We present the application of dual stable isotope analyses of NO 3 (δ 15 N-NO 3 and δ 18 O-NO 3 ) 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 δ 18 O-H 2 O, in addition to their δ 15 N-NO 3 and δ 18 O-NO 3 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 NO 3 stable isotopes are − 7.1 to + 9.2‰ and − 3.9 to + 13.2‰ for δ 18 O and δ 15 N, respectively. Interpretation of the stable isotope data characterizes 4 main sources of NO 3 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 NO 3 is derived from nitrified urea sources. During the other sampling periods (April and October), manure and sewage contribute more than 50% of river NO 3 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.

中文翻译：

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在高度城市化和农业密集的热带低地盆地，稳定同位素作为氮营养源识别的有效工具

我们介绍了 NO 3（δ 15 N-NO 3 和 δ 18 O-NO 3 ）双重稳定同位素分析的应用，以全面评估整个戴河流域 (DRB) 中硝酸盐的来源、分配和转化, 受农业和城市化影响严重的越南。除了它们的 δ 15 N-NO 3 和 δ 18 O-NO 3 特征外，还在 DRB 的 12 个位置采样了河水 δ 18 OH 2 O 的稳定同位素组成。在三个不同时期进行了样本收集，以捕捉区域天气和农业施肥制度的变化；四月（旱季和重点施肥期）、七月（雨季和另一个重点施肥期）和十月（没有区域施肥的雨季）。NO 3 稳定同位素的范围为 - 7.1 至 + 9.2‰ 和 - 3.9 至 + 13。δ 18 O和δ 15 N分别为2‰。稳定同位素数据的解释表征了 DRB 中 NO 3 的 4 个主要来源；(1) 源自集约化农业灌溉网络的硝化尿素肥料， (2) 从流域内浸出的土壤和地下水 (3) 粪便和污水输入（在下游河段更为普遍）和 (4) 从上游流入经道河汇入日河的红河。我们为 DRB 应用了一个混合模型，该模型由 4 个变量组成，代表这 4 个不同的来源。分区计算表明，7月施肥雨季，河道NO 3 45%以上来源于硝化尿素。在其他采样期间（4 月和 10 月），粪便和污水占河流 NO 3 的 50% 以上，它们来自 DRB 的中部，Day River 接收来自越南首都河内的生活污水。O 和 N 的稳定同位素数据表明，硝化过程在雨季比旱季更普遍，这主要发生在稻田农业区。总的来说，数据表明 DRB 中硝酸盐的损失是由于反硝化作用造成的，反硝化作用发生在河流受污染的河段，并在旱季占主导地位。本研究强调 (i) 生活垃圾在排放到 Day River 之前应进行处理，以及 (ii) 需要更好的集水区农业施肥实践，因为目前大部分肥料排放到河流中，这极大地影响了区域水质。