Carbon footprint of synthetic nitrogen under staple crops: A first cradle-to-grave analysis

More than half of the world’s population is nourished by crops fertilized
with synthetic nitrogen (N). However, N fertilization is a major source of
anthropogenic emissions, augmenting the carbon footprint (CF). To date, no
global quantification of the CF induced by N fertilization of the main
grain crops has been performed, and quantifications at the national scale
have neglected the CO2 assimilated by plants. A first Cradle-Grave life
cycle assessment was performed to quantify the CF of the N fertilizers’
production, transportation, and application to the field and the uses of
the produced biomass in livestock feed, human food, and biofuel
production. We quantified direct and indirect inventories emitted or
sequestered by the N fertilization of grain crops (wheat, maize, and
rice). Grain food produced with N fertilization had a net CF of 7.4 Gt
CO2eq. in 2019 after excluding the assimilated C in plant biomass, which
accounted for a quarter of the total CF. The Cradle (fertilizer production
and transportation), Gate (fertilizer application, and soil and plant
systems), and Grave (feed, food, biofuel, and losses) stages contributed
to the CF by 2, 11, and 87%, respectively. Although Asia was the top grain
producer, North America contributed 38% of the CF due to the greatest CF
of the Grave stage (2.5 Gt CO2eq.). The CF of grain crops will increase to
21.2 Gt CO2eq. in 2100, driven by the rise in N fertilization to meet the
growing food demand without actions to stop the decline in N use
efficiency. To meet the targets of climate change, we introduced an
ambitious mitigation strategy, including the improvement of N agronomic
efficiency (6% average target for the three crops) and manufacturing
technology, reducing food losses, and global conversion to healthy diets,
whereby the CF can be reduced to 5.6 Gt CO2eq. in 2100.