南水北调中线工程水源区硝酸盐氮污染形成的生态学机制及其调控

Nitrate nitrogen pollution control and governance is a greatest challenge for water quality security in the Danjiangkou catchment areas of the middle-route project of South-to-North water transfer. To fundamentally ease the nitrate pollution in the water source, insights into processes of nitrogen pollution and formation of ecological mechanism are urgently needed. Based on the status of natural, socioeconomic and ecological environment in the Danjiangkou reservoir area and its main tributaries, as well as the further monitoring information and analytical results as suggested herein on nitrogen pollution source, physicochemical and biochemical characteristics in water/sediment samples, and biological community structure, we would like to reveal the pathways of different forms of nitrogen in the water and sediments, the mechanism of coupling coordination between them, and the ecology reasons for the formation of nitrate pollution. Noting the special effects of the trace organic compounds on biological nitrogen transformation processes, attention would be paid to this cutting-edge issue with full consideration of the specific problems in the study area. As results, it is expected to construct optimal functional microbial communities for efficient degradation of both nitrogen and trace organic compounds. Furthermore, regulation countermeasures through microbial ecological restoration would be put forward in connection with the pollution source control strategy. This study is of significance to ensure the security of water quality in the water source of south-to-north water transfer project.

硝酸盐氮污染的控制与治理是南水北调中线工程丹江口水源区水质安全保障的重要挑战。要从根本上缓解水源区硝酸盐氮污染，必须深入揭示氮污染的过程与生态学形成机制。本项目拟基于丹江口库区和主要支流的自然、社会经济与生态环境现状，将氮污染源分析与水体/沉积物的物化、生化特征分析以及生物群落结构分析有机结合，揭示不同形态氮素在水体和沉积物中的转化途径以及不同氮转化作用的耦联协作机制，阐释硝酸盐氮污染形成的生态学原因。由于痕量有机物对生物氮转化过程具有特殊的影响作用，本课题拟结合水源区的具体问题对这一国际前沿问题展开深入研究，重点揭示痕量有机物影响氮素迁移转化的生态机制。在此基础上，构建氮素和痕量有机物高效降解菌群，结合污染源头控制策略提出微生物生态修复的调控途径，为保障南水北调中线水源地的水质安全提供重要的科技支撑。

针对南水北调中线工程丹江口水源区硝酸盐氮的污染问题，本课题在丹江口水库设置了12个代表性监测断面，于2016-2017年进行了4次大规模野外采样，系统开展了水样和沉积物样品中非生物要素（基本理化、营养盐、重金属、新兴有机污染物与δ15N等同位素）和生物要素（微生物群落）等指标的分析测试工作。基于野外监测和实验室分析，取得的创新性成果如下：（1）阐释了丹江口水源区氮素转化过程，发现氨氧化细菌（AOB）是水源区氨氧化速率的主要贡献者，丹江口水源区具有较强的硝化能力，在硝化充分的同时，会加重水库硝酸盐氮的负荷。传统反硝化过程是氮素异化代谢的主要过程，对库区氮损失的贡献约为57.1%。水源区51种氮转化功能基因在水体中的分布具有显著季节差异，在沉积物中无显著季节差异；其丰度沉积物中远高于水体，说明氮素的转化过程主要发生在表层沉积物。结合氮素入库出库通量的核算发现丹江口水源区硝酸盐氮积累的根本原因是库区现有的微生物反硝化能力不足以转化过量的入库氮素。（2）系统监测了可能影响反硝化过程的新兴污染物，其中抗生素、多溴联苯醚和全氟化合物在水体中浓度范围分别为17.2~200 ng/L、ND-287 ng/L和0.46-97.94 ng/L，在沉积物中的浓度范围分别为0.39~35.6 ng/g、ND-236 ng/g和0.07-1.62 ng/g，生态风险均较小。沉积物中Sb和Cr具有一定的生态风险，需要引起重视。进一步分析发现抗生素与反硝化过程具有较强的相关性，反硝化细菌是抗性基因的重要宿主。（3）基于对硝酸盐氮污染成因与影响因素的深入解析，提出了“源头”控制的基本思想，建立了土壤面源、工业废水和微污染水等不同氮素来源体系中以“硝化-反硝化”为核心的全过程调控和系统优化体系。在上述研究的基础上，提出丹江口水质保护的首要任务是进一步削减入库总氮负荷，建议今后应该重点加强水库“来水”硝氮与氨氮的“双控”，按照“硝化-反硝化”为核心的氮循环规律制定从源头出发的全过程水质保障体系。