Mechanism and Characteristics of Phosphorus Release from Sediments in Drawdown Zone Under Inundation/Drying Cycles

Phosphorus release from sediments significantly influences eutrophication in shallow lakes; however, its dynamics in drawdown zones under alternating inundation and drying cycles remain understudied. This study investigates the mechanisms of phosphorus release from sediments in the drawdown zone of Nansi Lake, a key reservoir along the eastern route of the South-to-North Water Diversion Project. Through field sampling and laboratory simulations, we analyzed the impact of inundation duration, physicochemical properties, and organic matter decomposition on phosphorus release. In Container a (first inundation period), phosphorus was rapidly released at the beginning of inundation, with total phosphorus (TP) in the overlying water increasing from 1.92 mg/L to 2.68 mg/L, and in the interstitial water from 8.45 mg/L to 15.24 mg/L. The second inundation period showed the highest phosphorus release, with TP reaching 3.61 mg/L in the overlying water and 21.51 mg/L in the interstitial water. Inorganic phosphorus dominated the release, with dissolved inorganic phosphorus (DIP) accounting for a higher proportion of TP than dissolved organic phosphorus (DOP). Changes in pH, oxidation-reduction potential (ORP), dissolved oxygen (DO), and total organic carbon (TOC) significantly influenced phosphorus distribution. The decomposition of organic matter during inundation increased dissolved organic matter levels, thereby affecting phosphorus release. These findings provide valuable insights into phosphorus dynamics and highlight the need for integrated management strategies to mitigate internal phosphorus loading and prevent eutrophication in Nansi Lake, offering guidance for water quality management and ecological protection in similar shallow lake systems.