Nitrogen flows associated with food production and consumption system of Shanghai.

The release of reactive nitrogen (N) from food production and consumption constitute the primary source of nitrogen pollution. However, nitrogen flows and the driving factors of food chain of Shanghai, China have not been previously studied. Here, we used a substance flow analysis model to analyze the changes in N inputs and outputs in agricultural production, livestock and poultry farming, and food consumption related to the Shanghai food chain between 2000 and 2018. The driving forces of N inputs, N use efficiency, and N surpluses/deficits in the food production and consumption system were also investigated. The results indicated that the main sources of N input in the food production and consumption system were nitrogen fertilizers, livestock and poultry feed from external sources, and plant-based foods, which accounted for 36.28-59.45% of N input in agricultural production, 37.32-76.57% of N input in livestock and poultry farming, and 35.38-59.37% of N input in food consumption, respectively. The main forms of N outputs were surplus nitrogen in the soil, excretal nitrogen from livestock and poultry animals, and excretal nitrogen from humans, which accounted for 38.2-48.89% of N output in agricultural production, 36.78-55.18% of N output in livestock and poultry farming, and 85.36% of N output in food consumption, respectively. From 2000 to 2018, the N inputs per unit area from agricultural production decreased at a rate of 20.42% before 2012, and then increased at a rate of 5.72%. Moreover, the N use efficiency of agricultural production component of Shanghai was at a low level, only 18.43-27.6%. Cultivation area of crops was the main driving forces of the N input to food production and consumption system. These results provide essential data for controlling nitrogen pollution caused by Shanghai food production and consumption, which can serve as a reference for administrative agencies in formulating policies.