Impact of Past and Future Nitrogen Deposition Pathways on the Terrestrial Carbon Sink and N2O emissions

Atmospheric nitrogen deposition has long been recognized as an important external driver of terrestrial carbon uptake by alleviating ecosystem nitrogen limitation. However, it also enhances terrestrial nitrous oxide (N₂O) emissions, leading to a potential climate trade-off under future mitigation pathways. Despite extensive research, the relative roles of anthropogenic emissions and climate change in regulating nitrogen deposition and their impacts on the terrestrial carbon sink and N₂O emissions remain poorly constrained at the global scale. Here, we quantify the impacts of past and future nitrogen deposition pathways on terrestrial carbon cycling and N₂O emissions from 1850 to 2100 by generating historical and future nitrogen deposition scenarios with the GEOS-Chem atmospheric chemistry transport model under the SSP1-2.6 and SSP3-7.0 pathways. We use these data and climate forcing from ISIMIP to drive a global carbon-nitrogen cycle model (ICON-Land in QUINCY configuration) in a factorial design to isolate climate and emission effects, while keeping the land cover fixed at 2014 land use conditions.

We find that increasing nitrogen deposition during the historical period (1850–2014) substantially enhanced terrestrial carbon uptake – contributing approximately 0.15 Pg C yr^-1 to the global land carbon sink – but also accounted for about 0.82 Tg N yr^-1 of terrestrial N₂O emissions. In the future period (2015-2100), declining nitrogen deposition under strong mitigation (SSP1-2.6) leads to a decline of the terrestrial carbon sink and a reduction of terrestrial N₂O emissions, whereas elevated nitrogen deposition under weak mitigation (SSP3-7.0) enhances both terrestrial carbon sequestration and N₂O emissions. Climate change further modulates these responses by altering nitrogen deposition patterns, amplifying both positive and negative feedbacks. These results highlight a fundamental trade-off within the nitrogen–carbon–climate system and underscore the importance of explicitly representing nitrogen processes in earth system carbon budget assessments and mitigation strategies.