Patterns change and determinants of urban nitrogen metabolism in Chinese megacities

Many studies on the spatiotemporal variations of urban nitrogen metabolism have been conducted widely; however, a detailed comparative study on the changing spatiotemporal patterns of nitrogen metabolism in megacities under a unified and consistent framework would provide a new direction for research. This study quantitatively estimated the changing patterns, metabolic characteristics, and driving forces of urban nitrogen metabolism in the four Chinese megacities of Beijing, Tianjin, Shanghai, and Chongqing from 2000 to 2016. These data were integrated into a coupled human-natural nitrogen flow model with high-quality database, a life cycle analysis combined with regional disparity characteristics, and a newly extended time serial logarithmic mean divisia index (LMDI) decomposition analysis. Results show that growth rates of all Nr inputs and outputs increased annually at varying degrees in the four megacities, with fossil fuel combustion and associated NOₓ emissions comprising over 50% of the flows in Beijing, Tianjin, and Shanghai. The changes in the Nr flow structure in Chongqing, which are mainly attributed to agricultural and industrial production processes, characterized its heavy reliance on primary and secondary industries. This indicates that local resource endowments control internal Nr metabolism and consequently lead to high exports of fertilizer and food commodities. Sankey diagrams show the continuous reduction of local Nr production capacity complemented by increased food and energy imports to meet growing Nr consumption in Beijing and Shanghai. In all cities, economic development is the most dominant factor contributing to the growth of Nr inputs, with partial offsets from the improvements in Nr production intensity effect. Different cities and industrial structures vary considerably in the effect that driving forces have on shaping Nr inputs. Since cities are hot spots of consumption and pollution in a region or country, identifying the abundant datas on knowing N flows between cities would exert a profound impact on scales ranging from regional to national. Understanding the Nr metabolism in these megacities could offer valuable insights for city planners in formulating comprehensive methods to promote sustainable Nr management and reduce environmental pressures.