The nitrogen footprints of China's major urban agglomerations: understanding regional challenges to advance sustainable development

The overgrowth of reactive nitrogen emissions (Nr, all species of nitrogen except N-2 gas) is a major cause of environmental pollution, especially in rapidly urbanizing regions. The nitrogen footprint (NF) indicator has been widely used to assess Nr losses occurring from the consumption of food and energy. We undertake the first attempt to apply NF methods to explore the spatial-temporal NF characteristics of major urban agglomerations in China between 2000 and 2019, and find that the highest level of annual NF (average 3868 Gg N yr(-1)) was produced by the Yangtze River Delta urban agglomeration (YRDUA), followed by the Beijing-Tianjin-Hebei urban agglomeration (BTHUA) (average 2657 Gg N yr(-1)). Their NF growth rates showed similar downward trends during the study period, while the Pearl River Delta urban agglomeration (PRDUA) (average 1528 Gg N yr(-1)) retained a higher growth rate. The average proportions of food NF (FNF) in BTHUA, YRDUA and PRDUA were 57.64%, 68.64% and 66.79%, respectively. Compared to the FNF, the energy NF gradually plays a more important role in China's urban agglomerations compared to other countries. Analysis of the underlying drivers showed that an increasing urbanization rate boosted the NF of YRDUA, and rising GDP per capita significantly contributed to the NF growths of BTHUA and PRDUA. Through scenario analysis, we found that shifting to healthy dietary patterns and a partial substitution of fossil fuels with clean energy, as well as improvements in rural wastewater treatment, could contribute to NF reductions by 2030. The largest potential NF reduction is predicted in PRDUA (29% reduction), followed by YRDUA (23% reduction) and BTHUA (18% reduction). The energy reduction scenario is considered to be the most realistic in reducing the NF. We demonstrate the potential of the NF as a tool for the assessment of sustainable development in urban agglomeration, which may prove instructive for broader research on sustainable Nr management.