Tracking Carbon and Ammonia Emission Flows of China's Nitrogen Fertilizer System: Implications for Domestic and International Trade.

China is the world's largest producer, consumer, and exporter of synthetic nitrogen (N) fertilizer. To assess the impact of domestic demand and international exports, we quantified the life-cycle COeq and ammonia (NH) emissions by tracking carbon (C) and nitrogen (N) flows from coal/gas mining through ammonia production to N fertilizer production, application, and export. In 2020, China's N fertilizer system emitted 496.04 Tg of COeq and 3.74 Tg of NH, with ammonia production and N fertilizer application processes contributing 36 and 85% of the life-cycle COeq and NH emissions, respectively. As the largest importers of N fertilizer, India, Myanmar, South Korea, Malaysia, and the Philippines collectively shifted 112.41 Tg of COeq. For every ton of N fertilizer produced and used in China, 16 t of COeq and 0.18 t of NH were emitted, compared to 9.7 t of COeq and 0.13 t of NH in Europe. By adopting currently available technologies, improving N fertilizer utilization efficiency and employing nitrification inhibitors could synergistically reduce COeq emissions by 20% and NH emissions by 75%, while energy transformation efforts would primarily reduce COeq emissions by 59%. The production of ammonia using green electricity or green hydrogen could significantly enhance the decarbonization of China's N fertilizer system.