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The present study aimed to investigate the nitrogen (N) and phosphorus (P) fertilization of continuous addition effects plant biomass, the physiological properties of leaves and the antioxi-dant enzyme activities of alfalfa (Medicago sativa L) in the northern Xinjiang region; including the no fertilization (CK), nitrogen fertilization (N, 120 kg·ha−1), phosphorus fertilization (with low amount of N) (P, 100 kg·ha−1 P and 23.5 kg·ha−1 N) and combined nitrogen and phosphorus fertilization (NP, 120 kg·ha−1 N and 100 kg·ha−1 P) on the K well supplied soil. After three consecutive years of the supply of N and P fertilization, samples were taken at the first flowering of alfalfa (four clippings in the total year) to determine its pigment concentration, stomatal aperture, antioxidant enzyme activity and hay yield. The results showed that NP fertilization promoted growth with a higher number of branches and hay yield of alfalfa, while N or P fertilization alone had a positive effect on the growth of alfalfa. However, P fertilization significantly increased the carotenoid (Car) content at the early flowering stage of alfalfa leaves (during four clippings) (p < 0.05), In addition, NP ferti-lization enhanced stomatal aperture, increased the antioxidant enzyme activity and decreased the oxidized substance at the early flowering stage of alfalfa leaves. The results showed that a N and P balance rather than an absolute amount of either enhanced the growth of alfalfa, and N or P fertili-zation affects physiological traits differently. We propose that NP fertilization increases the nutri-tional characteristics and physiological characteristics, enhancing the adaptive capacity of alfalfa and making it better adapted to external environmental changes. [ABSTRACT FROM AUTHOR]