The productivity of alfalfa is associated with a large amount of nitrogen (N) and phosphorus (P); the addition of exogenous N and P fertilizers can fully exploit the growth potential of alfalfa. However, there is uncertainty about the relationship between changes in alfalfa productivity and photosynthetic physiology and anatomy. We conducted field fertilization experiments on alfalfa in the second and third years under drip irrigation, as well as measurement of the photosynthetic physiology, anatomical structure and agronomic traits of alfalfa at different levels of N (0, 120 kg·ha⁻¹) and different levels of P₂O₅ (0, 50, 100 and 150 kg·ha⁻¹). The results showed that the dry matter yield (DMY), crude protein (CP), net photosynthetic rate (P_n) and specific leaf weight (SLW) were increased by 2.10~11.82%, 4.95~11.93%, 4.71~7.59% and 2.02~7.12% in the N application treatment compared with the non-N application treatment, while the DMY, CP, P_n and SLW were increased by 3.19~17.46%, 1.99~8.42%, 6.15~24.95% and 2.16~11.90% in the P application treatment compared with the non-P application treatment. N and P increase the thickness of the spongy tissue (ST) of alfalfa, which will facilitate the entry and exit of gas and water, and will further affect the photosynthetic indexes, such as stomatal conductance (G_s) and transpiration rate (T_r), of alfalfa leaves. Increased palisade tissue (PT) thickness will also enhance the adaptability of plant leaves to strong sunlight, thereby increasing the maximum net photosynthetic rate (P_max) and light saturation point (LSP). Fertilization treatment showed the highest utilization efficiency for low light and better adaptation to strong light, but the R_d decreased. The comprehensive scores of principal component analysis for anatomical structure, photosynthetic performance and agronomic traits were N₁P₂ > N₀P₂ > N₁P₃ > N₁P₁ > N₀P₃ > N₀P₁ > N₁P₀ > N₀P₀. Therefore, the application of N and P fertilizers contributed to the adaptive changes in alfalfa leaf anatomy and the improvement of photosynthetic capacity, which were beneficial to the improvement of alfalfa dry matter yield, growth traits and nutritional quality, with the most obvious improvement effect obtained with the application of 120 kg·ha⁻¹ of N and 100 kg·ha⁻¹ of P₂O₅.

Alfalfa productivity and cold resistance in different regions are influenced by the fall-dormancy level of alfalfa. However, it is unclear whether the stoichiometric characteristics and nutrient resorption efficiency in alfalfa leaves also vary with the fall-dormancy level. In order to further understand the differences in nutrient absorption and requirements of different fall-dormant alfalfa, we conducted field trials on 30 different fall-dormancy alfalfa cultivars for 2 consecutive years in 2020 and 2021. We investigated the concentrations of carbon, nitrogen, and phosphorus in mature and senescent alfalfa leaves; nutrient stoichiometry ratios; and the coupling relationship between nutrient reabsorption efficiency and dry matter yield. The differences in nutrient reabsorption, fall dormancy, and dry matter yield of different fall-dormant alfalfa, and the correlation between indicators were utilized to further analyze the regulatory mechanisms of nutrient reabsorption in different fall-dormancy alfalfa varieties. The results demonstrated that the nitrogen reabsorption efficiency (NRE) and phosphorus reabsorption efficiency (PRE) of leaves increased first and then decreased with the increase in fall dormancy, whereas the carbon reabsorption efficiency (CRE) showed the reverse tendency. Different fall-dormancy alfalfa varieties significantly affected the dry matter yield and nutrient absorption in the first cut, while the last cut had the lowest variable coefficient and impact. There was a significant decrease in the over-winter survival rate of alfalfa as the fall-dormancy level increased, whereas the over-summer survival rate was less affected by the fall-dormancy level. As the growth year increased, there was a significant decrease in the over-winter survival rate. Among mature leaves, the NRE and PRE showed a significant positive correlation with the C concentration, while they showed a significant negative association with the N and P concentrations. In the same cut, the dry matter yield decreased with the increase in CRE but increased with the increases in NRE and PRE, while there was no significant trend in dry matter yield and nutrient resorption efficiency (NuRE) between different cuts. Taken together, the alfalfa survival rate and dry matter yield were relatively better in the moderate fall-dormancy (fall-dormancy level, FD = 4, 5) types and fall-dormancy (FD = 3) type, with a corresponding increase in the reabsorption requirements for nitrogen and phosphorus.