Identification of the sources of different phosphorus fractions in lake sediments by oxygen isotopic composition of phosphate

Phosphorus (P) released from sediment driven by oxidation-reduction potential and enzymes can increase the concentrations of dissolved P in the overlying water, which trigger widespread algal blooms. Therefore, identification sediment P sources is critical for the management of P and restoration of eutrophic aquatic ecosystem. Sediment P fractions have been widely reported, whereas little is known regarding the source and pathway of different P fractions in eutrophic lake sediments. In this study, we applied chemical sequential extraction with oxygen isotopic compositions of phosphate (delta 18OP) to identify the inorganic P (Pi) source in the sediments of Lake Dianchi, China. Results show that contents of organic P (Po), HCl-Pi, NaOH-Pi, and NaHCO3-Pi of sediment were the main P fractions compared with that of sediment H2O-Pi, which was attributed to the bioavailable difference of these P fractions. Significant difference on the equilibrium values of delta 18OP (delta 18OP-eq), delta 18OP of sediment P fractions, and delta 18OP of external P was observed, which reflected the complex transformation processes of these P fractions in sediments. Further, the values of sediment delta 18ONaHCO3-Pi at some sites (17.2-22.5%o) deviated from delta 18OP-eq (14.7-19.0%o), and these values fell into the values of delta 18OP for external P (7.7-23.8%o), suggesting that sediment NaHCO3-Pi at these sites derived from external P. Comparison the values of sediment delta 18ONaOH-Pi (14.5-24.9%o) with delta 18OP-eq and delta 18O of external P sources suggested that sediment NaOH-Pi was not only from external P, but also from sedimentary Po remineralization. Pi released from sediment NaOH-Pi may be responsible for the higher values of sediment delta 18OHCl-Pi (15.2-20.8%o) than that of soils (13.1-15.3%o) and phosphate rock (8.9-12.6%o). Results gained from this study provided critical insights for the source and biogeochemical cycling of P across the sediment-water interface in the eutrophic lake.