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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 and Isao Koike
Biogeochemistry
Vol. 95, No. 2/3 (Sep., 2009), pp. 243-260
Published by: Springer
Stable URL: http://www.jstor.org/stable/40343518
Page Count: 18
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Longitudinal Distribution of Nitrate δ¹⁵N and δ¹⁸O in Two Contrasting Tropical Rivers: Implications for Instream Nitrogen Cycling
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Abstract

The longitudinal variations in the nitrogen (δ¹⁵N) and oxygen (δ¹⁸ O) isotopic compositions of nitrate (NO₃⁻), the carbon isotopic composition (δ¹³C) of dissolved inorganic carbon (DIC) and the δ¹³C and δ¹⁵N of paniculate 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} N_{NO_3 } $ became higher longitudinally in the freshwater reach of both rivers. The \[\delta ^{18} O_{NO3} \] also increased longitudinally in the river with a relatively steeper longitudinal gradient and a less cultivated watershed, while the \[\delta ^{18} O_{NO3} \] gradually decreased in the other river. A simple model for the \[\delta ^{15} O_{NO3} \] and the \[\delta ^{18} O_{NO3} \] 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 \[\delta ^{13} C_{DIC} \] and maximal pCO₂. The δ¹⁵Nof suspended particulate nitrogen (PN) varied in parallel to the \[\delta ^{15} N_{NO3} \] 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 \[\delta ^{15} O_{NO3} \] displayed a declining trend while the \[\delta ^{18} O_{NO3} \] 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 \[\delta ^{15} O_{NO3} \] and \[\delta ^{18} O_{NO3} \] however, the exact mechanism behind the observed trends is currently unresolved.

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