梯级开发对澜沧江氮迁移转化的影响机制及生态环境累积效应研究

Nitrogen is one of the most important biogenic substances in river systems, whose fate represents a key biogeochemical process in global circulation of material. Uneven distribution of nitrogen may lead to eutrophication of aquatic systems, severe greenhouse gas emission and other environmental problems. Till present, systematic investigation on the circulation of nitrogen and corresponding mechanisms in river systems is still lacking, especially on the fate of nitrogen and mechanisms behind, roles of microorganisms, identification of relevant environmental factors and the consequences of eco-environmental accumulation of nitrogen under varying eco-hydrological conditions which has been affected by hydropower cascade exploration in Southwest China, climate change and other human interference activities. In the present work, we take Lantsang River (the section in Yunnan province) as a representative river system to analyze the spatial and temporal distribution of nitrogen and the variation of its flux based on clarifying the evolution process of aquatic environment and its corresponding key driving factors under the impact of cascade development, so as to explore the impact of cascade development on the fate of nitrogen and corresponding working mechanism, and verify the evolution of the community structure of nitrogen related microorganisms in cascade reservoir area and river sections. Based on corresponding findings we plan to construct multi-dimensional model simulating and predicting the impact of cascade development on the fate of nitrogen and corresponding cumulative effect on the eco-environment. The findings from the present study is expected to be used to clarify the dynamic evolution of nitrogen in river systems in Southwest China, increase the abundance of microorganism information responsible for the circulation of nitrogen in plateau watersheds, and deliver theory evidence and technical supporting for protecting and managing trans-border rivers.

氮是河流系统中重要的生源物质，其迁移转化是生物地球化学循环的关键过程，氮循环失衡还可能引发水体富营养化、温室气体排放等环境问题。然而，目前仍然缺乏对河流完整氮循环过程和机制的系统研究，尤其针对我国西南水电梯级开发河流，气候变化及人为干预共同导致河流生态水文发生变化条件下的氮迁移转化机理、微生物介导作用及其环境影响因子的识别、生态环境累积效应的研究还很匮乏。本研究以澜沧江（云南段）为例，通过探明梯级工程影响下的河流水环境演变进程及关键驱动因子，分析氮的时空分异特征和通量变化过程，揭示梯级开发对氮迁移转化规律的影响和作用机制，阐明梯级库区与河段中氮循环功能微生物群落的演变规律，构建多维耦合模型预测梯级开发对澜沧江氮迁移转化过程的影响及生态环境累积效应。项目成果可以揭示西南复杂河流系统中氮的动态演进机理，丰富高原区流域氮循环过程的微生物学信息，为跨境河流的管理保护协同推进提供理论依据与技术支撑。

大型河流的梯级水电开发会对河流的生态环境影响会相互叠加，产生累积效应。氮作为主要的生源要素，在维系河流生态健康上发挥着无可替代的作用。然而梯级开发河流中氮的来源与归驱，转化关键过程、微生物作用机制及其生态环境累积效应仍不明确，难以实现对梯级水库生态效应的准确评估和河流水安全的科学保障。本研究以澜沧江梯级规划及开发河段为研究对象，采用地球化学、分子生物学等方法，瞄准“河流生源物质迁移转化及环境质量演变”科学目标，聚焦梯级开发河流氮来源、微生物转化机制及通量变化效应等核心内容。取得主要研究结论如下：.（1）采用氮氧同位素、水化学参数及贝叶斯混合模型分析了澜沧江梯级开发河段主要氮形态的时空变化规律，识别了水体中硝酸盐的主要来源，探讨了梯级水库中氮的迁移转化过程，定量解析了不同来源硝酸盐的贡献比例，揭示了河流-水库系统硝酸盐的生物地球化学过程及驱动因素。.（2）系统探讨了主要氮转化过程（好氧氨氧化、硝酸盐异化还原为铵、反硝化及厌氧氨氧化）微生物的丰度、潜在活性、多样性及种群结构，明晰了关键环境因子、生物地理学过程和水库工程特征对不同氮转化菌群的影响，给出了不同脱氮途径的相对贡献比率。.（3）采用水库氮质量平衡箱式模型和沉积物脱氮通量模型，估算了梯级水库不同来源氮的输入、输出通量，给出了梯级水库水-气界面N2O的释放通量和影响因素，表征了不同运行特征水库在氮输移过程中所起的“源/汇”作用和截留效果，系统给出了沉积物不同氮转化过程通量、微生物来源与去除途径。.（4）构建了水动力-氮迁移转化过程耦合的生态环境累积效应评价模型，揭示了梯级水库中氮的动态演进过程，定量预测了不同梯级开发工况下氮的累积效应，初步揭示了氮的非线性累积过程。.研究成果对揭示西南复杂河流系统中氮的动态演进机理，丰富梯级水库中氮循环过程的微生物学信息，为跨境河流的管理保护协同推进提供理论依据与技术支撑。