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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.

Abstract Background As a structurally and functionally important component in forest ecosystems, plant debris plays a crucial role in the global carbon cycle. Although it is well known that plant debris stocks vary greatly with tree species composition, forest type, forest origin, and stand age, simultaneous investigation on the changes in woody and non-woody debris biomass and their carbon stock with forest succession has not been reported. Therefore, woody and non-woody debris and carbon stocks were investigated across a subalpine forest successional gradient in Wanglang National Nature Reserve on the eastern Qinghai-Tibet Plateau. Results Plant debris ranged from 25.19 to 82.89 Mg∙ha− 1 and showed a global increasing tendency across the subalpine forest successional series except for decreasing at the S4 successional stage. Accordingly, the ratios of woody to non-woody debris stocks ranged from 26.58 to 208.89, and the highest and lowest ratios of woody to non-woody debris stocks were respectively observed in mid-successional coniferous forest and shrub forest, implying that woody debris dominates the plant debris. In particular, the ratios of coarse to fine woody debris stocks varied greatly with the successional stage, and the highest and lowest ratios were found in later and earlier successional subalpine forests, respectively. Furthermore, the woody debris stock varied greatly with diameter size, and larger diameter woody debris dominated the plant debris. Correspondingly, the carbon stock of plant debris ranged from 10.30 to 38.87 Mg∙ha− 1 across the successional series, and the highest and lowest values were observed in the mid-coniferous stage and shrub forest stage, respectively. Most importantly, the carbon stored in coarse woody debris in later successional forests was four times higher than in earlier successional forests. Conclusions The stock and role of woody debris, particularly coarse woody debris, varied greatly with the forest successional stage and dominated the carbon cycle in the subalpine forest ecosystem. Thus, preserving coarse woody debris is a critical strategy for sustainable forest management.