Agglomeration- and city-level resilience of the nitrogen metabolism network

Urban agglomerations face a dual challenge of nutrient accumulation and vulnerable industrial networks. Yet resilience assessment from a nitrogen metabolism perspective remains limited. Here, we developed an inter-city nutrient metabolism model to investigate how in- and transboundary nitrogen flow networks shape resilience at both city and agglomeration levels, using the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) as a case study. Results reveal highly uneven nitrogen output: upstream cities along the Pearl River bear disproportionate environmental burdens to supply food and material for the urban agglomeration; midstream cities act as regional consumption and distribution hubs; and downstream cities function as major consumption and emission centers. Such differentiated metabolic roles give rise to distinct resilience profiles across cities, with industrial diversity emerging as a key factor strongly correlated with overall resilience. Moreover, the urban agglomeration as a whole retains substantially higher stability (reflecting structural resilience) than any individual city. However, the functional resilience of the urban agglomeration is largely hindered by a few cities with weaker internal nitrogen cycling.