| Summary: | Differences in nitrogen (N) acquisition patterns between plant species are often reflected in the natural <sup>15</sup>N isotope ratios (δ<sup>15</sup>N) of the plant tissues, however, such differences are poorly understood for co-occurring plants in tropical and subtropical forests. To evaluate species variation in N acquisition traits, we measured leaf N concentration (%N) and δ<sup>15</sup>N in tree and understory plant species under ambient N deposition (control) and after a decade of N addition at 50 kg N ha<sup>−1</sup> yr<sup>−1</sup> (N-plots) in an old-growth subtropical forest in southern China. We also measured changes in leaf δ<sup>15</sup>N after one-year of <sup>15</sup>N addition in both the control and N-plots. The results show consistent significant species variation in leaf %N in both control and N-plots, but decadal N addition did not significantly affect leaf %N. Leaf δ<sup>15</sup>N values were also significantly different among the plant species both in tree and understory layers, and both in control and N-plots, suggesting differences in N acquisition strategies such as variation in N sources and dominant forms of N uptake and dependence on mycorrhizal associations among the co-occurring plant species. Significant differences between the plant species (in both control and N-plots) in changes in leaf δ<sup>15</sup>N after <sup>15</sup>N addition were observed only in the understory plants, indicating difference in access (or use) of deposited N among the plants. Decadal N addition had species-dependent effects on leaf δ<sup>15</sup>N, suggesting the N acquisition patterns of these plant species are differently affected by N deposition. These results suggest that co-occurring plants in N-rich and subtropical forests vary in their N acquisition traits; these differences need to be accounted for when evaluating the impact of N deposition on N cycling in these ecosystems.
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