Abstract: This paper reported the study on the effect of combined N applied at low level on the nitrogen fixation by alfalfa in monoculture and mixed culture with meadow rescue, and the effect on the absorption and utilization of indigenous soil nitrogen and nitrogen fertilizer. Amount of nitrogen fixed by alfalfa could be raised and duration of high peak of symbiotic nitrogen fixation activity could be extended when nitrogen fertilizer was applied reasonably. It was especially important for the early pastures or pastures with low supporting nitrogen capacity. Transfer of nitrogen fixed by alfalfa to meadow rescue occured in mixed culture. Nitrogen fixed from alfalfa was uptaken more easily than indigenous nitrogen in soil. planting alfalfa could raise soil fertility significantly. Meadow rescue may be able to fix nitrogen from the air in some way. When combined N was appropriately applied to soil, on which alfalfa and meadow fescue had been planted, it could promote increasing nitrogen fertility in soil.