, Volume 95, Issue 2–3, pp 243–260 | Cite as

Longitudinal distribution of nitrate δ15N and δ18O in two contrasting tropical rivers: implications for instream nitrogen cycling

  • Toshihiro Miyajima
  • Chikage Yoshimizu
  • Yoshie Tsuboi
  • Yoshiyuki Tanaka
  • Ichiro Tayasu
  • Toshi Nagata
  • Isao Koike


The longitudinal variations in the nitrogen (δ15N) and oxygen (δ18O) isotopic compositions of nitrate (NO3 ), the carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC) and the δ13C and δ15N of particulate organic matter were determined in two Southeast Asian rivers contrasting in the watershed geology and land use to understand internal nitrogen cycling processes. The \( \delta^{15} {\text{N}}_{{{\text{NO}}_{3} }} \) became higher longitudinally in the freshwater reach of both rivers. The \( \delta^{18} {\text{O}}_{{{\text{NO}}_{3} }} \) also increased longitudinally in the river with a relatively steeper longitudinal gradient and a less cultivated watershed, while the \( \delta^{18} {\text{O}}_{{{\text{NO}}_{3} }} \) gradually decreased in the other river. A simple model for the \( \delta^{15} {\text{N}}_{{{\text{NO}}_{3} }} \) and the \( \delta^{18} {\text{O}}_{{{\text{NO}}_{3} }} \) that accounts for simultaneous input and removal of NO3 suggested that the dynamics of NO3 in the former river were controlled by the internal production by nitrification and the removal by denitrification, whereas that in the latter river was significantly affected by the anthropogenic NO3 loading in addition to the denitrification and/or assimilation. In the freshwater-brackish transition zone, heterotrophic activities in the river water were apparently elevated as indicated by minimal dissolved oxygen, minimal δ13CDIC and maximal pCO2. The δ15N of suspended particulate nitrogen (PN) varied in parallel to the \( \delta^{15} {\text{N}}_{{{\text{NO}}_{3} }} \) there, suggesting that the biochemical recycling processes (remineralization of PN coupled to nitrification, and assimilation of NO3 -N back to PN) played dominant roles in the instream nitrogen transformation. In the brackish zone of both rivers, the \( \delta^{15} {\text{N}}_{{{\text{NO}}_{3} }} \) displayed a declining trend while the \( \delta^{18} {\text{O}}_{{{\text{NO}}_{3} }} \) increased sharply. The redox cycling of NO3 /NO2 and/or deposition of atmospheric nitrogen oxides may have been the major controlling factor for the estuarine \( \delta^{15} {\text{N}}_{{{\text{NO}}_{3} }} \) and \( \delta^{18} {\text{O}}_{{{\text{NO}}_{3} }} \), however, the exact mechanism behind the observed trends is currently unresolved.


Anthropogenic nitrogen loading Denitrification Estuary Nitrate Nitrification Tropical river 



This study was supported by the CREST (Core Research for Evolutional Science and Technology) program of Japan Science and Technology Agency, and Grant-in-Aid for Oversea Scientific Research (B) No.16405007 and Grant-in-Aid for Scientific Research (C) No.17510004 from Japan Society for the Promotion of Science. Ranong Coastal Resource Research Station of Kasetsart University and Had-Chao-Mai National Park Education Center kindly provided us with laboratory facilities. The authors gratefully acknowledge M. Nakaoka, C. Aryuthaka, Y. Monthum and T. Srisombat for management of field survey, and S. Pleum-arom, C. Jantharakhantee, T. Niyomsilpchai and N. Thongsin for assistance of field works. The authors were also benefitted by helpful discussion with K. Koba, N. Ohte, N. Ohkouchi, H. Ogawa and M. Tsutsumi on the results and implications of this study, and constructive comments by two anonymous reviewers for the earlier versions of the manuscript.


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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  • Toshihiro Miyajima
    • 1
    • 7
  • Chikage Yoshimizu
    • 2
    • 3
  • Yoshie Tsuboi
    • 1
    • 4
  • Yoshiyuki Tanaka
    • 1
    • 5
  • Ichiro Tayasu
    • 2
  • Toshi Nagata
    • 1
    • 2
  • Isao Koike
    • 1
    • 6
  1. 1.Ocean Research InstituteThe University of TokyoNakano, TokyoJapan
  2. 2.Center for Ecological ResearchKyoto UniversityHirano, OtsuJapan
  3. 3.Japan Science and Technology AgencyKawaguchi, SaitamaJapan
  4. 4.Kurume LaboratoryChemicals Evaluation and Research Institute, JapanMiyanojin, KurumeJapan
  5. 5.International College of Arts and SciencesYokohama City UniversityKanazawa, YokohamaJapan
  6. 6.University of the RyukyusNishihara, OkinawaJapan
  7. 7.Department of Chemical Oceanography, Ocean Research InstituteThe University of TokyoNakano, TokyoJapan

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