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 (δ18OP) 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 δ18OP (δ18OP-eq), δ18OP of sediment P fractions, and δ18OP of external P was observed, which reflected the complex transformation processes of these P fractions in sediments. Further, the values of sediment δ18ONaHCO3-Pi at some sites (17.2–22.5‰) deviated from δ18OP-eq (14.7–19.0‰), and these values fell into the values of δ18OP for external P (7.7–23.8‰), suggesting that sediment NaHCO3-Pi at these sites derived from external P. Comparison the values of sediment δ18ONaOH-Pi (14.5–24.9‰) with δ18OP-eq and δ18O 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 δ18OHCl-Pi (15.2–20.8‰) than that of soils (13.1–15.3‰) and phosphate rock (8.9–12.6‰). 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. •The δ18OP was employed to identify the sediment P source of a large eutrophic lake.•NaOH-Pi and HCl-Pi were from external P and sediment P fractions redistribution.•The δ18OP is an effective tool to identify the source and pathway of sediment P.