Deadwood decomposition is primarily attributed to wood-colonizing fungi and bacteria, driven mainly by intrinsic (e.g. tree species identity) rather than by extrinsic factors. A recent cross-ecosystem study, using gammasterilized wood blocks of different coniferous and deciduous tree species placed at 150 forest and 150 grassland sites, revealed that intrinsic factors most strongly influenced rate of decomposition. These results raised the question of whether the wood-colonizing microbial biodiversity follows similar assembly patterns. For this purpose, we used metabarcoding to analyse the fungal and bacterial communities colonizing the wood blocks. We discovered that the wood-colonizing communities were more strongly determined by extrinsic factors such as the ecosystem type and microclimate (air humidity, soil pH, soil moisture, soil temperature) than by intrinsic factors (tree species identity, wood pH, wood mass loss). Although overall these results seem to be more pronounced for fungi, both communities comprised highly specialized wood colonizers in both ecosystems. For instance, the fungal genus Mycena and the bacterial genus Granulicella were detected more frequently in forests, whereas Exophiala and Sphingomonas were more abundant in grasslands. Wood mass loss exhibited a stronger correlation with reduced fungal diversity, while bacterial richness displayed no association with mass loss, both within and across forest and grassland sites. However, the composition of both colonizers' communities was consistently linked to wood mass loss. Our study suggests that the environment selects distinct wood-colonizing communities that differ greatly in their decomposition efficiency; this result highlights the importance of crossecosystem analyses to assess ecological patterns.