Phosphorus and Its Release from Sediments in an Agricultural Reservoir in Neijiang City, China

Sediments play an important role in regulating the phosphorus concentration of the overlying water, especially for the agricultural reservoirs with the persistent input of external phosphorus by agricultural activities. Therefore, the sediment can significantly affect the process of reservoirs' eutrophication. In this study, an agricultural reservoir (Huanghezhen Reservoir) in Neijiang City, southeast of Sichuan Basin, China, was conducted to investigate the phosphorus fraction, adsorption, and release characteristics in sediments through field investigation and experimental studies. Results revealed significant enrichment of phosphorus in reservoir sediments, especially in the upstream and midstream, the sediment P contents vastly exceed the stander of heavy pollution (>650 mg/kg). Inorganic phosphorus accounted for 64.5% to 93.2% of total phosphorus, and calcium-bound phosphorus was the most significant fraction of Inorganic phosphorus, while the content of Fe/Al-bound phosphorus is fewer in a ratio of 3.5% to 32.4%. The results of the adsorption-desorption of phosphorus indicated that the adsorption capacity in sediments was weak, the phosphorus from agricultural activities was less adsorbed after entering the reservoir, and the sediment was a source of phosphorus in the Huanghezhen Reservoir. In addition, the content of released phosphorus in sediments positively agrees with the concentration of Fe/Al-bound phosphorus, showing that Fe/Al-bound phosphorus plays an effective indicator in the sediment phosphorus release potential. Moreover, the high water temperature and reduced environment would promote the release of sediment phosphorus. This study provided findings regarding sediment phosphorus fractions and adsorption-release characteristics, showing that the sediment phosphorus fractions, regional climate, and hydrogeological conditions should be comprehensively considered in establishing effective measures of prevention and controlling of eutrophication in agricultural reservoirs.