Purpose <p>Invasive alien plants disrupt soil carbon dynamics, yet the underlying mechanisms are poorly understood.</p> Methods <p>We investigated the impact of <i>Solidago canadensis</i> invasion on soil organic carbon (SOC) fractions, extracellular enzyme activities of carbon-, nitrogen-, and phosphorus-acquisition (EEA_C, EEA_N, and EEA_P), and microbial metabolic characteristics across invasion stages, including non-invasive stage (NI), early invasive stage (EI), intermediate invasive stage (II), the dominant invasive stage (DI), the complete invasive stage (CI). We simulated these stages using varying relative densities of <i>S. canadensis</i> and a native species (<i>Pterocypsela laciniata</i>).</p> Results <p>Dissolved organic carbon (DOC) at the EI decreased by 33.33% in the II stage and by 16.67% in the CI stage (<i>p</i> &lt; 0.05). In contrast, the heavy fraction of organic carbon showed no significant changes across the invasion stages (<i>p</i> &gt; 0.05). The EEA_C, EEA_N, and EEA_P showed substantial reductions, with higher values observed in the NI and EI stages, and progressively lower values in the II, DI, and CI stages (<i>p</i> &lt; 0.01). The microbial carbon use efficiency (CUE, the proportion of assimilated carbon allocated to microbial growth rather than respiration) emerged as a key predictor for multiple carbon fractions, including SOC, light fraction organic carbon (LFOC), readily oxidizable organic carbon (ROOC), and DOC. However, partial dependence and regression analyses showed non-linear relationships between CUE and these carbon fractions. Notably, SOC, LFOC, and ROOC exhibited a threshold increase at 0.55 of CUE, while DOC decreased significantly, suggesting a shift toward greater carbon accumulation in certain fractions.</p> Conclusion <p>These results emphasize the role of CUE in regulating soil carbon dynamics in invaded ecosystems.</p>