Changing nitrogen cascade in urban food system under rapid urbanization

As the most important source of anthropogenic reactive nitrogen (Nr), nitrogen (N) cascade and management in food system has become a focus of global research and policy debates. For China that consumes 1/3 of the global N... [ view full abstract ]

As the most important source of anthropogenic reactive nitrogen (Nr), nitrogen (N) cascade and management in food system has become a focus of global research and policy debates. For China that consumes 1/3 of the global N fertilizer, to secure domestic food supply while mitigating adverse environmental impacts from Nr remains a great challenge. Particularly, China is experiencing a significant transition of socioeconomic structure brought by rapid urbanization. What are the influences of growth of urban population, shift of diet pattern, change of lifestyle, as well as shrink of peri-urban arable land (due to urban sprawl) on N cycling of urban food system? Where are the opportunities for sustainable N management of urban food system in the context of rapid urbanization? Questions as such are gaining increasing concern.

Taking Xiamen, a rapidly urbanizing city in China, as an example, this research develops an N cascade model of urban food system by using substance flow analysis. The cascade model consists of four major sub-systems, namely crop production, animal production, household consumption and waste disposal. Nr fluxes and N use efficiencies (NUEs) were calculated to reveal N metabolic characteristics of urban food system, and data from 1993 to 2014 were collected to support a dynamic study. In addition, changing N cascade was coupled with socioeconomic dynamics in Xiamen city to elucidate empirically the impact of urbanization on N cycling of urban food system.

The results show that the total N input into the urban food system decreases from 29.36 Gg in 1993 to 24.10 Gg in 2014, with the applied N for local food production declining from 29.36 to 16.05 Gg while the net imported food N soaring from -0.74 to 8.05 Gg. During the same period, the total recycled N decreases from 7.34 to 4.84 Gg, and the recycling rate descends from 19.92% to 16.42%. Other than the scaling down of local agriculture, breakdown of nutrient recycling route caused by urbanized lifestyle is another reason behind. The total Nr losses into the environment decrease from 19.18 to 15.95 Gg, with roughly 32% into the atmosphere, 50% into surface water and 18% into land. It is noteworthy that Nr losses from production stage reduce from 15.92 to 9.40 Gg, whereas losses from consumption stage increase from 3.27 to 6.55 Gg, indicating transfer of pollution load from production to consumption end during urbanization process. The NUE of the whole food system in Xiamen (excluding food import) exhibits slightly downward trend during the study period, ranging between 14.77% and 27.50%. Besides conventional approach to improving NUE and decreasing Nr losses in agroecosystem, several recommendations are proposed for sustainable N management in urbanizing societies. These include improving nutrient recycling rate through promotion of urban agriculture and kitchen waste composting, reducing food waste, enhancing N-removal of sewage treatment, and constructing treatment system for rural household sewage.