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

Abstract. Increased reactive nitrogen (Nr, NO3− + NH4+ + dissolved organic nitrogen) emission from Asian continent poses profound threats on ecosystem safety from terrestrial throughout the ocean proper. To diagnose the sources of atmospheric Nr input and quantify its influence on marine nitrogen cycle of the South China Sea (SCS), an oligotrophic marginal sea adjacent to the emission hotspot China, we conducted measurements of dual isotopes of water-soluble nitrate (δ15NNO3 and δ18ONO3) and concentrations of major ions for bulk atmospheric deposition collected from Dongsha Island off south China. The δ15NNO3 and δ18ONO3 for bulk deposition ranged from −7.5‰ to +3.9‰ and ∼ +17‰ to +88‰, respectively. A relatively uniform low δ15NNO3 and high δ18ONO3 endmember were observed in winter. Non-sea-salt sulfate/calcium (nssSO42− and nssCa2+) peaked as the increasing nitrate depositional flux (one exception caused by typhoon), implying a pollution source of nitrate during high deposition. Meanwhile, the flux-weighted average of δ15NNO3 was −2.7± 2.3‰, resembling the isotopic signature of fossil fuel combustion in inland China. More variable dual isotopic values observed in July and September suggest relatively dynamics sources and conversion chemistry. During the period affected by the peripheral circumfluence of Typhoon Fanapi, a high nitrate deposition with uniform isotopic composition (δ15NNO3 of ~ −0.5‰ and δ18ONO3 of ∼ +19‰) was observed accompanying with low terrestrial constituents such as dust and pollutants (e.g. nssSO42− and nssCa2+). This high nitrate deposition was likely a natural endmember sourced from lightning. The summarized total atmospheric Nr deposition (AND) is ∼ 50 mmol N m−2 yr−1. If without this additional AND fertilization, CO2 release (currently 460 &pm; 430 mmol C m−2 yr−1) from the SCS would be doubled. Our study demonstrates that AND may serve as an important external Nr supply to the SCS yet difficult to separate from N-fixation (−2‰ to 0‰) due to isotopic similarity and high N : P ratio. More studies related to isotopic composition of nitrogen speciation and their relative contributions as well as the role of typhoon-induced input to AND are required.

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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

Biogeosciences ◽

10.5194/bg-11-1833-2014 ◽

2014 ◽

Vol 11 (7) ◽

pp. 1833-1846 ◽

Cited By ~ 37

Author(s):

J.-Y. T. Yang ◽

S.-C. Hsu ◽

M. H. Dai ◽

S. S.-Y. Hsiao ◽

S.-J. Kao

Keyword(s):

South China Sea ◽

Isotopic Composition ◽

Atmospheric Deposition ◽

South China ◽

Dry Deposition ◽

Wet Deposition ◽

Hot Spot ◽

Inorganic Nitrogen ◽

Water Soluble ◽

China Sea

Abstract. Increased reactive nitrogen (Nr, NO3− &plus; NH4&plus;+ &plus; dissolved organic nitrogen) emission from the Asian continent poses profound threats to ecosystem safety from terrestrial throughout the ocean proper. To quantify atmospheric Nr input, diagnose its sources, and evaluate influence on marine nitrogen cycle of the South China Sea (SCS), an oligotrophic marginal sea adjacent to the emission hot spot China, we conducted measurements of concentrations of nitrate and ammonium as well as other major ions and dual isotopes of nitrate (δ15NNO3 and δ18ONO3) in atmospheric deposition collected from Dongsha Island off southern China. The δ15NNO3 and δ18ONO3 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 similar flux-weighted mean in δ15NNO3 (−2.6‰) yet a significantly higher mean in δ18ONO3 (+78.8‰). The dual isotope ratios showed an anti-correlation and an inverse seasonality; the δ15NNO3 values were higher in summer compared to those in winter, while the δ18ONO3 values were higher in winter than those in summer. In winter, not only dual isotopic compositions of nitrate but also the ammonium and nitrate dry deposition fluxes were relatively uniform, demonstrating a persistent influence of fossil fuel combustion sourced from Asian continental outflows via the northeasterly monsoon winds. More variable isotopic values in summer likely suggest varying sources and dynamical formation processes of dry deposition nitrate. Biomass burning and lightning are suggested to be responsible for the 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 in dry deposition but ammonium slightly higher in wet deposition. If not including this additional fertilization of atmospheric inorganic nitrogen deposition to enhance the carbon sequestration, CO2 release out of the SCS would be double than that of the present amount, 460 ± 430 mmol C m−2 year−1. Our study demonstrates that atmospheric deposition may serve as an important external Nr supplier to the SCS; however it is difficult to separate the isotopic signal from N2 fixation (−2–0‰) due to their similarity in δ15NNO3. More studies related to isotopic composition of nitrogen speciation in atmospheric Nr deposition, their relative contributions and source identification, and the role of typhoons in the SCS are required.

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