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• In a natural boreal forest, we collected samples from downy birch lying deadwood. • The decomposition of deadwood was visually assessed using a five-class system. • We described the abundance and community composition of four microbial groups. • We evaluated the potential involvement of the deadwood in climate change. Deadwood plays an important role in the forest ecosystems, providing nutrients and habitat for a wide range of organisms, preventing soil erosion, and improving carbon storage. Microorganisms are primary agents in wood decomposition. The aim of the present research is to describe the changes in diversity, structure and abundance of microbial communities over downy birch lying deadwood decomposition in a boreal forest under natural conditions. This study also included investigations on the potential involvement of deadwood in climate change. The decomposition of deadwood was visually assessed using a five-class system. The microbial community diversity and composition were assessed with polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprinting. Real time PCR was used to evaluate the absolute and relative microbial abundance. The potential involvement of deadwood in climate change was assessed by in situ -measuring of the carbon dioxide (CO 2) emissions from downy birch lying deadwood and from soil. The results obtained indicate that deadwood represents a substrate whose physico-chemical and microbiological property change with time. Higher diversity of fungi, bacteria, and archaea were recorded in the decay class 5. Conversely, actinobacteria showed the lowest values of diversity in decay class 5. We observed a succession of dominant microbial taxa over the decomposition progress. Overall, the abundance of each microbial group increases with the advance of decomposition. Among the estimated physico-chemical properties, nitrogen content, that increased with decay, and pH were the most important candidate drivers of microbial community composition and abundance. CO 2 emissions were recorded higher in the decay class 5 and in soil of plots with the highest amount of lying deadwood. Bacteria dominated the microbial community and may play a more important role in the late stages of wood decomposition conversely to fungi and actinobacteria that are assumed to be primary involved in early stages of wood colonization. Archaea are shown to be an integral and dynamic component of decaying wood biota. The presence of large amounts of deadwood may affect greenhouse gas (GHG) emissions, especially in the event of an increase in temperatures that could reduce the carbon (C) sink capacity of the boreal forests. [ABSTRACT FROM AUTHOR]