We measured the abundance of a nitrogen (N) isotope (¹⁵N) in two subtropical forests in China to study the cycling of input N (deposition and addition). Plant leaves in both forests were ¹⁵N-depleted relative to the atmospheric standard, likely as an imprint from ¹⁵N-depleted deposition. Plant ¹⁵N changed into ¹⁵N of the added N, indicating incorporation of N into plants. Thus, interpretations of ecosystem ¹⁵N from high-N-deposition regions need to include data on the ¹⁵N deposition signature.