Nitrogen flows associated with food production and consumption system of Shanghai

The release of reactive nitrogen (N_r) 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_r 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_r inputs, N_r use efficiency, and N_r surpluses/deficits in the food production and consumption system were also investigated. The results indicated that the main sources of N_r 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_r input in agricultural production, 37.32–76.57% of N_r input in livestock and poultry farming, and 35.38–59.37% of N_r input in food consumption, respectively. The main forms of N_r 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_r output in agricultural production, 36.78–55.18% of N_r output in livestock and poultry farming, and 85.36% of N_r output in food consumption, respectively. From 2000 to 2018, the N_r 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_r 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_r 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.