Fig. 1. Life cycle assessment of GHG emissions from manufacturing and field use of N fertilizer in China and weighted emission factors of main processes (system boundaries are described in the main text). Atmospheric nitrogen (N2) is combined with hydrogen using energy from fossil fuels. The produced NH3 is reacted with CO2, nitric acid, hydrochloric acid, or phosphoric acid to produce different N fertilizer products. These fertilizers are transported by various means before being applied to croplands. The solid line represents the materials and N fertilizer flow. The broken line represents GHG exchanges between the fertilizer chain and the atmosphere.