enriched. The values of εprop were −3.5 ‰. It is concluded that the degradation of organic carbon via propionate to acetate and CO2 involves only a little isotope fractionation. The results further indicate a major contribution of Syntrophobacter-type propionate fermentation under sulfidogenic conditions and Smithella-type propionate fermentation under methanogenic conditions. This interpretation is consistent with data regarding the microbial community composition published previously for the same soils.