Changes in soil bacterial communities and functional groups beneath coarse woody debris across a subalpine forest successional series

Although the role of coarse woody debris (CWD) in nurturing biodiversity has been well documented, there is little understanding of how CWD with varying diameters across the forest succession series affect the composition and diversity of the soil bacterial community. Therefore, this study investigated the soil bacterial community composition and biodiversity beneath debris with varying decay classes and diameters in six different successional forests, which constituted a subalpine forest series in the Wanglang National Nature Reserve in the eastern Qinghai–Tibet Plateau, China. The composition and functional groups of the soil bacterial communities in this study varied greatly with forest succession, decay classes and debris diameters. The alpha diversity of these communities decreased gradually along the succession gradients, whereas slight differences were observed among decay class and diameters. The soil bacterial communities beneath CWD were dominated by Proteobacteria and Acidobacteria under all soil conditions, and their relative abundance only varied greatly with the forest succession. The soil bacterial community structure varied significantly among different forest succession and diameters, but not decay classes. The functional groups were dominated by Chemoheterotrophy and aerobic chemoheterotrophy under all soil conditions. Nitrogen-fixing bacterial taxa and functional groups were enriched in the highly decayed classes. The soil nitrogen concentration and C:N ratio were negatively correlated with nitrogen-fixing bacterial communities. The soil temperature and CWD stock were significantly correlated with the diversity and composition of bacterial communities in soils beneath CWD. In conclusion, CWD with varying diameters and decay levels across different succession stages harbor some unique bacterial phyla and genera, and the association of nitrogen-fixing bacteria with highly decayed wood debris concluded that the existence of CWD in forest ecosystem could stimulate the ecosystem nitrogen fixation.