Functional Traits of Trees and Shrubs Drive Soil Carbon Storage in Rocky Desertification Areas via Direct and Fungal-Mediated Pathways

Vegetation restoration is an effective measure to improve soil carbon storage in rocky desertification areas. However, the underlying mechanisms driving differences in soil carbon storage among different vegetation types remain unclear. In this study, we selected four typical vegetation types (monospecific broadleaf plantation, mixed conifer–broadleaf plantation, monospecific coniferous plantation, and natural shrubland) from the comprehensive control zone for moderately rocky desertification of Xuyong County in the Chishui River Basin. We investigated the effects of vegetation patterns on soil carbon storage through tree layer functional traits, tree diversity, shrub layer functional traits, shrub diversity, and soil fungal/bacterial diversity. The results showed that soil carbon storage was highest in monospecific broadleaf plantations, followed by mixed conifer–broadleaf plantations, then natural shrubland, and it was lowest in monospecific coniferous plantations. Significant differences were mainly observed in the 0–10 cm and 10–20 cm soil layers, while no significant difference was found in the 20–30 cm layer. Tree layer community-weighted mean of leaf nitrogen content, soil fungal Shannon–Wiener index, and shrub layer community-weighted mean of specific leaf area were the core positive drivers of soil carbon differentiation. Among these, tree layer community-weighted mean of leaf nitrogen content was the most important factor, whereas the regulatory effect of shrub layer community-weighted mean of specific leaf area intensified with increasing soil depth. Furthermore, tree layer community-weighted mean of leaf nitrogen content and shrub layer community-weighted mean of specific leaf area not only directly promoted soil carbon accumulation but also indirectly promoted it by enhancing soil fungal diversity. In contrast, the effects of tree and shrub layer diversity and soil bacterial diversity were negligible. This study demonstrates that increasing soil carbon storage in rocky desertification ecosystems depends on the direct effect of soil fungal diversity, as well as the direct effects of tree and shrub layer functional traits and their indirect effects via regulating soil fungal diversity. We recommend that in moderately rocky desertified areas, priority should be given to tree species with high leaf nitrogen content; on sites unsuitable for afforestation, promoting natural shrublands with high specific leaf area can effectively enhance carbon sequestration capacity.