Driving forces of nitrogen flows and nitrogen use efficiency of food systems in seven Chinese cities, 1990 to 2015

The effects of population growth (PG), dietary changes (DC), native rural-to-urban migration (NM), migration from regions distant from the cities (M), and agricultural patterns and practices (AP) on N use in food systems and the food trade, and on apparent and virtual nitrogen (N) and N use efficiencies (NUE), at the city scale, are not well understood. Here we selected seven Chinese cities as the study subjects, analyzed the food trade effects on apparent and virtual N inputs and NUE, and quantified the relative magnitudes of these factors on N inputs to cities' food systems during 1990–2015, by designing several scenarios. Our results show that food-sink cities are relying more and more on external food and feed, but in 2015 they transferred 33.8–74.9% of their N input for food or feed productions to areas outside their boundaries, and the food trade showed different effects on the virtual N cost of food N consumption. Apparent NUEs of food systems were 33.1–74.9% higher than those calculated from virtual N costs in Beijing, Tianjin, Shanghai, Lanzhou and Xiamen in 2015. But in cities that export large amounts of food and feed—for example, Chongqing and Changchun—apparent NUE was underestimated by 4.0–46.4% relative to virtual NUE. Native PG, DC, NM, M, and AP accounted for 1.2–14.1%, −6.6–30.0%, 0.6–8.2%, −7.7–131.0%, and −43.8–12.8%, respectively, of the increase in virtual N inputs associated with cities' food systems in 2015, compared to 1990. Our study concludes that M, DC, and AP changes should be considered for mitigating N input in these Chinese cities, and virtual N exports induced by the food trade should also be included if the city is a net food exporter. Selective food trade could help improve the NUE of cities' food systems, and virtual NUE should be used as an indicator, rather than apparent NUE. [Display omitted] •Apparent and virtual N input and driving forces were analyzed in seven cities.•Virtual N input was significantly higher than apparent N input in majority of cities.•Food trade has some effect on virtual N cost of cities' food N consumption.•Migration, dietary changes and agricultural practices are the key drivers of N input.•Virtual NUE is an accurate indicator for cities' food systems' NUE comparisons.