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

Abstract

This paper takes the Fuyang River in the North China Plain as an example, utilizing an optimized phosphate oxygen isotope (δ¹⁸O_P) preprocessing method and extensively collecting watershed phosphorus (P) source and overlying water δ¹⁸O_P data. Under the premise of validating the application of δ¹⁸O_P, 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 δ¹⁸O_P 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 δ¹⁸O_P’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.