Abstract: This study systematically examines the differential impacts of A. melanoxylon and Eucalyptus plantations on soil fertility, with particular emphasis on their respective mechanisms in soil property modification, microbial community restructuring, and nutrient cycling efficiency. The results show that A. melanoxylon enhances soil organic matter accumulation and nitrogenphosphorus cycling efficiency through synergistic rhizobial nitrogen fixation and litter input. Its mixed cultivation patterns elevate soil pH value and increase soil porosity, demonstrating superior soil structure modification. Compared to Eucalyptus monocultures, A. melanoxylon plantations significantly promote beneficial microbial proliferation and acid phosphatase activity, and exhibit stronger ecological adaptability in nutrient-deficient environments. The sustainable nutrient input system driven by biological nitrogen fixation, coupled carbon-nitrogen decomposition of litter, and root complementarity in mixed systems achieves significant improvement in soil nutrient turnover efficiency, effectively mitigating nutrient depletion in fast-growing plantations. This study constructs a“structure-process-function” theoretical framework for comparing the effects of A. melanoxylon and Eucalyptus plantations on soil fertility. The results provide scientific guidance for selecting Eucalyptus alternatives and optimizing mixed cultivation patterns in degraded subtropical forest restoration, while informing the transition towards ecologically sustainable plantation management. Future studies should prioritize quantifying the frost resistance ability of A. melanoxylon under low-temperature site conditions and the long-term stability of the mixed system, and determine its suitable site conditions and the optimal suitable habitats.