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Mixed-species plantations of trees with N-fixing species have the potential of promoting forest productivity and soil fertility. However, few studies in the literature have addressed the advantages of mixed-species plantations of leguminous trees over monocultures of leguminous trees based on in situ inventories over a long time period. Here, we monitored the dynamics of tree community composition, vegetation biomass, soil nutrients, and soil microbial phospholipid fatty acids (PLFAs), in an <i>Acacia mangium</i> monoculture plantation during 33 years of development and compared it with a mixed-species plantation of <i>A. mangium</i> associated with 56 native species which were underplanted 14 years after the initial establishment. Leaf N and phosphorus (P) concentrations of three main species in the overstory and understory of the <i>A</i>. <i>mangium</i> monoculture were measured. Our results showed that the soil organic carbon (SOC), total nitrogen (TN), and available phosphorus (AP) concentrations significantly increased over time during the approximately thirty years of <i>A</i>. <i>mangium</i> monoculture plantation, while the disadvantages were associated with new species regeneration and the increment of vegetation biomass. In the <i>A</i>. <i>mangium</i> monoculture plantation, leaf N concentration of <i>A</i>. <i>mangium</i><b>,</b><i>Rhodomyrtus tomentosa,</i> and <i>Dicranopteris dichotoma</i> continuously increased from 21 to 31 years, while the leaf P concentration of <i>A</i>. <i>mangium</i> and <i>R. tomentosa</i> decreased. The mixed-species plantations of <i>A</i>. <i>mangium</i> with native tree species had lower SOC and soil TN concentrations, more new tree species recruitment in the understory, and faster vegetation biomass increment than the <i>A</i>. <i>mangium</i> monoculture. However, the PLFAs of soil microbial groups were slightly different between the two types of plantations. We conclude that improved soil N nutrient condition by <i>A. mangium</i> monoculture benefits N absorption by <i>A</i>. <i>mangium</i>, <i>R</i>. <i>tomentosa,</i> and <i>D. tomentosa</i>, while low soil AP limits P absorption by <i>A</i>. <i>mangium</i> and <i>R</i>. <i>tomentosa</i>. Meanwhile, transforming the <i>A</i>. <i>mangium</i> monoculture into a mixed-species plantation via the introduction of multiple native species into the <i>A</i>. <i>mangium</i> monoculture decreases SOC and TN concentrations but the advantages include improving forest regeneration and maintaining forest growth in a long-term sequence. These findings provide useful and practical suggestions for managing forest monocultures of <i>A</i>. <i>mangium</i> in subtropical regions.