Riverine Phosphorus Source Apportionment Based on the Phosphate Oxygen Isotope in the Ankang Catchment, Shaanxi, China

The Ankang catchment, the primary water-producing area in the upper reaches of the Danjiangkou Reservoir catchment, experiences immense pressure on its water environmental carrying capacity. However, existing studies remain unable to differentiate between anthropogenic and natural inputs of riverine phosphorus (P), constraining catchment nutrient management. To address the issue of riverine P sources in the Ankang catchment, this study, based on data from three hydrological phases, applied a Bayesian phosphate oxygen isotope mixing model for the first time to quantify the natural and anthropogenic sources of riverine P. Subsequently, statistical methods were employed to reveal the mechanisms controlling riverine P export, thereby providing a scientific basis for riverine P management. Quantitative apportionment results showed that riverine P was primarily derived from natural sources (48.6%) in the normal phase. During the wet phase, the proportion of natural sources increased in some tributaries due to enhanced precipitation. The contribution of agricultural sources increased to 46.5% during the dry phase, driven by a new agricultural production cycle. Correlation analysis indicates that conserving water and increasing water use efficiency lower wastewater loading, and that decreasing fertilizer application or improving fertilizer use efficiency contributes to reducing agricultural inputs.