Effect of Different Mixing Patterns on Carbon and Nitrogen Dynamics During the Decomposition of Deadwood in Subtropical Forest Ecosystems

As global forest areas decline and face increased risk from extreme events, optimizing forest types for long-term stability becomes crucial. However, empirical evidence for the effects of mixing methods on carbon and nitrogen dynamics in forest ecosystems remains limited. This study investigates five forest types in Southern China: the Tsuga longibracteata W.C.Cheng pure forests, the Tsuga longibracteata–hardwood mixed forests, the Tsuga longibracteata–Phyllostachys edulis (Carr.) J.Houz. mixed forests, the Tsuga longibracteata–Rhododendron simiarum Hance mixed forests, and the Tsuga longibracteata–hardwood–Rhododendron simiarum mixed forests (the tree species are all dominant community species). We examined one monoculture and four mixed forests, categorized into pure tree forests and tree–shrub mixed forests, and categorized by tree species richness levels of 1, 2, and 3. We measured carbon (C) and nitrogen (N) content, along with the C:N, of coarse woody debris (CWD) at various decay stages and in the adjacent topsoil (0–10 cm) to analyze decomposition rates and their effects on soil nutrients. Our results indicate that the C content and density of CWD differed significantly among forest types (p < 0.001). The Tsuga longibracteata–Phyllostachys edulis mixed forest exhibited the highest C and N content in CWD, but the lowest in adjacent topsoil, alongside the fastest decomposition rate. Soil C content and the C:N ratio showed highly significant differences among forest types (p < 0.001), and N content showed a significant difference (p < 0.05). Optimal outcomes occurred at a species richness level of 2, as excessive or insufficient species richness can diminish decomposition rates. The ecological benefits of tree–shrub mixed forests surpassed pure tree forests. Overall, these findings suggest that mixed forests do not always provide greater ecological advantages than pure forests, and that improper mixing can deplete soil.