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Increased reactive nitrogen (Nr, NO3− + NH4++ + dissolved organic nitrogen) emission from the Asian continent poses profoundthreats to ecosystem safety from terrestrial throughout the ocean proper. Toquantify atmospheric Nr input, diagnose its sources, and evaluateinfluence on marine nitrogen cycle of the South China Sea (SCS), anoligotrophic marginal sea adjacent to the emission hot spot China, weconducted measurements of concentrations of nitrate and ammonium as well asother major ions and dual isotopes of nitrate (δ15NNO3and δ18ONO3) in atmospheric deposition collected fromDongsha Island off southern China. The δ15NNO3 andδ18ONO3 for dry deposition averaged at−2.8‰ and +58.8‰, ranging from −7.5 to+3.7‰ and from ∼ +17 to +88‰,respectively. Wet deposition, although with limited samples, showed a similarflux-weighted mean in δ15NNO3 (−2.6‰) yeta significantly higher mean in δ18ONO3 (+78.8‰). Thedual isotope ratios showed an anti-correlation and an inverse seasonality;the δ15NNO3 values were higher in summer compared tothose in winter, while the δ18ONO3 values were higherin winter than those in summer. In winter, not only dual isotopiccompositions of nitrate but also the ammonium and nitrate dry depositionfluxes were relatively uniform, demonstrating a persistent influence offossil fuel combustion sourced from Asian continental outflows via thenortheasterly monsoon winds. More variable isotopic values in summer likelysuggest varying sources and dynamical formation processes of dry depositionnitrate. Biomass burning and lightning are suggested to be responsible forthe observed higher δ15NNO3 values in summer.Atmospheric nitrate and ammonium deposition together was estimated to be∼50 mmol N m−2 year−1, with the dominance of nitrate indry deposition but ammonium slightly higher in wet deposition. If not including this additional fertilization of atmospheric inorganic nitrogen deposition toenhance the carbon sequestration, CO2 release out of the SCS would bedouble than that of the present amount, 460 ± 430 mmol C m−2year−1. Our study demonstrates that atmospheric deposition may serve asan important external Nr supplier to the SCS; however it isdifficult to separate the isotopic signal from N2 fixation(−2–0‰) due to their similarity inδ15NNO3. More studies related to isotopic compositionof nitrogen speciation in atmospheric Nr deposition, theirrelative contributions and source identification, and the role oftyphoons in the SCS are required

biogeosciences

Isotopic composition of water-soluble nitrate in bulk atmospheric deposition at Dongsha Island: sources and implications of external N supply to the northern South China Sea