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 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 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. Whereas butyrate and CH₄ were ¹³C-depleted relative to propionate, acetate and CO₂ were ¹³C-enriched. The isotopic enrichment factors (ε_prop) 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 ε_prop were -3.5 ‰. It is concluded that degradation of organic carbon via propionate to acetate and CO₂ involves only 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 of the microbial community composition published previously for the same soils.