Mixing N-fixing trees with native species is promoted as a sustainable strategy for enhancing the functions of plantation ecosystems compared to pure N-fixing plantations. However, how such mixed plantations influence soil microbial communities and soil multifunctionality under different climate conditions remains unclear. Here, we examined the 24-year effects of Robinia pseudoacacia (RP), an N-fixing tree, mixed with native deciduous (e. g., Prunus sibirica) and coniferous trees (e.g., Platycladus orientalis and Pinus tabuliformis), on microbial communities and evaluated their relationship with soil multifunctionality in two contrasting climate regions (semihumid and semi-arid). In semi-arid regions, all three mixed RP plantations had significantly higher bacterial, fungal, and protist richness, microbial network complexity, C-N-P-cycling gene abundance and soil multifunctionality compared to pure RP plantation. Elevated soil P levels in mixed plantations promoted microbial Nand P-cycling gene diversity and fungal symbiont richness, which were predominantly responsible for the 92.3 % increase in soil multifunctionality in semi-arid regions. By contrast, in semi-humid regions, microbial structure and functional indicators did not differ significantly between mixed and pure forests, likely because soil nutrient contents (as microbial substrates) varied little between the two forest types. Since RP-Pinus tabuliformis mixed forests exhibited the highest microbial diversity, nutrient-cycling gene abundance, and soil multifunctionality, planting N-fixing species and native coniferous species may benefit soil functioning, especially in semi-arid regions. Overall, the effects of N-fixing-tree mixed plantations on microbial communities and associated soil multifunctionality are strongly climate-dependent, with microbial functional diversity being a key determinant of soil multifunctionality. These findings provide policymakers with compelling evidence to support naturebased solutions for sustainable plantation management.