Foliar Nano-Selenium Modulates Metabolic and Antioxidant Responses in Alfalfa (Medicago sativa L.): Integration of Pot and Field Evidence

Alfalfa (<i>Medicago sativa</i> L.), as a globally crucial high-quality forage crop, frequently suffers from yield reduction and quality deterioration due to environmental stressors such as drought and salt. Nano-selenium (NSe) offers a viable solution to mitigate this challenge. However, the multi-level regulatory mechanisms of NSe in alfalfa remain unclear. Foliar NSe modulates nitrogen metabolism, antioxidant defense, and rhizosphere microbial community collaboration to enhance alfalfa yield and quality. Pot experiments demonstrated that foliar NSe (1-20 mg/L) enhanced seedling growth, elevated nutrient biosynthesis (soluble protein, amino acids), and boosted antioxidant capacity via activation of superoxide dismutase and glutathione peroxidase. Metabolomics in field trials revealed shoot-root metabolic partitioning: shoots were upregulated in α-linolenic acid metabolism (jasmonic acid, methyl jasmonate), while roots enriched amino acid biosynthesis (proline, arginine), achieving a synergistic enhancement between aboveground and belowground processes. Microbial community analysis indicated Actinobacteria enrichment and elevated soil urease activity in NSe-treated groups. These findings demonstrate that NSe coordinates carbon-nitrogen metabolism with antioxidant pathway activation to synergistically enhance alfalfa growth performance and nutritional quality.