The environmental and socioeconomic trade-offs of importing crops to meet domestic food demand in China

China increasingly relies on agricultural imports, driven by its rising population and income, as well as dietary shifts. International trade offers an opportunity to relieve pressures on resource depletion and pollution, such as nitrogen (N) pollution, while it poses multiple socioeconomic challenges, such as food availability. To quantify such trade-offs considering the roles of different crop types, we developed a unique crop-specific N budget database and assessed the impacts of the crop trade on multiple sustainability concerns including N pollution caused by crop production, crop land area, independence of food supply, and trade expenditures. We quantified the "virtual" N inputs and harvested areas, which are the amount of N inputs and land resources used in exporting countries for China's crop import. In addition, we proposed the concepts of "alternative" N inputs and harvested area to quantify the resources needed if imported crops were produced in China. By comparing results from "alternative" and "virtual" concepts, we assessed the role of trade in Chinese crops over the past 30 years (i.e., 1986-2015) in alleviating N pollution and saving cropland in China and the world. Crop imports accounted for 31% of Chinese crop N consumption in 2015, and these crop imports eased the need for an additional cropland area of 62 million ha. It also avoided an N surplus by 56 and 36 Tg (Tg = 109 kg) for China and the world respectively but led to $621 billion crop trade expenditures over the 30-year period. The N pollution damage avoided by crop imports in economic terms was priced at $22 ±16 billion in 2015, which is lower than the crop trade expenditures but may be surpassed in the future with the development of the Chinese economy. Optimizing a crop trade portfolio can shift domestic production from N-intensive crop production (e.g., maize, fruits, and vegetables) to N-efficient crop production (e.g., soybeans), and consequently mitigate an N surplus by up to 12%. Improving N use efficiency for individual crops can further increase the mitigation potential of N surplus to 30-50%, but requires technology advancement and policy incentives.