The importance of climate change and nitrogen use efficiency for future nitrous oxide emissions from agriculture

Nitrous oxide (N₂O) is an important greenhouse gas and ozone depleting substance. Previous projections of agricultural N₂O (the dominant anthropogenic source) show emissions changing in tandem, or at a faster rate than changes in nitrogen (N) consumption. However, recent studies suggest that the carbon dioxide (CO₂) fertilization effect may increase plant N uptake, which could decrease soil N losses and dampen increases in N₂O. To evaluate this hypothesis at a global scale, we use a process-based land model with a coupled carbon-nitrogen cycle to examine how changes in climatic factors, land-use, and N application rates could affect agricultural N₂O emissions by 2050. Assuming little improvement in N use efficiency (NUE), the model projects a 24%-31% increase in global agricultural N₂O emissions by 2040-2050 depending on the climate scenario - a relatively moderate increase compared to the projected increases in N inputs (42%-44%) and previously published emissions projections (38%-75%). This occurs largely because the CO₂ fertilization effect enhances plant N uptake in several regions, which subsequently dampens N₂O emissions. And yet, improvements in NUE could still deliver important environmental benefits by 2050: equivalent to 10 Pg CO₂ equivalent and 0.6 Tg ozone depletion potential.