Nitrogen transformation and drivers in response to hydrological variability in the Yangtze subterranean estuary

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 NO₃⁻ 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.