Optimizing nitrogen management enhances photosynthesis and alfalfa productivity in jujube-alfalfa intercropping

Although alfalfa is capable of nitrogen fixation, the optimal nitrogen (N) application rate for jujube-alfalfa intercropping systems in arid oasis regions remains unclear. This study evaluated four N application rates (0, 80, 160, and 240 kg/ha) to determine their effects on alfalfa photosynthesis, yield, and quality. Measurements of gas exchange and chlorophyll fluorescence revealed that the 160 kg/ha treatment (N2) significantly enhanced photosynthetic performance, particularly at the early flowering stage-a critical period when peak photosynthetic activity directly governs biomass accumulation and final yield. At this stage, the net photosynthetic rate (Pn) in the first and second harvests increased by 31.1% and 42.8%, respectively, while the photosynthetic performance index (PIabs) improved by 37.7%-102.2%. Compared with the unfertilized control, the N2 treatment significantly increased fresh forage yield by 23.8%-45.8% and dry matter yield by 18.4%-29.7%; crude protein content rose by up to 9.6% in the second harvest. Yield was significantly positively correlated with Pn, transpiration rate (Tr), the maximum photochemical efficiency of PSII (Fv/Fm), and PIabs. In conclusion, a nitrogen application rate of 160 kg/ha is recommended to maximize alfalfa yield and quality in this intercropping system.