Aims <p>Fall dormancy (FD) in alfalfa (<i>Medicago sativa</i> L.) is a physiological adjustment strategy that affects forage yield, winter survival rates and spring regrowth. This study aimed to determine whether FD could drive alfalfa–soil interactions by modulating soil nutrient dynamics and the microbial community structure.</p> Methods <p>We assessed the&#xa0;root traits, soil properties, and microbial communities (bacteria/fungi) of alfalfa cultivars representing dormant (D), semi-dormant (SD), and non-dormant (ND) FD types during both growing and dormant seasons in semi-arid northwestern China.</p> Results <p>Our results indicated that total nitrogen (TN), soil organic carbon (SOC), and total phosphorus (TP) content increased with FD level during dormancy, whereas TN, sucrase activity, and protease activity generally decreased during the growing season. Furthermore, both the bacterial and fungal diversity indices were lower in the dormant season than in the growing season. Microbial co-occurrence networks revealed that the proportion of positive bacterial-fungal correlations increased with increasing FD level in the dormant season, while negative correlations decreased. During the dormant season, FD exerted highly significant positive effects on soil nutrients, enzymes, and root nutrition but significantly reduced alfalfa biomass and microbial biomass. Conversely, FD positively influenced root nutrients and microbial biomass during the growing season.</p> Conclusions <p>Our results highlight the importance of FD-driven rhizosphere effects on nutrient cycling and soil microbial communities, particularly during dormancy. This study provides valuable information for understanding complex FD-driven plant–soil interactions in alfalfa cultivation systems.</p> Graphical Abstract <p></p>