Assessing the impact of an explicit representation of the nitrogen cycle on SIF and GPP dynamics across European sites

The representation of gross primary production (GPP) in land surface models remains highly uncertain, despite GPP being a key driving component of the terrestrial carbon cycle (Gier et al., 2024). These uncertainties mainly arise from both the lack of direct measurements of GPP above the leaf scale and an incomplete representation of plant physiological processes (in terms of both parameter values and equations), in particular the links between carbon assimilation and nutrient availability.

Solar-induced chlorophyll fluorescence (SIF) has therefore emerged as a proxy of photosynthetic activity and of GPP by terrestrial ecosystems (Li et al., 2018). To further constrain parameters controlling photosynthetic activity, satellite-based SIF observations can be assimilated (from the TROPOSIF product, and, in the near future, the FLEX fluorescence product), as SIF provides information on plant physiological traits that regulate photosynthetic activity and GPP.

A fluorescence module previously developed for ORCHIDEE (Bacour et al., 2019) enables the simulation and assimilation of SIF observations. The ORCHIDEE-N land surface model now includes an explicit representation of the nitrogen cycle (Vuichard et al., 2019), allowing a more mechanistic description of photosynthesis through nitrogen limitations on key leaf traits controlling GPP, such as chlorophyll and Rubisco contents.

Integrating the fluorescence module into a model that explicitly represents leaf nitrogen limitation is expected to improve the simulation of both SIF and GPP by providing a more realistic description of chlorophyll content and photosynthetic capacity. In this study, an updated fluorescence module is implemented in ORCHIDEE-N to consistently link nitrogen availability, SIF, and photosynthetic activity. 

We present a first intercomparison of these two model versions (with and without the nitrogen cycle) based on the seasonal cycles of GPP and SIF at seven observational sites in Europe. These sites are drawn from the AustroSIF database (Martini et al., in prep.), which integrates in situ measurements of eddy-covariance fluxes (used to estimate GPP), SIF, and pulse-amplitude modulated fluorescence measurements. 

So far, neither the fluorescence model parameters nor those of the nitrogen-explicit module have been optimised in this new version. This preliminary study paves the way for assimilating both site-level data and satellite-derived SIF retrievals to further constrain the model.

Bacour, C., Maignan, F., et al. (2019). Improving estimates of gross primary productivity by assimilating solar‐induced fluorescence satellite retrievals in a terrestrial biosphere model using a process‐based SIF model. Journal of Geophysical Research: Biogeosciences, 124(11), 3281-3306.

Gier, B. K., et al. (2024). Representation of the terrestrial carbon cycle in CMIP6. Biogeosciences, 21(22), 5321-5360.

Li, X., et al. (2018). Solar‐induced chlorophyll fluorescence is strongly correlated with terrestrial photosynthesis for a wide variety of biomes: First global analysis based on OCO‐2 and flux tower observations. Global change biology, 24(9), 3990-4008.

Martini, D., et al., (in prep.). AustroSIF — A compilation of combined passive and active fluorescence data at flux tower sites across Europe.

Vuichard, N., et al. (2019). Accounting for carbon and nitrogen interactions in the global terrestrial ecosystem model ORCHIDEE (trunk version, rev 4999): Multi-scale evaluation of gross primary production. Geoscientific Model Development, 12(11), 4751-4779.