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Natural abundance of 15N (δ15N) in plants and soilscan provide time-integrated information related to nitrogen (N) cyclingwithin ecosystems, but it has not been well tested in warm and humidsubtropical forests. In this study, we used ecosystem δ15N toassess effects of increased N deposition on N cycling in an old-growthbroad-leaved forest and a secondary pine forest in a high-N-deposition areain southern China. We measured δ15N of inorganic N in input andoutput fluxes under ambient N deposition, and we measured N concentration (%N) andδ15N of major ecosystem compartments under ambient deposition andafter decadal N addition at 50 kg N ha−1yr−1, which has aδ15N of −0.7 ‰. Our results showed that the totalinorganic N in deposition was 15N-depleted (−10 ‰) mainly dueto high input of strongly 15N-depleted NH4+-N. Plant leaves inboth forests were also 15N-depleted (−4 to −6 ‰). Thebroad-leaved forest had higher plant and soil %N and was more15N-enriched in most ecosystem compartments relative to the pine forest.Nitrogen addition did not significantly affect %N in the broad-leavedforest, indicating that the ecosystem pools are already N-rich. However,%N was marginally increased in pine leaves and significantly increased in understoryvegetation in the pine forest. Soil δ15N was not changedsignificantly by the N addition in either forest. However, the N additionsignificantly increased the δ15N of plants toward the 15Nsignature of the added N, indicating incorporation of added N into plants.Thus, plant δ15N was more sensitive to ecosystem N inputmanipulation than %N in these subtropical forests. We interpretthe depleted δ15N of plants as an imprint from the high and15N-depleted N deposition that may dominate the effects offractionation that are observed in most warm and humid forests. Fractionationduring the steps of N cycling could explain the difference between negativeδ15N in plants and positive δ15N in soils, and theincrease in soil δ15N with depths. Nevertheless, interpretation ofecosystem δ15N from high-N-deposition regions needs to include dataon the deposition 15N signal

biogeosciences

Nitrogen input &amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N signatures are reflected in plant &amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N natural abundances in subtropical forests in China