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Natural abundance of ¹⁵N (<i>δ</i>¹⁵N) in plants and soils can provide time-integrated information related to nitrogen (N) cycling within ecosystems, but it has not been well tested in warm and humid subtropical forests. In this study, we used ecosystem <i>δ</i>¹⁵N to assess effects of increased N deposition on N cycling in an old-growth broad-leaved forest and a secondary pine forest in a high-N-deposition area in southern China. We measured <i>δ</i>¹⁵N of inorganic N in input and output fluxes under ambient N deposition, and we measured N concentration (%N) and <i>δ</i>¹⁵N of major ecosystem compartments under ambient deposition and after decadal N addition at 50 kg N ha⁻¹yr⁻¹, which has a <i>δ</i>¹⁵N of −0.7 ‰. Our results showed that the total inorganic N in deposition was ¹⁵N-depleted (−10 ‰) mainly due to high input of strongly ¹⁵N-depleted NH₄⁺-N. Plant leaves in both forests were also ¹⁵N-depleted (−4 to −6 ‰). The broad-leaved forest had higher plant and soil %N and was more ¹⁵N-enriched in most ecosystem compartments relative to the pine forest. Nitrogen addition did not significantly affect %N in the broad-leaved forest, indicating that the ecosystem pools are already N-rich. However, %N was marginally increased in pine leaves and significantly increased in understory vegetation in the pine forest. Soil <i>δ</i>¹⁵N was not changed significantly by the N addition in either forest. However, the N addition significantly increased the <i>δ</i>¹⁵N of plants toward the ¹⁵N signature of the added N, indicating incorporation of added N into plants. Thus, plant <i>δ</i>¹⁵N was more sensitive to ecosystem N input manipulation than %N in these subtropical forests. We interpret the depleted <i>δ</i>¹⁵N of plants as an imprint from the high and ¹⁵N-depleted N deposition that may dominate the effects of fractionation that are observed in most warm and humid forests. Fractionation during the steps of N cycling could explain the difference between negative <i>δ</i>¹⁵N in plants and positive <i>δ</i>¹⁵N in soils, and the increase in soil <i>δ</i>¹⁵N with depths. Nevertheless, interpretation of ecosystem <i>δ</i>¹⁵N from high-N-deposition regions needs to include data on the deposition ¹⁵N signal.