Effects of Freeze-Thaw Cycles on Phosphorus from Sediments in the Middle Reaches of the Yarlung Zangbo River

The effect of the freeze-thaw process is an important factor in soil nutrient changes and erosion enhancement. Sediments in the middle reaches of the Yarlung Zangbo River are likely affected by the daily freeze-thaw cycles in winter. Examining the freeze-thaw effects of phosphorus from sediments in this area is of great significance for protecting the structure and safety of the ecosystem. The freeze-thaw process of sediments in the middle reaches of the Yarlung Zangbo River was simulated through laboratory experiments, and different phosphorus contents and particle states were synchronously detected and analyzed. The results show that freeze-thaw cycles can accelerate phosphorus migration and release in the sediments, and the total amount of phosphorus release increases by 12%. After being subjected to freeze-thaw cycles, the sediment particles were broken, and the competition between ions for adsorption sites reduced phosphorus adsorption onto the sediments from the middle reaches of the Yarlung Zangbo River. The organic matter on the sediment surface was also broken down, and the energy dispersive spectroscopy (EDS) results showed that the combined ions that were released competed for the adsorption sites on the particle surfaces, thereby promoting phosphorus release. Among the different forms of phosphorus, aluminum-bound phosphorus (Al-P) and iron-bound phosphorus (Fe-P) are the two most released phosphorus forms by the freeze-thaw process. Although the contents of Al-P and Fe-P only account for 2.41% of the total phosphorus content, both phosphorus forms are biologically available, and freeze-thaw cycles may increase the risk of nutrient loss. This research may provide information for the study of phosphorus in river ecosystems in areas subjected to freeze-thaw cycles.