Li Dongsheng, Zhao Yunduo, Liu Zhongfang
State Key Laboratory of Marine Geology, Tongji University, Shanghai, 200092, China.
Key Laboratory of State Forestry Administration on Soil and Water Conservation, Beijing Forestry University, Beijing, 100083, China.
J Environ Manage. 2025 Sep;391:126474. doi: 10.1016/j.jenvman.2025.126474. Epub 2025 Jul 4.
Nutrient transport by submarine groundwater discharge (SGD) is a crucial factor for marine eutrophication, but studies have revealed that subterranean estuaries (STEs) are active regions of nitrogen removal. Thus, understanding the nitrogen transformation and drivers in STEs is a fundamental prerequisite for an accurate assessment of the nitrogen flux resulting from SGD. Nitrogen transformation is driven primarily by microorganisms, and hydrological conditions influence the structure and function of microbial communities. We investigated the nitrogen transformation potential and environmental responses in various hydrological units (the upper saltwater plume and the mixed area above/below the silty clay layer) of the Yangtze subterranean estuary. The major N transformation genes were nrfA + nirB, nasA and napA, accounting for 36.8 %, 24.2 % and 12.4 %, respectively. Our results indicated that the process of dissimilatory nitrate reduction to ammonium (DNRA) is the crucial mechanism for nitrogen conversion in the Yangtze STE, which accounts for approximately 35 % of the nitrogen transformation and is greater than denitrification. Additionally, the proportions of DNRA in the total NO reduction were greater than 83 % across various aquifers. Interestingly, the nitrogen transformation proportions were similar among the three hydrological units of the STE, but the factors influencing nitrogen transformation differed. Therefore, it is necessary and crucial to divide the hydrological regions within STEs to accurately understand nitrogen transformation, enabling the regulation of SGD-derived nitrogen fluxes through hydrological processes or environmental condition management. Our results will improve the knowledge of the nitrogen cycle in STEs and help to formulate effective measures to protect coastal ecological environments.
通过海底地下水排放（SGD）进行的营养物质运输是海洋富营养化的一个关键因素，但研究表明，地下河口（STE）是氮去除的活跃区域。因此，了解STE中的氮转化及其驱动因素是准确评估SGD产生的氮通量的基本前提。氮转化主要由微生物驱动，水文条件影响微生物群落的结构和功能。我们研究了长江地下河口不同水文单元（上部盐水羽状体以及粉质粘土层上方/下方的混合区域）中的氮转化潜力和环境响应。主要的氮转化基因是nrfA + nirB、nasA和napA，分别占36.8%、24.2%和12.4%。我们的结果表明，异化硝酸盐还原为铵（DNRA）过程是长江STE中氮转化的关键机制，约占氮转化的35%，且大于反硝化作用。此外，在各个含水层中，DNRA在总硝酸盐还原中的比例均大于83%。有趣的是，STE的三个水文单元中的氮转化比例相似，但影响氮转化的因素不同。因此，有必要且至关重要的是划分STE内的水文区域，以准确了解氮转化，从而通过水文过程或环境条件管理来调节SGD产生 的氮通量。我们的结果将增进对STE中氮循环的认识，并有助于制定保护沿海生态环境的有效措施。
