Dynamic nitrogen metabolism in urban environments: Cross-sectoral and cross-media insights for sustainable management

The effective management of nitrogen pollution in urban environments presents unique challenges due to the cross-sectoral and cross-media characteristics of nitrogen migration and transformation. Using Macao as a case study, this research dynamically simulates urban nitrogen metabolism from 2000 to 2021 to explore its trends, drivers, and environmental implications. Through Element Flow Analysis, a cross-sectoral and cross-media nitrogen metabolism model was developed to identify critical pathways and key sectors in nitrogen dynamics. Results reveal a significant increase in nitrogen input, output, and environmental load during the study period. The food sector dominated urban nitrogen metabolism, contributing 46.35% of total nitrogen inputs, with an average annual input exceeding 3000 tons. Soil and water bodies experienced cumulative N inputs of approximately 5700 tons and 28000 tons, respectively, while atmospheric nitrogen flows decreased by 10.49%. The study identified insufficient wastewater treatment capacity and pipeline leakage as primary contributors to nitrogen emissions, alongside significant deficiencies in organic waste recovery. Driving force analysis highlights population growth and shifts as dominant factors influencing nitrogen metabolism. Ecological Network Analysis further reveals complex interactions within the nitrogen cycle, emphasizing the need for integrated urban management strategies. This study underscores the urgency of coordinated cross-sectoral and cross-media approaches to mitigate urban nitrogen pollution, providing actionable insights for sustainable urban resource management and policy development.