Interpretation of anthropogenic impacts (agriculture and urbanization) on tropical deltaic river network through the spatio-temporal variation of stable (N, O) isotopes of NO⁻₃^*

* Originally presented at the IAEA International Symposium on Isotope Hydrology: Revisiting Foundations and Exploring Frontiers, 11–15 May 2015, Vienna, Austria.

ABSTRACT

For the first time, the dual isotope approach was applied to trace the sources of impacts and to identify the governing biogeochemical processes in a river network in the tropical deltaic region of the Red River (Vietnam). Our long term surveys concluded that water in this river network was severely impacted by anthropogenic activities. Analysis has shown strong spatio-temporal variation of nitrate isotopes; ranges of δ¹⁵N– and δ¹⁸O– were from −5 to 15 ‰ and from −10 to 10 ‰, respectively. Average values of δ¹⁵N– and δ¹⁸O– in the dry season, when fertilizer is applied, were 3.54 and 3.15 ‰, respectively. In the rainy season, the values changed to 6.41 and −2.23 ‰, respectively. Denitrification and biological assimilation were active throughout the year, but were especially enhanced during fertilization time. Mineralization of domestic organic matter and consequent nitrification of mineralized were the dominant processes, particularly during the rainy period.

KEYWORDS:

• Biological assimilation
• denitrification
• dual isotope approach
• isotope fractionation
• isotope hydrology
• nitrification
• nitrogen-15
• oxygen-18
• Vietnam

Acknowledgements

Special gratitude is sent to Assoc. Prof. Nishida Kei (ICRE, University of Yamanashi, Japan) for help in the stable isotope analysis. We also thank the two anonymous reviewers for the valuable advice to improve the discussion. Appreciation is sent to Prof. Alexander Scheeline, University of Illinois, for editing this paper.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

* Originally presented at the IAEA International Symposium on Isotope Hydrology: Revisiting Foundations and Exploring Frontiers, 11–15 May 2015, Vienna, Austria.

Additional information

Funding

The experiments conducted in this study were funded by the NAFOSTED [program no. 104.04-2013.37]. The paper was written with support from the CRP program ‘Application and Development of Isotope Techniques to Evaluate Human Impacts on Water balance and Nutrient Dynamics of Large River Basins’ [No. 18443] and the VAST program [No. VAST07.04/15-16].