Using a Multi-isotope Approach and Isotope Mixing Models to Trace and Quantify Phosphorus Sources in the Tuojiang...

Identifying phosphorus (P) sources is critical for solving eutrophication and controlling P in aquatic environments. Phosphate oxygen isotopes (δ¹⁸O_p) have been used to trace P sources. However, the application of this method has been greatly restricted due to δ¹⁸O_P values from the potential source having wide and overlapping ranges. In this research, P sources were traced by combining δ¹⁸O_p with multiple stable isotopes of nitrogen (δ¹⁵N), hydrogen (δD), and dissolved inorganic carbon (δ¹³C). Then, a Bayesian-based Stable Isotope Analysis in R (SIAR) model and IsoSource model were used to estimate the proportional contributions of the potential sources in the Tuojiang River. δ¹⁸O_p was not in equilibrium with ambient water, and statistically significant differences in the δ¹⁸O_p values were found between the potential sources, indicating that δ¹⁸O_p can be used to trace the P sources. δ¹⁵N, δD, and δ¹³C could assist δ¹⁸O_p in identifying the main sources of P. The SIAR and IsoSource models suggested that industrial and domestic sewage was the largest contributor, followed by phosphate rock and phosphogypsum and agricultural sewage. The uncertainty of the calculation results of the SIAR model was lower than that of the IsoSource model. These findings provide new insights into tracing P sources using multiple stable isotopes in watersheds.