Combined Micronutrient and Microbial Inoculant Application Improves Bur Clover Yield and Quality While Reshaping Rhizosphere Microbial Communities Under Greenhouse Conditions

Micronutrient limitation and rhizosphere imbalance often constrain the productivity and quality of leafy vegetables in intensively managed greenhouse soils. This study evaluated the effects of conventional fertilization (CK), micronutrient supplementation (Mi), and micronutrient supplementation combined with a compound microbial inoculant (MM) on bur clover (Medicago polymorpha L.) yield, quality, rhizosphere chemical properties, and soil microbial communities. Compared with CK, Mi increased yield by 26.53%, whereas MM increased yield by 40.77%. MM also significantly increased SPAD, soluble protein, and soluble sugar, while reducing plant nitrate content by 22.86%; Mi had no significant effect on nitrate reduction. MM decreased soil pH from 8.62 to 8.34 and increased EC, available P and K, water-soluble Ca, Mg, and K, and available Zn and B, indicating improved rhizosphere chemical conditions. Mantel analysis showed that yield and plant nitrate were significantly associated with several soil variables. MM also markedly reshaped rhizosphere microbial communities, with clear treatment separation for both bacteria and fungi. The bacterial community was significantly explained by selected soil variables, whereas the fungal model was not significant. Overall, micronutrient supplementation mainly promoted yield, while its combination with microbial inoculation further improved rhizosphere conditions, crop quality, and nitrate control.