Basal Molybdenum Application Enhances Growth and Nitrogen Fixation of Hairy Vetch in Acidic Soil

Basal molybdenum (Mo) application may alleviate micronutrient constraints on legume growth and biological nitrogen fixation in acidic soils, but its integrated effects on hairy vetch ( Vicia villosa Roth) and soil nutrient processes remain insufficiently characterized. A field experiment was conducted in acidic cropland using five basal Mo application rates: 0 (CK), 0.3 (T1), 0.6 (T2), 0.9 (T3), and 1.2 (T4) kg Mo ha ⁻¹ . Root morphology, nodulation, nitrogenase activity, biomass accumulation, plant nutrient status, soil chemical properties, microbial biomass, and exploratory path relationships were evaluated. Mo application markedly promoted root development and enhanced nodule formation and nitrogenase activity, indicating an improved potential for symbiotic N ₂ fixation. Under T4, total root length, root surface area, and root volume reached 644.42, 99.81, and 1.32 cm ³ , representing increases of 92.1%, 102.9%, and 175.0% relative to CK, respectively, while the nodule number and nitrogenase activity increased by 144.9% and 115.8%, respectively. Mo application also increased biomass production. Fresh yield was highest under T4 (19264.73 kg ha ⁻¹ ), whereas the dry yield was highest under T3 (2487.50 kg ha ⁻¹ ). Plant N concentration peaked under T2, increasing by 32.7% compared with CK, and plant Mo concentration reached 1.25 mg kg ⁻¹ under T4. Soil pH, inorganic N, and microbial biomass C and N increased following Mo application, whereas the responses of soil organic matter, total P, and available P were relatively limited. Under T4, microbial biomass C and N increased by 165.3% and 128.4%, respectively. Exploratory path analysis suggested that Mo promoted plant elemental accumulation and N assimilation mainly by enhancing root development and increasing soil microbial biomass. These findings indicate that rational Mo management can improve hairy vetch productivity and soil N supply in acidic cropland. Considering both dry‐matter yield, nitrogen fixation efficiency, and economic input, a basal Mo application rate of 0.6–0.9 kg Mo ha ⁻¹ (T2–T3) is recommended as an indicative optimal range for hairy vetch production in acidic soils, with quadratic dose–response analysis indicating diminishing dry‐matter returns above 0.9 kg Mo ha ⁻¹ .