Carbon and water fluxes of the boreal evergreen needleleaf forest biome constrained by assimilating ecosystem carbonyl sulfide flux observations

Boreal forests absorb a significant amount of atmospheric CO₂ through gross primary production (GPP), representing about 20% of the global GPP. However, direct observations of GPP over the whole boreal region are not available as plant photosynthetic rate cannot be measured at scales larger than the leaf scale. At large scales, Land Surface Models (LSMs) can simulate GPP but the lack of direct GPP measurements makes it challenging to evaluate and improve the GPP representation in LSMs. In addition, boreal forests are highly sensitive to environmental changes, impacting gas exchanges and leading to high uncertainties in GPP estimates simulated by LSMs or obtained from data driven methods. Carbonyl sulfide (COS) has emerged as a promising proxy to infer GPP estimates or to better constrain GPP representation in LSMs. Because COS is absorbed by vegetation following the same diffusion pathway as CO₂ during photosynthesis and not emitted back to the atmosphere, implementing a mechanistic representation of vegetation COS uptake in LSMs allows using COS data to constrain GPP representation. In this study, we performed ecosystem COS flux and GPP assimilations to constrain the COS and GPP related parameters in the ORCHIDEE LSM. We focused on Hyytiälä forest, where the longest time-series of ecosystem COS flux measurements was reported. We found that assimilating ecosystem COS fluxes increases the estimated net ecosystem COS uptake by 14%. However, a persistent underestimation of the ecosystem COS flux seasonal amplitude after data assimilation points towards structural errors in the COS model, possibly related to COS internal conductance representation. In comparison with an assimilation of GPP only, adding ecosystem COS flux assimilation leads to a stronger reduction in the stomatal conductance, highlighting the potential of COS to inform stomatal diffusion. Consequently, including COS data in the assimilations also impacts the resulting latent heat flux and water use efficiency. Finally, we scaled up this assimilation framework to the boreal region and found that the joint assimilation of COS and GPP fluxes increased the modeled vegetation COS uptake up to 18%, but not the GPP budget. This contrasts with previous inversion studies that simultaneously increase vegetation COS uptake and GPP budgets based on a linear relationship relating the two.