Impact of Water-Sediment Regulation Operation on Nitrogen Concentration, Transformation and Sources in the Lower Yellow River

The Yellow River (YR) has the highest suspended sediment concentration in the world, with its water and sediment exerting a significant influence on nutrient transport and transformation processes. The periodic regulation of water and sediment by the Xiaolangdi Dam, has significantly altered downstream water and sediment transport. This study examined the impact of the Xiaolangdi Dam’s 2023 water-sediment regulation on nitrogen dynamics in the lower Yellow River (LYR). Surface water, suspended sediment, and deposited sediment samples were collected at seven downstream stations to analyze changes in nitrogen concentration, sources, and transformation processes. As the water regulation stage progresses, the (total nitrogen) TN concentration in the water phase decreased, while that of NO3--N increased slightly. Concurrently, the inorganic nitrogen concentration in the suspended phase also declined. As the sediment regulation stage progresses, the TN and NO3−-N concentrations in the water phase continued to decrease, while the inorganic nitrogen concentration in the suspended phase showed an initial increase followed by a decrease. As the early stage of sediment regulation progresses, ammonia concentrations decreased, while nitrate concentrations increased and δ18O-NO3− value decreased indicated nitrification occurred. As the late stage of sediment regulation progresses, nitrate concentrations decreased and the δ15N-NO3− value increased, indicated denitrification occurred. The TN flux during water-sediment regulation reaches 41.5 kt (14.6% of the annual flux). During the water-sediment regulation stage, the main nitrate sources were manure and sewage. This contribution peaked at 54.2% during the sediment regulation stage. The research results provide a scientific basis for the relationship between water and sediment changes and nitrogen output changes in the LYR.