Nitrogen input 15N-signatures are reflected in plant 15N natural abundances of sub-tropical forests in China
Other literature type
Gurmesa, Geshere Abdisa
Natural abundance of <sup>15</sup>N (δ<sup>15</sup>N) 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 δ<sup>15</sup>N 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 δ<sup>15</sup>N of inorganic N in input and output fluxes under ambient N deposition, and N concentration (N %) and δ<sup>15</sup>N of major ecosystem compartments under ambient and after decadal N addition at 50 kg N ha<sup>−1</sup> yr<sup>−1</sup>. Our results showed that the N deposition was δ<sup>15</sup>N-depleted (−12 ‰) mainly due to high input of depleted NH<sub>4</sub><sup>+</sup>-N. Plant leafs in both forest were also δ<sup>15</sup>N-depleted (−4 to −6 ‰). The old-growth forest had higher plant and soil N %, and was more <sup>15</sup>N-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 δ<sup>15</sup>N was not changed significantly by the N addition in both forests. However, the N addition significantly increased the δ<sup>15</sup>N of plants toward the <sup>15</sup>N signature of the added N (~ 0 ‰), indicating incorporation of added N into plants. Thus, plant δ<sup>15</sup>N was sensitive to ecosystem N input manipulation although N % was unchanged in these N-rich sub-tropical forests. We interpret the depleted δ<sup>15</sup>N values of plants as an imprint from the high and δ<sup>15</sup>N-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 δ<sup>15</sup>N values from high N deposition regions need to include data on the deposition δ<sup>15</sup>N signal.