Evaluation of N-limitation effect in 1% CO2 scenario

Rising atmospheric CO2 enhances the land carbon (C) uptake, providing a negative feedback mechanism for atmospheric CO2. At the same time, CO2-driven warming of land and air temperature tends to weaken land carbon storage, providing a primarily positive feedback on climate (i.e. intensifying climate change). The magnitude of these effects is, beside others, mediated by the nitrogen (N) content in land, which attenuates the land C response to atmospheric CO2 and climate [Kou-Giesbrecht et al., 2025]. Comprehensive Earth System Models (ESMs) have been developed to project effects from different feedbacks on Earth’s climate change, but to date not all ESMs take into account effects from the coupled C-N cycles.

ICON is a state of the art ESM [Jungclaus et al., 2022], yet its initial land surface model (LSM) implementation JSBACHv4.3 [Schneck et al., 2022] does not include a representation of the N-cycle. Recently, the QUINCY model [Thum et al., 2019] was integrated into the ICON framework. While the geophysical processes of the initial LSM JSBACHv4.3 are taken over, the new QUINCY configuration provides an alternative representation of the vegetation and biogeochemical processes, including a more realistically representation of vegetation structure (e.g. by coupling the LAI to the available carbon) and a comprehensive representation of the terrestrial N-cycle processes.

In the current study, we apply ICON in its ICON-XPP configuration [Müller et al. 2025] and with QUINCY as configuration for ICON-Land to evaluate the N-effect on land C uptake under conditions of 1%CO2 increase in the atmosphere. For this purpose, two numerical AMIP experiments (sea surface temperature and sea ice are prescribed) were carried out for the period of 1850-2019. In one experiment only C cycle was considered, in another - C and N cycles. The modelling results will be analysed to evaluate a possible N limitation effect under the conditions of increasing atmospheric CO2.