Key factors controlling nitrogen transformation and efflux at the sediment–water interface: a comparative study from different trophic reservoirs.

Purpose: Dam construction can impact nitrogen accumulation in reservoir sediments and may potentially lead to internal release due to intricate nitrogen cycling at the sediment–water interface (SWI), exacerbating nitrogen loading in the water body. This study aims to clarify the transformation and transportation processes and key influencing factors of nitrogen species at the SWI in deep reservoirs. Methods: Two consecutive sediment cores were collected from Baihua Reservoir (BHR) and Hongjiadu Reservoir (HJD) in July 2019 and were stratified on-site. Pore water was extracted via centrifugation for water-soluble inorganic nitrogen measurement. Adsorbed inorganic nitrogen was extracted using 2 mol L⁻¹ KCl. Inorganic nitrogen forms were measured by an analytical continuous flow analyzer (SEAL Analytical Limited, Germany). Particulate organic nitrogen (PON) samples were obtained after the removal of inorganic carbon and inorganic nitrogen with 2 mol L⁻¹ KCl and 0.5 mol L⁻¹ HCl. Stable isotopes (δ¹⁵N-PON) and the ratios of C/N were measured via a stable isotopic mass spectrometer (MAT-253) and an elemental analyzer (Elementar, Rhine main, Germany). Fick's First Law and Grey correlation analysis (GCA) were employed to estimate the diffusion fluxes and controlling factors. Results: The sediment nitrogen concentrations in BHR were influenced by higher-level anthropogenic activities, as indicated by the comparison of nitrogen concentrations and δ¹⁵N-PON values (averaging 9.13‰ and 5.39‰ in BHR and HJD, respectively). Dissolved oxygen (DO) was the key factor affecting nitrogen transformation processes at the SWI. Mineralization and nitrification were dominant in HJD, while BHR exhibited mineralization, denitrification, and potential anaerobic ammonia oxidation (anammox). Livestock waste and soil organic matter were common sources of PON in both reservoirs. Moreover, the contribution of phytoplankton in BHR was also significant due to high-level eutrophication. Evaluations of diffusion fluxes revealed that both sediments in BHR and HJD acted as sources of water-soluble ammoniacal nitrogen (NH₄⁺-N) for internal release (0.97 and 0.27 mg m⁻² day⁻¹ in BHR and HJD, respectively). Additionally, adsorbed NH₄⁺-N exhibited high-level diffusion fluxes in both BHR (15.81 m⁻² day⁻¹) and HJD (3.37 m⁻² day⁻¹). Furthermore, GCA revealed that pH, T, and DO trigger the desorption of adsorbed nitrogen, leading to an increase in the nitrogen load in the overlying water. Conclusion: Sediments in eutrophic reservoirs experience high-level nitrogen inputs. Complex nitrogen cycling processes at the SWI and environmental factors (pH, DO, and T) may lead to significant internal releases. Further studies on the fate of nitrogen at the SWI in different eutrophic-level reservoirs are still necessary. [ABSTRACT FROM AUTHOR]