N2O emissions from the global agricultural nitrogen cycle - current state and future scenarios

Reactive nitrogen (N-r) is not only an important nutrient for plant growth, thereby safeguarding human alimentation, but it also heavily disturbs natural systems. To mitigate air, land, aquatic, and atmospheric pollution caused by the excessive availability of N-r, it is crucial to understand the long-term development of the global agricultural N-r cycle. For our analysis, we combine a material flow model with a land-use optimization model. In a first step we estimate the state of the N-r cycle in 1995. In a second step we create four scenarios for the 21st century in line with the SRES story-lines. Our results indicate that in 1995 only half of the N-r applied to croplands was incorporated into plant biomass. Moreover, less than 10 per cent of all N-r in cropland plant biomass and grazed pasture was consumed by humans. In our scenarios a strong surge of the N-r cycle occurs in the first half of the 21st century, even in the environmentally oriented scenarios. Nitrous oxide (N2O) emissions rise from 3 Tg N2O-N in 1995 to 7-9 in 2045 and 5-12 Tg in 2095. Reinforced N-r pollution mitigation efforts are therefore required.