Determination of Nutrient Fluxes Across the Sediment-Water Interface in a Nitrate-Rich Reservoir

This study was to quantify the interfacial flux of nutrients and to explore the potential effects of nitrate on phosphate release from sediments in a nitrate-rich reservoir. Typical sediment samples were collected from the Tianzhuang Reservoir. Porewater profiles of nutrients (Nitrate, ammonium, and soluble reactive phosphorus) and diffusive fluxes of these nutrients across the sediment-water interface were determined. The interfacial flux of NH₄ ⁺-N, NO₃ ^--N, and PO₄ ^3--P were 141.39~175.83 mg/(m²ldrd), -43.68~-42.50 mg/(m²ldrd), and 0.14 mg/(m²ldrd), respectively. The results show that the sediment is an important source for NH₄ ⁺-N, and a sink for NO₃ ^--N. The high nitrate rate into sediments indicates that denitrification is actively occurring in the anoxic surflcial sediments. In addition, the increase of phosphate concentration coincides with the decrease of nitrate concentration in the porewaters of sediments ranging from the interface down to about 7.5 cm. These evidence suggest that nitrate in overlying water may retard the phosphate release from the redox-sensitive sediments. This retard effect has to be considered when countering nitrogen pollution by decreasing external loadings of nitrate.