Abstract: In order to understand whether the elevated CO2 does alleviate the toxic effect on tomato root system due to high NH4+-N concentration in nutrient solutions,a growth chamber experiment was conducted to determine the influence of NH4+/NO3- ratio and elevated CO2 concentrations on the pH in nutrient solution,growth and root vigor system of tomato seedling roots.Tomato(Lycopersicon esculentum Mill var.Hezuo 906) was grown in pots with nutrient solutions varying in NH4+∶NO3- ratio(0∶1,1∶3,1∶1,3∶1 and 1∶0) in growth chambers with ambient(360 L/L) or elevated CO2 concentrations(720 L/L).It enhanced the changes of pH with promotion of growth time and CO2 enrichment.At two CO2 levels,pH increased with whole NO3--N in nutrient solution and decreased in other treatments and enhanced the decrease of pH with increasing proportion of NH4+-N in nutrient solutions.The decrease in pH value with only NH4+-N in nutrient solution was higher than the increase in pH with only NO3--N in nutrient solution. Elevated CO2 increased the dry weight of shoots and roots,root vigor system, total absorbing area,active absorbing area of tomato seedlings.All measurement indices above of tomato seedlings were increased in the elevated CO2 treatment with increasing proportion of NO3- in nutrient solutions and the dry weight of shoots,dry weight of roots,root vigor system,total absorbing area and active absorbing area were up to 65.8%,78.0%,18.9%,12.9% and 18.9% higher at elevated than at ambient CO2 concentration,depending on NH4+/NO3- ratio.The results indicated that tomato seedling roots may benefit best from CO2 enrichment when 100% NO3--N in nutrient solutions is supplied,and elevated CO2 did not inhibit the adverse effects on tomato seedling roots when 100% NH4+-N in nutrient solution is supplied.