Straw amendment is a common practice for improving the fertility of rice field soils, but it also enhances production of the greenhouse gas methane. To quantify carbon flux partitioning and priming effects due to straw amendment, we measured δ 13 C in CH 4 and CH 4 precursors produced in anoxic slurries of soil from Italy, China and Thailand after addition of straw from either rice (C3 plant) or maize plants (C4 plant), which have different δ 13 C signatures. The δ 13 C values of the CH 4 , acetate and CO 2 produced were similar when expressed as the difference to the δ 13 C value of the straw applied. These results indicated that the 13 C-isotopic fractionation involved in methanogenic decomposition was similar for rice straw and maize straw. However, measurement of CH 4 produced in soil without or with straw showed that isotopic fractionation during methanogenic degradation of straw was smaller than during degradation of soil organic matter. Isotopic fractionation during hydrogenotrophic methanogenesis, measured in the presence of methyl fluoride, with straw was also smaller than with soil organic matter. The results show that 13 C-isotopic analysis after application of rice straw and maize straw is a convenient approach for quantifying carbon flux partitioning during methanogenic degradation of straw and soil organic matter. In our experiments, straw degradation accounted for most of the CH 4 production and caused a negative priming effect on the methanogenic degradation of soil organic matter.