Effects of Water and Nitrogen Regulation on Alfalfa (Medicago sativa L.) Production Performance Through Optimization of Nitrogen Metabolism

Water and nitrogen application rates directly affect crop yield formation and protein accumulation. Nitrogen metabolism, as a key physiological process linking water and nitrogen supply with crop growth, plays a critical role in regulating nitrogen uptake, transformation, and accumulation. However, the functional relationships among different nitrogen metabolism indicators in mediating the formation of high-quality and high-yield alfalfa under water–nitrogen regulation remain unclear. In this study, alfalfa (Medicago sativa L.) was subjected to four nitrogen application levels [N0 (0 kg·hm−2), N1 (80 kg·hm−2), N2 (160 kg·hm−2), N3 (240 kg·hm−2)] and four irrigation gradients [severe deficit (W0, 45–60% θf), moderate deficit (W1, 55–70% θf), mild deficit (W2, 65–80% θf), and full irrigation (W3, 75–90% θf), where θf represents field capacity]. The relationships between water–nitrogen regulation and alfalfa nitrogen metabolism and productive performance were analyzed. The results showed that (1) both irrigation amount and nitrogen application rate significantly affected the activities of leaf nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and soluble protein (SP) content (p < 0.05). Aboveground nitrogen content (TN) initially increased and then decreased with increasing irrigation and nitrogen application, reaching its maximum under W2N2, with leaves being the primary site of aboveground nitrogen accumulation. (2) Under W2N2, alfalfa yield and crude protein accumulation (CP-a) both reached their maximum values, averaging 10.22 t·ha−1 and 1187.10 kg·ha−1, respectively. (3) Structural equation modeling (SEM) indicated that water–nitrogen regulation primarily influenced yield and CP-a formation through its effects on GS activity and TN. Based on these findings, a comprehensive evaluation model (GS–TN–Yield–CP-a) was constructed, and the preliminary suitable water–nitrogen regulation ranges for high-quality and high-yield alfalfa were identified as an irrigation amount of 69.4–85.9% θf and a nitrogen application rate of 102.6–222.3 kg·hm−2. These results can provide a theoretical basis for high-quality and high-yield alfalfa management in arid and semi-arid regions, but further verification through field experiments is still required.