Overestimating annual human impacts on riverine nutrient export by neglecting watershed potential loads

Excessive nitrogen (N) and phosphorus (P) inputs from intensive agriculture and rapid urbanization have become major drivers of persistent eutrophication in large lake basins worldwide, posing significant challenges to water quality management even under strict nutrient control on contemporary anthropogenic input. Using the Poyang Lake basin as a case study, we calculated Net Anthropogenic Nitrogen Input (NANI) and Net Anthropogenic Phosphorus Input (NAPI) for 32 sub-basins during 2020-2023 and estimated corresponding riverine nutrient exports. A multifactor mixed-effects regression model was then employed to characterize the relationship between anthropogenic nutrient inputs and riverine nutrient exports. Based on this framework, we applied the concept of Watershed Potential Load (WPL), defined as the persistent nutrient export at the basin outlet that remains after excluding the effects of annual anthropogenic inputs. The results indicate that NANI has significantly decreased, primarily due to reduced nitrogen fertilizer application, while NAPI increased due to the increased use of phosphate and compound fertilizers. Model-based estimates suggest that approximately 27.2% of total nitrogen and 68.7% of total phosphorus exports were attributed to WPL. Partial least squares structural equation modeling (PLS-SEM) revealed that atmospheric deposition is the predominant source of nitrogen WPL, while legacy phosphorus accumulation is the largest contributor to phosphorus WPL. These findings indicate that reducing short-term anthropogenic nutrient inputs alone may be insufficient, because long-term accumulated nutrient loads sustain elevated baseline exports and weaken management effectiveness.