Nitrogen and phosphorus metabolism together lead to a continuous increase in the environmental pollution risk in Minnan-Triangle cities

High-intensity human activities at urban scale profoundly affect the nitrogen and phosphorus cycle. However, few studies analyzed their coupled characteristic in flow process and the associated ecological risks. In this study, a 32-node model and coupling indicators were developed from a metabolic perspective to analyze the N and P budget, flow, coupling characteristics and the ecological risks arising from their emissions in Minnan-Triangle cities. Results showed that from 2000 to 2021, the N and P budget for each city continued to grow but sustained imbalance. The residential nodes of Xiamen, agricultural nodes of Zhangzhou, and industrial nodes of Quanzhou accounted for more than 10% of the total budget during the metabolic process. Nitrogen and phosphorus collectively involved in 62% of the flows number and the N/P ratio exhibited an increasing pattern along the industrial supply chain. The N/P ratio increased from 4:1 in upstream agricultural entities to 8:1 in the end- consumption entities. Emissions from high energy-consuming nodes led to a 17%, 140% and 80% increase in the environmental pollution risk in Xiamen, Zhangzhou, and Quanzhou. The standardized and unified framework and the coupling indicators constructed in this study provide a basis for future academic research. While the combination of risk assessment with process analysis and the consideration of coupling characteristic facilitate the proposal of targeted regulatory measures for key nodes or pathways.