The nitrogen isotopic compositions of plant tissue could reflect its uptake of and preference for ammonium or nitrate. However, various factors may influence the field-collected delta N-15 values under field condition, which causes the interpretation problematic. The spatial variation of nitrogen (N) concentrations and the isotopic compositions were investigated in the soils and tissues of Chinese prickly ash from the southwest China to the east China. The objectives were to investigate the variation in soil and tissue delta N-15 values and N forms taken up by the plant. The leaf and root delta N-15 values varied significantly in response to the pattern of soil delta N-15 values. The difference in delta N-15 values between the leaves and roots was 2.57% and may be caused by an increase in the transport of unassimilated NO3- and NH4+ to the leaves. Leaf nitrogen was significantly and positively correlated with leaf potassium and negatively related to leaf calcium. Because potassium is the favoured counter-cation for nitrate transport in the xylem, the enrichment of N-15 in leaf relative to root induced by preferenced uptake of nitrate should be accompanied by significant and positive relationship of leaf nitrogen with leaf potassium concentrations. These results suggest that Chinese prickly ash prefers NO3- over NH4+.