Migration and release potential of nitrogen at the sediment–water interface in lakes in cold and arid regions

Lake eutrophication in cold and arid regions is showing a deepening trend in recent years, posing a serious threat to the regional ecological environment. The occurrence characteristics, bioavailability, sorption–desorption characteristics, and release risk of sediment nitrogen in the Ulanor Wetland, located in the Hulun Lake basin of China, were investigated by combining field investigation, laboratory simulation experiments, and multiple technologies, including diffusive gradients in thin films and high-resolution dialysis technology. The total nitrogen (TN) in the water overlying the sediment bed (i.e., overlying water) ranged from 1.44 to 2.65 mg/L. Dissolved inorganic nitrogen was the main form of TN in overlying water, and ammonia nitrogen (NH₄⁺–N) in the pore water at the sediment–water interface was higher than that in the overlying water. Surface sediment TN content ranged from 695.37 to 2,344.77 mg/kg, with acid-dissolved nitrogen as the main component, and can cause the lowest level of ecotoxic effect. The maximum and equilibrium adsorption amounts of sediment NH₄⁺–N ranged from 0.269 to 1.017 mg/g and 0.0132–0.0382 mg/g, respectively. The bioavailability and transport capacity of sediment nitrogen were relatively weak, but a release risk was still observed.