Abstract: The C:N:P ecological stoichiometry of plant-soil systems is closely related to plant growth and protein synthesis, and would thus be a lens to examine differences in either ecological strategy or environmental adaptability. However, few studies have assessed how forage crops adapt to saline-alkali soils under the C:N:P stoichiometry framework. We evaluated the ecophysiological adaptability of 20 domestic and foreign alfalfa (Medicago sativa) cultivars grown in slightly saline-alkali soils of Hetao Plain, Ningxia, by analyzing plant C:N:P stoichiometry, yield, and quality. We further explored the relationships of plant-microbe-soil C:N:P ecological stoichiometry and other environmental factors with the yield and quality of the alfafa (the difference between cultivars and between adaptations were not considered). The adaptability of the studied cultivars was divided into three categories by cluster analysis: Category I (high adaptability): 4 cultivars, including Gongnong 5#; Category II (moderate adaptability): 7 cultivars, including Zhongmu 1#; Category III (low adaptability): 9 cultivars, including Algonquin. Plant, microbe, and soil C:N:P stoichiometry had lower capacity to explain the variation in yield. However, plant C:N:P stoichiometry could explain the variations in quality. Yield was primarily influenced by soil available P, K⁺, and NH₄⁺-N, whereas quality was mainly determined by plant C:N, C:P, and N:P ratios. In summary, alfalfa yield is primarily affected by the soil nutrient supply, whereas quality is primarily controlled by the balance of C, N and P in plants.

Key words: alfalfa, ecological stoichiometry, Hetao Plain, improvement of saline-alkali land, physicoecological adaptation