Biogeosciences Discuss., 10, 9661-9695, 2013
www.biogeosciences-discuss.net/10/9661/2013/
doi:10.5194/bgd-10-9661-2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG.
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
J.-Y. T. Yang1,2,*, S.-C. Hsu2, M. Dai1, S. S.-Y. Hsiao2, and S.-J. Kao1,2
1State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
2Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan
*All experiments were accomplished at Research Center for Environmental Changes

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 ± 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.

Citation: Yang, J.-Y. T., Hsu, S.-C., Dai, M., Hsiao, S. S.-Y., and Kao, S.-J.: 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 Discuss., 10, 9661-9695, doi:10.5194/bgd-10-9661-2013, 2013.
 
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