Abstract: Abstract: In order to further understand the potential mechanisms of grassland structure and function in response to soil phosphorus deficiency under future CO2 concentration increase, we examined the effects of CO2 concentration increase and phosphorus deficiency on the stomatal traits and leaf gas exchange parameters of annual ryegrass with environmental growth chambers, where the CO2 concentration was accurately controlled at 400 μmol/mol or 800 μmol/mol and meanwhile these grasses were fertilized with six phosphorus levels including 0.004 mmol/L (P0.004), 0.012 mmol/L (P0.012), 0.02 mmol/L (P0.02), 0.06 mmol/L (P0.06), 0.1 mmol/L (P0.1) and 0.5 mmol/L (P0.5). The results showed that: 1) The CO2 concentration increase significantly decreased the stomatal density of plants under lower phosphorus levels, but increased the stomatal density of annual ryegrass grown at higher phosphorus levels (0.1 mmol/L and 0.5 mmol/L); Meanwhile, CO2 concentration increase obviously decreased the stomatal openness of annual ryegrass, and made the spatial distribution pattern of stomata more regular when plants were grown at the phosphorus level of 0.06 mmol/L. 2) The CO2 concentration increase substantially enhanced the net photosynthetic rates (Pn) of annual ryegrass treated with higher phosphorus levels (0.1 and 0.5 mmol/L), but reduced the Pn of plants subjected to lower phosphorus levels, and thus increased the water use efficiency (WUE) of annual ryegrass at high phosphorus levels. 3) The responses of chlorophyll contents to CO2 concentration increase were different among the six phosphorus levels, and CO2 concentration increase substantially changed the ratio of chlorophyll a/b at higher phosphorus levels. 4) The total plant biomass and allocation between aboveground and belowground were obviously changed by phosphorus deficiency, and CO2 concentration increase featured CO2 fertilization effect on the aboveground biomass of annual ryegrass at higher phosphorus levels. These results suggested that the responses of stomatal traits and leaf gas exchange efficiency to CO2 concentration increase were obviously asymmetry between low and high phosphorus levels. These grasses under higher phosphorus levels optimized the leaf gas exchange efficiency by increasing the stomatal density and stomatal openness as well as regulating the spatial distribution pattern of stomata, and thus plants might benefit from CO2 fertilization effect under CO2 concentration increase. By contrast, annual ryegrass plants subjected to low phosphorus levels down-regulated the morphological traits of stoma and the regular pattern of distribution, and decreased leaf gas exchange efficiency of annual ryegrass to adapt the low phosphorus conditions under high CO2 concentration. Our results may have significant importance on further understanding the potential mechanisms of grassland ecosystem structure and function in response to CO2 concentration increase and phosphorus deficiency under future climate change.