Irrigation, Phosphorus Application, and Inoculation Improve the Yield and Nutritional Quality of Alfalfa by Regulating Its Photosynthetic Performance

Water and phosphorus (P) scarcity severely limit the high-quality production of alfalfa (Medicago sativa L.) in arid regions. To clarify the synergistic regulatory effects of irrigation, P application, and phosphate-solubilizing bacteria (PSB) inoculation, this study used a three-factor completely randomized design. Three irrigation levels (W1: 40–45%, W2: 55–60%, W3: 70–75% of field capacity), two P application levels (P1: 50 mg kg−1, P2: 100 mg kg−1), and two inoculation treatments (J0: no inoculation, J1: inoculation with Bacillus mucilaginosus) were set to explore their interactive effects on photosynthesis, growth, and nutritional quality of alfalfa. The results showed that under mild drought, high-P combined with PSB inoculation significantly increased plant height (PH), chlorophyll content (Chl), and leaf instantaneous water-use efficiency (WUEi) while maintaining stable dry matter yield (DM). This treatment also increased crude protein (CP) content and relative feed value (RFV), and decreased neutral and acid detergent fiber contents, thus improving forage quality. In conclusion, under mild drought (55–60% of field capacity), P fertilization at 100 mg·kg−1 combined with PSB inoculation enhances photosynthetic performance, water- and phosphorus-use efficiencies, and improves yield and quality. It provides a theoretical reference for water-saving and high-quality alfalfa production in arid regions.