Nitrogen input 15N-signatures are reflected in plant 15N natural abundances of sub-tropical forests in China.

Natural abundance of ¹⁵N (δ¹⁵N) in plants and soils can provide integrated information on ecosystem nitrogen (N) cycling, but it has not been well tested in warm and humid sub-tropical forests. In this study, we examined the measurement of δ¹⁵N for its ability to assess changes in N cycling due to increased N deposition in an old-growth broadleaved forest and a secondary pine forest in a high N deposition area in southern China. We measured δ¹⁵N of inorganic N in input and output fluxes under ambient N deposition, and N concentration (N%) and δ¹⁵N of major ecosystem compartments under ambient and after decadal N addition at 50 kg N ha^-1 yr^-1. Our results showed that the N deposition was δ¹⁵N-depleted (-12 ‰) mainly due to high input of depleted NH₄⁺-N. Plant leafs in both forest were also δ¹⁵N-depleted (-4 to -6 ‰). The old-growth 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 both forests, indicating that the ecosystem pools are already N-rich. Soil δ¹⁵N was not changed significantly by the N addition in both forests. However, the N addition significantly increased the δ¹⁵N of plants toward the ¹⁵N signature of the added N (~ 0 ‰), indicating incorporation of added N into plants. Thus, plant δ¹⁵N was sensitive to ecosystem N input manipulation although N % was unchanged in these N-rich sub-tropical forests. We interpret the depleted δ¹⁵N values of plants as an imprint from the high and δ¹⁵N-depleted N deposition. The signal from the input (deposition or N addition) may override the enrichment effects of fractionation during the steps of N cycling that are observed in most warm and humid forests. Thus, interpretation of ecosystem δ¹⁵N values from high N deposition regions need to include data on the deposition δ¹⁵N signal. [ABSTRACT FROM AUTHOR]