Chinese fir- Phoebe mixture increases microbial diversity and functional potential by accelerating SOM decomposition and increasing nutrient availability

Near-natural silviculture practices have gained prominence as sustainable forestry strategies for optimizing stand site conditions and enhancing forest productivity. The performance of these ecosystem processes is closely related to soil microorganisms. However, there is still a lack of systematic characterization of how microorganisms respond to such silvicultural interventions, and this critical knowledge gap limits our ability to predict long-term forest ecosystem trajectories. Moreover, the specific functions of microbial communities in mixed plantations remain largely unknown. Here, we analyzed the compositional and functional profiles of bacterial and fungal communities across a gradient of stand types, including nine near-natural mixed Cunninghamia lanceolata and Phoebe bournei plantations (a 3 × 3 factorial design with three densities per species) and one pure C. lanceolata stand. The results showed that mixed planting enhanced bacterial and fungal diversity and altered their community structures by accelerating soil organic matter decomposition and total nitrogen mineralization, while increasing soil pH, available phosphorus, and available potassium. Notably, mixed planting increased the relative abundance of bacteria involved in carbon and nitrogen metabolism, along with saprophytic and symbiotic fungi, while reducing that of pathogenic fungi. Moreover, microbial community variation was stronger among mixed planting treatments than between soil depths, with reduced C. lanceolata density exerting a greater influence than increased P. bournei density. These findings underscore that tree diversity and planting density collectively determine soil microbial assembly. Consequently, this study provides empirical evidence to predict the impact of near-natural silviculture on microbial communities and offers insights for optimizing the management of C. lanceolata plantations.

IMPORTANCE

Driven by western forest management concepts of “Nature-Approximating Forestry,” near-natural afforestation has emerged as a key strategy for improving forest quality and productivity, especially in mixed conifer-broadleaf afforestation. Soil microorganisms constitute a vital component of forest ecosystems, serving as a primary driver of ecosystem functioning. Our research demonstrated that mixed conifer-broadleaf forests (Chinese fir- Phoebe ) can increase soil bacterial and fungal diversity and functional potential in pure C. lanceolata plantations by accelerating organic matter decomposition and increasing soil pH, available phosphorus, and available potassium levels.