Aims This study addressed a microbial composition of East Siberian larch forest bed soil and focused on major nitrogen-fixing bacterial consortia showing the highest capacity to induce acetylene reduction. Methods and Results The ability of a soil microbial consortium to effectively reduce acetylene was investigated using gellan gum medium. In parallel, each microbial component in the medium was analysed by a 16S rRNA gene-targeting denaturing gradient gel electrophoresis (DGGE), together with an attempt to isolate pure culture of the culturable eubacteria. Six strains of the isolated bacteria were characterized as diazotrophs, and Burkholderia xenovorans was found to be one of the major nitrogen fixer in the bacterial community cultured from the BC2 horizon soil microbiota. Co-culturing of either a diazotrophic Pseudomonas sp. with a nondiazotrophic Luteibacter sp. or a diazotrophic Luteibacter sp. with a nondiazotrophic bacterium B.xenovorans resulted in a marked elevation in acetylene reduction over that observed in the pure culture of each diazotroph. Conclusions A promotion of acetylene reduction in mixed bacterial cultures, particularly when the cultures included Luteibacter, was clearly observed due to an increase in the bacterial cell population. Significance and Impact of the Study Our study, which showed that some unculturable bacteria including anaerobic Clostridium sp. that can provide major contribution to nitrogen fixation, may provide key information useful in identifying the 'missing link' in the nitrogen cycle in the cryogenic soil ecosystem of boreal forests.