Development of a plant carbon-nitrogen interface coupling framework in a coupled biophysical-ecosystem-biogeochemical model (SSiB5/Triffid/DayCent-SOM v1.0): Its parameterization, implementation, and evaluation

Abstract. Plant and microbial nitrogen (N) dynamics and N availability regulate the photosynthetic capacity and capture, allocation, and turnover of carbon (C) in terrestrial ecosystems. Studies have shown that a wide divergence in representations of N dynamics in land surface models leads to large uncertainty in the biogeochemical cycle of the terrestrial ecosystems and then in climate simulations as well as the projections of future trajectories. In this study, a plant C-N interface coupling framework is developed and implemented in a coupled biophysical-ecosystem-biogeochemical model (SSiB5/TRIFFID/ DayCent-SOM v1.0). The main concept and structure of this plant C-N framework and its coupling strategy are presented. This framework takes more plant N-related metabolism processes into account. For instance, plant resistance and self-adjustment is represented by a dynamic C/N ratio for each plant functional type (PFT). Furthermore, when available N is less than plant N demand, plant growth is restricted by a lower maximum carboxylation capacity of Rubisco (Vmax) level, reducing gross primary productivity (GPP). In addition, a module for plant respiration rates is introduced by adjusting the respiration with different rates at different plant components for the same N concentration. Since insufficient N can potentially give rise to lags on plant phenology, phenology scheme is also adjusted with a lag factor related to N processes. All these considerations ensure a more comprehensive incorporation of N regulations to plant growth and C cycling. This new approach has been tested systematically to assess the effects of this coupling framework and N limitation on the terrestrial carbon cycle. Long term measurements from both flux tower sites with different PFTs and global satellite-derived products are employed as references to assess these effects. The results show a general improvement with the new plant C-N coupling framework, with more consistent emergent properties, such as GPP and leaf area index (LAI), compared to observations. The main improvements occur in tropical Africa and boreal regions, accompanied by a decrease of the bias in global GPP and LAI by 16.3 % and 27.1 %, respectively.