Sources and Cycling of Phosphorus in the Sediment of Rivers along a Eutrophic Lake in China Indicated by Phosphate Oxygen Isotopes

Sources and cycling of phosphorus in the sediment of inflow rivers (FL, TY and QY) along the eutrophic Chaohu lake, China were discussed by using the oxygen isotope compositions of phosphorus fractions of Fe/Al–Pi and Ca–Pi (SMT) and Fe–Pi, Auth-Pi, and Detr-Pi (SEDEX). The δ18OFe/Al-Pi (11.9‰–19.6‰) and δ18OCa-Pi (9.6‰–18.2‰) in FL, TY, and QY river sediments deviated far from the isotopic equilibrium (δ18Op-equ, 19.4‰–21.5‰) in the Chaohu watershed. Depletion of δ18OFe/Al-Pi in river sediments indicated a likely source of 18O-depleted phosphates from the hydrolysis of organic phosphorus. The δ18OCa-Pi values in FL sediments ranged from 9.6‰ to 13.7‰, indicating phosphate source from weathered soils; while those in TY and QY ranged between 11.5‰ and 18.2‰, indicating the Ca–Pi was partly inherited from agricultural fertilizers and partly taken from domestic sewage. Significant correlations (r = 0.85, P < 0.01) between δ18OFe/Al-Pi and δ18OCa-Pi of FL, TY, and QY river sediments indicated common phosphate sources for Fe/Al–Pi and Ca–Pi, and thus suggested the new formation of secondary Ca-bound phosphates. These recycling of phosphorus in FL, TY, and QY indicated by δ18OFe/Al-Pi and δ18OCa-Pi were supported further by the δ18OFe-Pi, δ18OAuth-Pi and δ18ODetr-Pi in new TY sediments collected independently. This study provides a new perspective on understanding the sources and cycling of phosphorus in aquatic environments.