Abstract: Symbiosis between arbuscular mycorrhizal fungi (AMF) and forage plants plays an important role in the utilization efficiency of plant nutrients in grassland. Mixed sowing of legumes and grasses can effectively increase grassland yield through biological N fixation, and the addition of appropriate amounts of external nitrogen and phosphorus can also further promote herbage growth. In order to better manage the utilization efficiency of grassland in the future, the optimal combination of plant species, AMF and nitrogen and phosphorus will need to be known. Therefore, an experiment was conducted with the presence or absence of arbuscular mycorrhizal fungi (+/- AMF) as the main treatment, and different nitrogen and phosphorus fertilizer additions (N₀, no added N or P; N₁, 2.0 g N and 0.2 g P·m^-2; N₂, 20.0 g N and 2.0 g P·m^-2) as secondary treatments, to study the plant growth and photosynthesis response of coexisting alfalfa communities. It was found that: 1) AMF significantly improved the net photosynthetic rate of Leymus chinensis and Medicago sativa. 2) The light response curve indicated that the net photosynthetic rate of L. chinensis and M. sativa increased with increase in light intensity, and the net photosynthetic rate of +AMF L. chinensis was higher than -AMF L. chinensis. 3) AMF significantly increased the chlorophyll content of L. chinensis and M. sativa. Under different nitrogen and phosphorus application rates, the chlorophyll content of +AMF L. chinensis and M. sativa +AMF was higher than that of the -AMF treatments. 4) The application rate of nitrogen and phosphorus and the AMF treatment significantly affected the biomass of L. chinensis and M. sativa. In this research, treatment that optimized herbage production of coexisting communities of alfalfa-grass mixed grassland was +AMF+N₂.

Key words: photosynthetic characteristics, net photosynthetic rate, chlorophyll, total fresh weight