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Long-term interactions between vegetation succession and soil quality in alfalfa fields were investigated in the semi-arid Loess Plateau of China. The successional ecosystem is composed of six different planting-year alfalfa fields (namely, 3–5–9–15–21–25 years). The yield, cover, and height of alfalfa increased from 194 g dry hay m−2, 55.6%, and 52.8 cm in the 3-year stage to 406 g dry hay m−2, 85.5%, and 72.5 cm in the 9-year stage, respectively. Subsequently, they tended to decrease and reached their respective minimum levels at the 25-year stage. The density of alfalfa continued to decrease but the plant species richness increased with successional age. Microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass C-to-N ratio and soil basal respiration (BR) at 0–20 cm soil depth decreased steadily from 3 years to 9 years, but increased from 15 years to 25 years. The values for metabolic quotient (qCO2) were the opposite of the above four parameters along the succession gradient....