Identifying sources and cycling of phosphorus in the sediment of a shallow freshwater lake in China using phosphate oxygen isotopes.

Biotic and abiotic pathways for the transformation of phosphorus (P) in the sediment of Taihu Lake, a eutrophic shallow freshwater lake in southeastern China, were studied using the oxygen isotope ratios of phosphate (δ18O P) along with sediment chemistry, X-ray diffraction, and 57Fe-Mössbauer spectroscopic methods. The results showed that δ18O P values of sediment P pools significantly deviated from equilibrium and thus allowed distinguishing potential P sources or pathways of transformation. Isotope values of authigenic P being lighter than equilibrium suggests the re-mineralization of organic matter and subsequent precipitation of apatite as the major pathway of formation of authigenic P. The δ18O P values of the Al-bound P pool (18.9–23.5‰) and ferric Fe-bound P (16.79–19.86‰) could indicate potential terrestrial sources, but the latter being closer to equilibrium values implies partial overprinting of potential source signature, most likely due to reductive dissolution and release of P and followed by partial biological cycling before re-sorption/re-precipitation with newly formed ferric Fe minerals. Oxic/anoxic oscillation and dissolution/re-precipitation reactions and expected isotope excursion are corroborated by sediment chemistry and Mössbauer spectroscopic results. These findings provide improved insights for better understanding the origin and biogeochemical cycling of P associated with eutrophication in shallow freshwater lakes. Unlabelled Image • Sediment P pools provide information on the size of labile P and recalcitrant P sinks. • δ18O P values can differentiate sources and pathways of formation of P pools. • Isotopes of Al bound P is a better recorder of terrestrial P sources than Fe-bound P pool. [ABSTRACT FROM AUTHOR]