Abstract
We present the application of dual stable isotope analyses of NO3 (δ15N-NO3 and δ18O-NO3) to provide a comprehensive assessment of the provenance, partitioning, and conversion of nitrate across the Day River Basin (DRB), Vietnam, which is heavily impacted by agriculture and urbanization. Stable isotope compositions of river water δ18O-H2O, in addition to their δ15N-NO3 and δ18O-NO3 signatures, were sampled at 12 locations in the DRB. Sample collection was conducted during three different periods to capture changes in regional weather and agricultural fertilization regimes; April (the dry season and key fertilization period), July (the rainy season and another key fertilization period) and October (the rainy season with no regional fertilization). Ranges of NO3 stable isotopes are − 7.1 to + 9.2‰ and − 3.9 to + 13.2‰ for δ18O and δ15N, respectively. Interpretation of the stable isotope data characterizes 4 main sources of NO3 in the DRB; (1) nitrified urea fertilizer derived from an intensive agricultural irrigation network, (2) soil and groundwater leaching from within the basin (3) manure and sewage inputs (which is more prevalent in downstream river sections) and (4) upstream inflow from the Red River which discharges into the Day River through the Dao River. We applied a mixing model for the DRB consisting of 4 variables, representing these 4 different sources. The partition calculation shows that during the fertilization and rainy period of July, more than 45% of river NO3 is derived from nitrified urea sources. During the other sampling periods (April and October), manure and sewage contribute more than 50% of river NO3 and are derived from the middle portion of the DRB, where the Day River receives domestic wastewater from the Vietnamese capital, Hanoi. Stable isotope data of O and N reveal that nitrification processes are more prevalent in the rainy season than in dry season and that this predominantly takes place in paddy field agricultural zones. In general, data demonstrate that nitrate loss in the DRB is due to denitrification which takes place in polluted stretches of the river and dominates in the dry season. This study highlights that (i) domestic waste should be treated prior to its discharge into the Day River and (ii) the need for better catchment agricultural fertilization practices as large portions of fertilizer currently discharge into the river, which greatly impacts regional water quality.
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Acknowledgements
This work was supported by the RCUK-NAFOSTED [Grant Numbers NE/P014577/1]; and the NAFOSTED [Grant Number 105.08-2014.26]. Stable isotope analysis was performed as part of the IAEA-CRP program ‘Isotopes to Study Nitrogen Pollution and Eutrophication of Rivers and Lakes—F32007’. This paper is written with a financial support from the Graduate School of Science and Technology, Vietnam Academy of Science and Technology (GUST.STS.ĐT2017-ST02).
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Appendix: Calculation of δ18O-NO3 based on water oxygen and dissolved oxygen isotopes
Appendix: Calculation of δ18O-NO3 based on water oxygen and dissolved oxygen isotopes
Nitrification occurs as a two-step process whereby ammonia is first converted to nitrite and the produced nitrite is then converted to nitrate. During the bacterial nitrification process, the biogeochemical sources of oxygen atoms are dioxygen (O2) and water (H2O). O2 is incorporated during the oxidation of ammonia to hydroxylamine (NH2OH), while H2O is incorporated during the oxidation of both hydroxylamine to nitrite and nitrite to nitrate. While the ratio of 1:2 oxygen atoms from O2 and H2O implied by these observations is commonly used to interpret the oxygen isotopic content of nitrate derived from bacterial nitrification (Kendall 1998; Burns and Kendall 2002; Wankel et al. 2006), the utilization of this ratio involves the assumptions that exchange and fractionation of oxygen isotopes during nitrification are minimal. Recent works (e.g. Casciotti et al. 2007; Casciotti et al. 2010; Buchwald and Casciotti, 2010) have presented oxygen isotopic exchange and fractionation during nitrification. In general, during bacterial ammonia oxidation, the produced δ18O-NO2 is computed as:
In which χAOB, εk,O2,εk,H2O,1, εeq, are respectively the fraction of nitrite oxygen atoms that have equilibrated with H2O during ammonia oxidation, the kinetic isotope effect for O2 incorporation, the kinetic isotope effect for H2O incorporation by hydroxylamine oxidoreductase, and the equilibrium isotope effect for nitrite equilibration with H2O.
Then, during bacterial nitrite oxidation, δ18O-NO3 is estimated as (exchange of oxygen atoms between nitrite and water is minimal; Buchwald & Casciotti, 2010):
whereas εk,H2O,2 is the kinetic isotope effect for water incorporation by nitrite oxidoreductase.
Literature review has shown that χAOB, εk,O2+ εk,H2O,1, εeq, and εk,H2O,2 are respectively + 0.15 ± 0.1‰, + 26.3 ± 7.7‰, + 14‰, and + 15.5 ± 3.8‰ (Casciotti et al. 2007; Casciotti et al. 2010; Buchwald & Casciotti, 2010).
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Luu, T.N.M., Do, T.N., Matiatos, I. et al. Stable isotopes as an effective tool for N nutrient source identification in a heavily urbanized and agriculturally intensive tropical lowland basin. Biogeochemistry 149, 17–35 (2020). https://doi.org/10.1007/s10533-020-00663-w
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DOI: https://doi.org/10.1007/s10533-020-00663-w