Multiscale Quantitative Identification of P Sources in a Watershed during the Rainy Season Using a Phosphate Oxygen Isotope

This paper takes the Fuyang River in the North China Plain as an example, utilizing an optimized phosphate oxygen isotope (δ18OP) preprocessing method and extensively collecting watershed phosphorus (P) source and overlying water δ18OP data. Under the premise of validating the application of δ18OP, this study enhances the Bayesian mixing model priors using the end-member mixing model and iteratively corrects the total phosphate load to achieve quantitative identification of P sources at both the watershed and administrative division scales during the rainy season. It also summarizes a methodological framework for the application of δ18OP in watersheds. The results show that the total phosphate load at the watershed scale amounted to 241.36 kg/d, with contributions from the tributary inflow, sediments, riparian soils, wastewater treatment plant (WWTP) effluents, and street dust, accounting for 38.8, 23.4, 17.3, 16.2, and 4.3%, respectively. Variations in P source contributions and loads across administrative divisions reflect regional economic and land-use diversity. P management strategies should adopt region-specific approaches, considering diverse land uses and their effects on P transport. This study demonstrates δ18OP’s effectiveness in quantitatively identifying multiscale P sources and calculating phosphate loads during the rainy season, providing technical support for precise watershed phosphate load reduction.