Synergistic Regulatory Effects of Water–Nitrogen Coupling on Osmotic Regulation, Yield, and Forage Quality of Alfalfa

Water scarcity and poor soil fertility are major limiting factors constraining agricultural production in the arid and semi-arid regions of Northwest China. Water-nitrogen synergistic regulation is an important approach to improving crop growth and enhancing agricultural productivity. In this study, four irrigation levels-severe water deficit (W1: 45-65% θ_f), moderate water deficit (W2: 55-70% θ_f), mild water deficit (W3: 65-80% θ_f), and full irrigation (W4: 75-90% θ_f)-and four nitrogen application rates-no nitrogen (N0, 0 kg·ha^-1), low nitrogen (N1, 80 kg·ha^-1), medium nitrogen (N2, 160 kg·ha^-1), and high nitrogen (N3, 240 kg·ha^-1)-were established to systematically analyze the effects of water-nitrogen coupling on osmotic adjustment substances, yield, and forage quality of alfalfa (<i>Medicago sativa</i> L.) leaves. The results showed that: (1) Proline (Pro) content increased significantly with intensified water deficit, with W1 being 82.29% higher than W4 on average. Soluble protein (SP) and soluble sugar (SS) contents increased with increasing water availability, with their average values under W4 being 26.50% and 36.92% higher than those under W1, respectively. Increasing nitrogen application significantly improved the accumulation of osmotic adjustment substances, with Pro reaching the lowest value at N2, SP peaking at N2, and SS peaking at N3. (2) Yield increased significantly with higher irrigation, and increased first and then decreased with nitrogen application. Yield under W4 was 94.20% higher than under W1, and N2 increased yield by 12.45-50.65% compared with other nitrogen levels. (3) Under the W4N2 treatment, crude protein (CP) content and relative feed value (RFV) increased by 34.54% and 51.10%, respectively, compared with W1N0, while acid detergent fiber (ADF) and neutral detergent fiber (NDF) decreased by 28.74% and 24.44%, respectively. (4) Correlation analysis indicated that Pro content was significantly positively correlated with ADF and NDF but negatively correlated with yield, CP, and RFV. In contrast, SP and SS contents were significantly negatively correlated with ADF and NDF and positively correlated with yield, CP, and RFV. (5) Principal component analysis identified that the combination of full irrigation (W4: 75-90% θ_f) and medium nitrogen application (N2, 160 kg·ha^-1) optimizes both yield and forage quality by balancing osmotic adjustment substances.