雅鲁藏布江中下游含氮化合物的分布特征及来源解析. (Chinese)

As one of the rivers with the highest elevation in the world, the Yarlung Zangbo River concentrates more than half of the water resources in Tibet. With the development of cascade hydropower stations in the Yarlung Zangbo River, human activities have changed the ecological environment and affected the nitrogen cycle in the river. In order to clarify the distribution characteristics of nitrogenous compounds and quantitatively analyze their sources, eleven sampling points were set up in the middle and lower reaches of the Yarlung Zangbo River and its main tributaries (Nianchu River, Lhasa River and Niyang River), and water samples were collected in March (the dry season) and August (the wet season) of 2018. The concentrations of dissolved oxygen (DO), total dissolved nitrogen (TDN), NO 3 - , NO 2 - and NH 4 + were measured. Dissolved organic nitrogen (DON) was calculated as the difference between TDN and inorganic N. The occurrences and spatial-temporal distribution of nitrogenous compounds in the water samples of the rivers were analyzed, and the main sources of NO 3 - in the wet season were determined by the dual isotope analyses. The results showed that the DON was the main form of TDN. The concentrations of TDN ranged from 1.3 to 2.5 mg/L in the dry season and from 1.1 to 2.0 mg/L in the wet season, and showed a downward trend along the river. The NO 3 - -N was the main form of inorganic nitrogen in the water, with an average proportion of 25.6% in TDN, and NH 4 + -N accounted for 17.5% in TDN, while NO 2 - -N was not detected at the most sampling sites. The results of nitrogen and oxygen isotope analyses showed that the values of δ 15 N-NO 3 - ranged from -9.17‰ to 22.68‰, and δ 18 O-NO 3 - from -13.25‰ to 8.24‰ in the wet season. The NO 3 - mainly came from rainfall and fertilizer, organic nitrogen in soil, and sewage and feces. Based on the Stable Isotope Analysis in R (SIAR) model, the contribution rates of nitrate from different sources were quantitatively calculated. In the Shigatse and Lhasa sections, NO 3 - in the water body mainly came from the organic nitrogen in soil and the NH 4 + in rainfall and fertilizer, with more than 70% of contribution rate. The main sources of NO 3 - in the water in the upstream and downstream of the cascade power stations were NH 4 + in rainfall and fertilizer, as well as sewage and feces, which accounted for a similar proportion, totally contributing more than 80% of the sources of NO 3 - . In the Nyingchi section, the NH 4 + in rainfall and fertilizer contributed the most to NO 3 - , accounting for 60%-70%, followed by the organic nitrogen in soil with 22%-28% of contribution rate. A large amount of farmland was distributed along the Yarlung Zangbo River and its tributaries. The nitrogen fertilizer applied in the farmland flowed into the river along with the surface runoff through rainfall, which was an important source of nitrogen in the water body of the Yarlung Zangbo River. Due to the reclamation of gentle slopes along the river and the erosion of cultivated soil, the organic nitrogen in soil was another important source of NO 3 - in the water of the Yarlung Zangbo River, especially in the wide valley section. Higher contribution rate of sewage and feces to NO 3 - in the canyon reach may be related to the development and construction of the cascade hydropower stations in the region. According to the change of DO contents and theoretical and measured values of δ 18 O-NO 3 - , nitrification and denitrification in the Yarlung Zangbo River are the main factors affecting the forms of nitrogen and the composition of nitrogen and oxygen isotopes. The results in the present study can provide data for the pollution control of nitrogenous compounds in the rivers in plateaus and drinking water security in Tibet. [ABSTRACT FROM AUTHOR]

作为世界上海拔最高的大河之一，雅鲁藏布江集中了西藏地区一半以上的水资源。随着梯级水电站的开发，人 类活动的影响大大增加，可能改变雅鲁藏布江及周边的生态环境，进而对河流氮循环产生影响。为深入理解水体含氮化 合物的赋存演变规律，定量分析其来源，在雅鲁藏布江中下游及其主要支流设置 11 个采样点，研究了雅鲁藏布江中下游 水体中含氮化合物赋存形式和时空分布特征，结果表明：溶解性有机氮（Dissolved Organic Nitrogen，DON）和硝态氮 （NO 3 - -N）是溶解性总氮（Total Dissolved Nitrogen，TDN）的主要形式，枯水期 TDN 的浓度高于丰水期，沿程呈现下 降趋势。结合氮氧同位素技术和稳定同位素模型对水体中的 NO 3 - 进行了源解析。δ 15 N-NO 3 - 和 δ 18 O-NO 3 - 的变化范围分别 为-9.17‰～22.7‰和-13.3‰～8.24‰。NO 3 - 主要来自降雨及肥料、土壤有机氮以及污水和粪便。在日喀则段和拉萨段， 水体 NO 3 - 主要来自土壤有机氮和降雨及肥料中 NH 4 + ，二者贡献率超过 70%；在山南段和林芝段，降雨及肥料中 NH 4 + 对 水体 NO 3 - 的贡献率最大，达到 43%～70%。水体硝化和反硝化作用是影响氮素赋存形态和氮氧同位素组成的主要因素。 [ABSTRACT FROM AUTHOR]

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