Enrichment of atmospheric CO₂ and N deposition generally stimulates plant photosynthetic activity and enhances C allocation for plant symbionts such as mycorrhizae. This in turn may modify plant species interactions through differentially impacting plant nutrient acquisition. We examined how elevated CO₂ and N deposition affects plant growth and whether mycorrhizae mediate interactions of a highly mycorrhizal forb (Plantago lanceolata L.) with different varieties of non-mycorrhizal plant (Festuca arundinacea Schreb) in a microcosm experiment. Plants were grown at ambient (400 mmol mol^-1) and elevated CO₂ (ambient + 330 mmol mol^-1) levels with or without N inputs. The ¹⁵N isotope tracer was introduced to quantify the mycorrhizally mediated N acquisition of plants. Elevated CO₂ significantly increased the shoot biomass ratio of P. lanceolata to F. arundinacea particularly when N was low. It also increased mycorrhizal colonization of P. lanceolata roots, but did not show any effects on F. arundinacea. Elevated CO₂ enhanced ¹⁵N and total N uptake of P. lanceolata but had no effects on N acquisition of F. arundinacea. Nitrogen inputs significantly reduced the shoot biomass ratio of P. lanceolata to F. arundinacea with greater reduction under elevated CO₂ than under the ambient control. These results suggest that elevated CO₂ may enhance the competitive ability of mycorrhizal over non-mycorrhizal plants but N deposition may favor non-mycorrhizal plants under future atmospheric CO₂ and N deposition scenarios.