Abstract: The effect of NO₃^--N and NH₄⁺-N and their different supply ratios on the growth of Fraxinus mandshurica seedlings was studied by sand culture in greenhouse.The highest seedling growth was obtained at a NH₃^--N/NH₄⁺-N ratio of 75/25 in nutrient solution.The growth of seedlings declined with the increasing ratio of NH₄⁺-N to NO₃^--N. Excessive NH₄⁺-N inhibited root growth,and the root/shoot ratio of the seedlings significantly decreased as the NH₄⁺-N/NO₃^--N ratio was over 50%.Increasing NH₄⁺-N in nutrient solution led to the decrease of net photosynthetic rate,the increase of phosphorus content in plant tissues,and the decrease of Zn and Fe contents.The pH value of cultural medium was affected markedly by nitrogen form,which was increased with increasing NO₃^--N and decreased with increasing NH₄⁺-N in nutrient solution.The pH value was up to 7.22 in the medium contained NO₃^--N alone,while decreased tO₄.20 only with NH₄⁺-N.