Purpose <p><i>Cunninghamia lanceolata</i> (Chinese fir), a prominent fast-growing conifer in southern China, faces increasing concerns regarding soil degradation in monocultures. Introducing native broad-leaved trees like <i>Phoebe bournei</i> through underplanting is a key close-to-nature management strategy, yet the short-term impacts of underplanting density on soil multifunctionality (SMF) have yet to be fully elucidated. This study examined how underplanting density influences SMF responses.</p> Materials and methods <p>We examined a 29-year-old <i>C. lanceolata</i> plantation where <i>P. bournei</i> was underplanted at five density levels (0, 300, 600, 900, and 1200 stems·ha^− 1). Twelve months after density adjustment, we assessed soil physicochemical properties, bacterial community structure, and five key exoenzymes driving C, N, and P biogeochemical cycles across rhizosphere and bulk zones. Modeling via Random Forest and partial least squares path modeling (PLS-PM) was employed to evaluate the dictating factors of SMF shifts, specifically comparing the relative importance of bacterial community composition, diversity, and resource availability in both soil compartments.</p> Results and discussion <p>Our results demonstrated that SMF and soil quality index (SQI) exhibited significant but divergent responses to underplanting density. Medium-high underplanting density (900−1200 stems·ha^− 1) maximized SMF and bacterial alpha diversity in both soil compartments. PLS-PM indicated that bacterial community composition and diversity, rather than resource availability, dictated the initial shifts in SMF. Specifically, the relative abundance of Proteobacteria and Acidobacteriota predominantly regulated ecosystem functions. These patterns highlight that while soil chemical properties change slowly, microbial functions and diversity respond rapidly to management interventions. The findings suggest that early-stage functional recovery is primarily driven by microbial community shifts rather than immediate changes in total nutrient pools, illustrating the sensitivity of soil microbes to silvicultural adjustments.</p> Conclusions <p>We conclude that a medium-high underplanting density of <i>P. bournei</i> (900−1200 stems·ha^− 1) is most effective in accelerating the recovery of soil ecosystem multifunctionality during early forest conversion. This strategy optimizes microbial-mediated functions, providing a practical basis for close-to-nature management. These results emphasize the potential of density control in mitigating soil degradation in subtropical coniferous plantations.</p>