Nitrogen adsorption and release characteristics of sediments in the Jingjiang reach in the middlestream of the Yangtze River

As a critical protection zone in the Yangtze River Basin, the water quality of the Ching River, located in the middle reaches of the Yangtze River, has long been a research focus in aquatic environmental science. To clarify the quantitative relationship between nitrogen concentrations in overlying water and sediments, and to elucidate the regulatory mechanism of sediments on water column nitrogen levels, sediment samples were collected, and a series of laboratory experiments, including nitrogen adsorption-desorption kinetic tests and adsorption thermodynamic analyses, were carried out. The results indicate that along the flow direction, the equilibrium adsorption capacity of sediments for ammonia nitrogen presents an increasing trend, while the equilibrium release capacity shows a decreasing tendency. Elevated temperature and pH values exert an inhibitory effect on the ammonia nitrogen adsorption capacity of sediments, whereas the increase in sediment organic matter content enhances the adsorption performance. The calculated equilibrium nitrogen concentration (ENC₀) of the sediments is higher than the ammonia nitrogen concentration in the overlying water, suggesting that the sediments in the Ching River pose a potential risk of endogenous nitrogen release.