A unified modeling and understanding of the canopy CO2, COS and SIF processes with a process-based model

Photosynthesis is a fundamental ecosystem process coupled with terrestrial cycles of energy, carbon and water. As it is difficult to directly measure photosynthesis by partitioning the exchange of carbon dioxide (CO₂) between plants and surrounding air, how photosynthesis responds to a variety of environmental drivers across temporal scales remains unclear. Carbonyl sulfide (COS) fluxes, and Sun-induced Chlorophyll Fluorescence (SIF), have been recently suggested as a promising proxy to infer photosynthesis and track stomatal processes at the ecosystem scale. However, the link between COS fluxes, SIF and CO₂ uptake as well as stomatal opening varied with environmental factors across temporal scales remained unstudied given the tight coupling of leaf water and carbon fluxes. We first developed the CoupModel for simultaneous modeling of the COS, CO₂, and SIF, and explicitly considered the mesophyll conductance in mediating COS and CO₂ diffusion in leaf. By combining the long-term observations of the COS, CO₂ fluxes as well as satellited-retrieved SIF from a boreal forest site with CoupModel, we disentangled the impacts of multiple environmental factors on COS, CO₂ and SIF. Our results suggested leaf uptake of COS, SIF, gross primary productivity and transpiration show different response to variation in climatic controlling factors. We also demonstrated that the leaf uptake of COS is similar to CO₂ on one hand mainly under light and temperature sufficient conditions, e.g., growing-season and daytime. On the other hand, the leaf uptake of COS under the light and temperature limited conditions such as non-growing season and nighttime is existing and different from CO₂, as non-negligible uptake of COS occurs while the CO₂ uptake is close to zero due to absence of photosynthesis. In summary, our study provides new insights into the controlling factors of COS-CO₂-SIF and changes in COS-CO₂-SIF relationships across temporal scales. We suggest that more mechanistic study for the ecosystem uptake of COS across multiple time scales is necessary for better utilizing COS to constrain the ecosystem water and carbon fluxes.