Introducing alfalfa (Medicago sativa L.) into degraded lands is a cost-effective strategy to restore ecosystem functioning. However, the long-term persistence of alfalfa can diminish productivity and disrupt soil nutrient balance, raising concerns about the sustainability of these systems. Here, we quantified soil carbon (C), nitrogen (N), and phosphorus (P) stoichiometry, identified its environmental determinants, and evaluated its role in regulating soil multifunctionality (SMF) across 112 alfalfa grassland sites on the Loess Plateau of China. Average soil C:N, C:P, and N:P ratios (0-20 cm) were 8.6 +/- 0.8, 14.4 +/- 4.5, and 1.7 +/- 0.5, respectively. The C:P and N:P ratios were positively correlated with vegetation cover, aboveground biomass of native species, species richness, soil organic C, but not with alfalfa or total plant biomass. Structural equation modeling revealed that species richness, microbial biomass C, and soil moisture were the dominant drivers of soil C:P and N:P ratios. Soil stoichiometry, particularly the C:P ratio, exerted a stronger influence on SMF than alfalfa biomass, with both direct effects and indirect effects mediated through plant abundance and species richness. These findings identify soil stoichiometry as a key mechanism linking vegetation and microbial processes to SMF. We argue that improving soil stoichiometry-through practices such as balanced fertilization and enhanced plant diversity-will be essential to optimize nutrient use efficiency and sustain soil functioning in alfalfa grasslands.