Abstract: Biological nitrogen fixation (BNF), nitrogen transfer and the contribution of nitrogen derived from air to nitrogen yield were investigated in a semiarid area of China using four grasslegume mixtures: Leymus chinensis-Astragalus adsurgens, L. chinensis-Lespedeza daurica, L. chinensis-Medicago ruthenica, and L. chinensis-M. sativa cv. Aohan. The nitrogen yield of the L. chinensis-Astragalus adsurgens and of the L. chinensis-M. sativa cv. Aohan were significantly greater than that of L. chinensis monoculture and they had an improved grassland quality. The BNF of the four legumes was 35%-40%, suggesting that it was an important nitrogen resource for the growth of legumes. The amount of nitrogen transferred from legume to L. chinensis was 2-12 kg N/(ha·year), about 5%-24% of the nitrogen yield of L. chinensis. Legume BNF promoted the nitrogen resources of grassland ecosystems, contributing 20.30%, 17.78%, 11.29% and 31.77% to total nitrogen yield for A. adsurgens, L. daurica, M. ruthenica, and M. sativa cv. Aohan, respectively.