Simple Summary
It is vital to understand how mixing different tree species can make forests healthier, but the specific ways this works underground are not fully clear. In this study, we compared the soil microbial community from pure larch forests with mixed forests where larch grows together with birch or spruce. We collected leaves and soil to understand how different tree combinations influence soil microorganisms. We found that mixing larch with birch increased leaf quality and soil nutrients, such as nitrogen and phosphorus, and supported a richer and more interactive community of soil bacteria that help transform nutrients. Mixing larch with spruce mainly increased carbon and phosphorus storage but supported fewer soil fungi, especially favoring fungi that form close partnerships with tree roots. Our results show that bacteria are influenced indirectly by changes in leaf traits and soil nutrients, while fungi are more directly shaped by the type of tree present. These findings help explain how tree diversity can be used to improve soil fertility, nutrient cycling, and the long-term sustainability of managed forests.
Abstract
The mixing of tree species influences soil microbial communities and ecosystem functioning, yet the underlying mechanisms remain inadequately understood. This study aimed to elucidate how different tree species mixtures regulate soil microbial community structure and ecological functions and to disentangle the relative roles of leaf functional traits, soil nutrients, and tree species identity in shaping bacterial and fungal assemblages. Leaf and soil samples were collected from 15 plots (20 m × 30 m) established in pure Larix principis-rupprechtii plantations (PL) and mixed Larix-Betula platyphylla (MLB) and Larix-Picea asperata (MLP) stands in the Saihanba Mechanical Forest Farm, China. Principal coordinate analysis, co-occurrence network analysis, and partial least squares path modeling were employed to assess changes in microbial community structure, network organization, and functional potential. Our results showed that the MLB stand mainly improved leaf nitrogen content (LNC), specific leaf area (SLA), and the concentrations of total nitrogen (STN) and phosphorus (STP) in the soil. The MLP stand preferentially promoted carbon and phosphorus accumulation in both leaves and soil. The MLB stand exhibited higher bacterial Chao1 richness, whereas the MLP stand showed reduced fungal diversity. The MLB supported a more complex bacterial network enriched with keystone taxa involved in nitrification and nitrate reduction, while MLP displayed a less complex bacterial network and a higher relative abundance of ectomycorrhizal fungi. Path analyses revealed that tree species mixtures shaped bacterial community structure largely via changes in leaf functional traits and soil conditions. Bacterial functional potential was primarily driven by improvements in soil nutrient availability. In contrast, fungal assemblage organization and functional expression were directly governed by the identity of the mixed tree species. These insights provide a foundation for improving soil fertility and nutrient cycling in managed forests via strategic species diversification.