A comparison of isotope fractionation of carbon and hydrogen from paddy field rice roots and soil bacterial enrichments during CO2/H2 methanogenesis

To better understand the isotope biogeochemistry of paddy field CH₄, we investigated carbon and hydrogen isotope fractionation during CO₂ reduction by a methanogenic community enriched from California paddy field soil and rice plants. Results from analyses of terminal restriction fragment length polymorphism (T-RFLP) and sequences of the archaeal small-subunit (SSU) rRNA-encoding genes (rDNA) showed a difference in methanogenic community structure between the soil (dominated by Methanobacteriaceae) and roots (dominated by Methanospirillaceae) which was essentially the same for sampling dates 15 and 99 days after flooding (DAF). CO₂/H₂ methanogenesis by these microbial communities produced CH₄ with different isotope ratios and fractionation factors (α factors). The carbon isotope α factors in an open system with a continuous supply of 0.5% H₂ were 1.050 ± 0.002 and 1.057 ± 0.001 for soil and root enrichment cultures at 15 DAF, and 1.052 ± 0.0.002 and 1.059 ± 0.002 for soil and root enrichment cultures at 99 DAF, respectively. These α factors are similar to, but distinct from values previously obtained from cultures of mesophilic methanogens and are larger than calculated values (1.045) for paddy soil. Fractionation of hydrogen isotopes was also studied in a closed system under 80% H₂. The difference in α factors between soil and root enrichment cultures remained clear. The hydrogen isotope fractionations between culture water and the product CH₄ were −327 ± 14‰ and −319 ± 18‰ for soil enrichments, and −389 ± 17‰ and −382 ± 21‰ for root enrichments at 15 DAF and 99 DAF, respectively.