Spatial nutrient load analysis based on output coefficient method and land use pattern identification: a case study of Shiyan City

The application rate of nitrogen and phosphate fertilizers has increased significantly in China over the past few decades. However, the over-dozed nutrients also lead to the serious problem of eutrophication in inland and coastal waters. The control of nitrogen and phosphorus nutrients is generally considered to be an important part of reducing lake eutrophication and cyanobacteria blooms. Currently, the accurate identification of nutrient sources and the quantitative deconstruction of the spatial representation of excess nutrient concentration still require special attention. Based on the statistical data of pollutants in Shiyan City and the relevant monitoring reports of eco-environmental protection departments, the source composition and spatial load distribution of nitrogen and phosphorus nutrients were analyzed by the output coefficient method and land use pattern identification. The results showed that total nitrogen and total phosphorus of Shiyan City were mainly from non-point sources, accounting for 81.84% and 80.08%, respectively, and point sources accounted for less. The total nitrogen pollution load mainly came from farmland runoff, accounting for 45.72% of the total, followed by dry and wet deposition pollution sources, accounting for 11.43% of the total. The total phosphorus pollution load also mainly came from farmland runoff, accounting for 30.51% of the total, followed by soil erosion, accounting for 22.28% of the total. The highest load intensity of total nitrogen into the river was 22.71 t/km², and the highest load intensity of total phosphorus into the river was 5.22 t/km².