Anthropogenic intensification of urban reactive nitrogen inputs and potential to mitigate nitrogen pollution in Guangzhou, China

Human activities intensify reactive nitrogen (Nr) inputs to the urban system. Excess Nr loss negatively affects environment and human health. Compared with large-scale studies, greater attention should be paid to the urban system as a burgeoning and important Nr creation source. In this study, we investigated the anthropogenic effects on urban Nr inputs (1995-2015) and assessed the potential to mitigate Nr pollution in Guangzhou, China. Here, we show that compared to the national level, the Nr input structure in urban systems is greatly dependent on external Nr inputs, massive Nr emissions during fossil fuel combustion and increasing Haber-Bosch N fixation associated with industrial synthetic production. The relatively slow increase in Nr inputs with rapid GDP growth shows a decoupling of N with economic development. Under baseline conditions, Nr inputs in 2050 can be assumed to rise to 112-138% of the 2015 value. Under the mitigation scenario involving reducing Nr emissions from fossil fuel combustion, increasing recycling rate of industrial N, controlling per capita demand for food and materials and improving N use efficiency, the Nr pollution possibly decreases to 52-64% of the 2015 levels. The combined mitigation scenario suggests that the biophysical potential for a sustainable future for urban Nr use exists. This study provides decision-makers with a novel view and rational strategies for urban N management.