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

Abstract

The longitudinal variations in the nitrogen (d 15 N) and oxygen (d 18 O) isotopic compositions of nitrate (NO 3 -), the carbon isotopic composition (d 13 C) of dissolved inorganic carbon (DIC) and the d 13 C and d 15 N 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 d 15 N NO 3 became higher longitudinally in the freshwater reach of both rivers. The d 18 O NO 3 also increased longitudinally in the river with a relatively steeper longitudinal gradient and a less cultivated watershed, while the d 18 O NO 3 gradually decreased in the other river. A simple model for the d 15 N NO 3 and the d 18 O NO 3 that accounts for simultaneous input and removal of NO 3 -suggested that the dynamics of NO 3 -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 NO 3 -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 d 13 C DIC and maximal pCO 2 . The d 15 N of suspended particulate nitrogen (PN) varied in parallel to the d 15 N NO 3 there, suggesting that the biochemical recycling processes (remineralization of PN coupled to nitrification, and assimilation of NO 3 --N back to PN) played dominant roles in the instream nitrogen transformation. In the brackish zone of both rivers, the d 15 N NO 3 displayed a declining trend while the d 18 O NO 3 increased sharply. The redox cycling of NO 3 -/NO 2 -and/or deposition of atmospheric nitrogen oxides may have been the major controlling factor for the estuarine d 15 N NO 3 and d 18 O NO 3 , however, the exact mechanism behind the observed trends is currently unresolved.

Differences in Stream Water Nitrate Concentrations between a Nitrogen-Saturated Upland Forest and a Downstream Mixed Land Use River Basin

“…where R is the 15 N/ 14 N or 18 O/ 16 O ratio of the NO3 − of a sample and the international standard (atmospheric N2 for nitrogen and the Vienna Standard Mean Ocean Water (VSMOW) for oxygen, respectively).…”

“…It has been demonstrated that anthropogenic nitrate originating from wastewaters and fertilizers has a differing range of nitrogen isotope ratio in NO 3 − (δ 15 N) values, so the isotope tracer technique is particularly useful for assessing nitrogen pollution in rivers. Stable isotopes of riverine NO 3 − , such as δ 15 N and oxygen isotope ratio in NO 3 − (δ 18 O) values have been used to identify NO 3 − sources in surface water [15,16]. Increases in both δ 15 N and δ 18 O values indicate N removal processes including denitrification [17,18] and assimilation by microbial activities [19,20].…”

“…Stable isotopes of riverine NO 3 − , such as δ 15 N and oxygen isotope ratio in NO 3 − (δ 18 O) values have been used to identify NO 3 − sources in surface water [15,16]. Increases in both δ 15 N and δ 18 O values indicate N removal processes including denitrification [17,18] and assimilation by microbial activities [19,20]. Therefore, the use of stable isotopes of these riverine NO 3 − should be a powerful tool for inferring the N source and removal process of NO 3 − in downstream water.…”

Nitrate sources and dynamics in a salinized river and estuary – a δ<sup>15</sup>N–NO<sub>3</sub><sup>−</sup> and δ<sup>18</sup>O–NO<sub>3</sub><sup>−</sup> isotope approach

“…There is evidence that O can exchange between H 2 O and intermediate compounds of nitrification (Andersson et al, 1982;DiSpirito and Hooper, 1986;Kool et al, 2007). Since the δ 18 O of estuarine water is expected to be higher than that of river water (Miyajima et al, 2009)…”

“…More than concentration data alone, the combined use of N (δ 15 N) and O (δ 18 O) isotopes of NO − 3 has provided a powerful tool to investigate NO − 3 dynamics and identify NO − 3 sources in estuaries (Middelburg and Nieuwenhuize, 2001;Sebilo et al, 2006;Wankel et al, 2006;Dähnke et al, 2008;Miyajima et al, 2009). Therefore, in the present study, a combined approach based on the mixing curves of DIN concentration versus salinity and δ 15 N-and δ 18 O-NO − 3 is applied to (1) identify potential dominant NO …”