The influence of crop and chemical fertilizer combinations on greenhouse gas emissions: A partial life-cycle assessment of fertilizer production and use in China

Different crops and fertilizer combinations result in different greenhouse gas (GHG) emissions. To investigate GHG emissions among crops with different chemical fertilizer production and use processes, we use a partial life-cycle based greenhouse gas footprint approach to evaluate total GHG emissions, emissions per unit area, and emissions per unit yield of nine crops receiving applications of seven chemical fertilizers for the period 1998 to 2016 in China. Our results show that the life-cycle GHG emissions in China increased by 35% over this 18-year period. Chemical fertilizer use has a greater influence than chemical fertilizer production, as it emitted over half of total GHG emissions. Due to different fertilizer types/uses and cultivated areas, GHG emissions varied among crops. The three main grain crops including rice, wheat, and maize contributed half of the total GHG emissions, mainly due to their larger cultivated areas than other crops. Crops with smaller cultivated areas but larger fertilizer application rates also contributed nonnegligible GHG emissions. For example, vegetable production increased nearly two-fold and accounted for about 20% of total GHG emissions, due to high applications of urea, NPK compound fertilizer, and diammonium phosphate. Fruit crops contributed both much higher GHG emissions and GHG emissions per cultivated area than most other crops, resulting from high application intensities of urea and NPK compound fertilizer. Among the chemical fertilizers, urea generated the greatest GHG emissions (~60% of the total GHG emissions) due not only to its relatively high application across the crops, but also its relatively high GHG emission intensities for both production and application. Finally, we explore potential to mitigate the life-cycle GHG emissions of chemical fertilizers nationally, through varying fertilizer application rates, fertilizer types, and cultivated areas. For example, our scenarios modify the cultivated areas of the three main grain crops, reduce urea and NPK compound fertilizer for fruits and vegetables, and substitute some urea and diammonium phosphate with superphosphate and potassium chloride. Our study highlights the importance of different chemical fertilizer and crop combinations. It can help to mitigate the GHG emissions comprehensively and objectively.