Abstract. The uptake of carbonyl sulfide (COS) by terrestrial plants is linked to photosynthetic uptake of CO2 by a shared diffusion pathway. Applying COS as a photosynthesis tracer in models requires an accurate representation of biosphere COS fluxes, but these models have not been extensively evaluated against field observations of COS fluxes. In this paper, the COS flux as simulated by the Simple Biosphere Model, version 4 (SiB4) is updated with the latest mechanistic insights and evaluated with site observations from different biomes: one evergreen needleleaf forest, two deciduous broadleaf forests, three grasslands, and two crop fields spread over Europe and North America. To account for the effect of atmospheric COS mole fractions on COS biosphere uptake, we replaced the fixed COS mole fraction originally used in SiB4 with spatially and temporally varying COS mole fraction fields. The lower COS mole fractions in the late growing season reduces COS uptake rates in agreement with observations. We also replaced the empirical soil COS uptake model in SiB4 with a mechanistic model that represents both uptake and production of COS in soils, which improves the match with observations over agricultural fields and fertilized grassland soils. SiB4 was capable of simulating the diurnal and seasonal variation of COS fluxes in the boreal, temperate and Mediterranean region. The daytime vegetation COS flux is on average 8 ± 27 % underestimated, albeit with large variability across sites. On a global scale, our model modifications caused a drop in the COS biosphere sink from 922 Gg S yr−1 in the original SiB4 model to 753 Gg S yr−1 in the updated version. The largest drop in fluxes was driven by lower atmospheric COS mole fractions over regions with high productivity, which highlights the importance of accounting for variations in atmospheric COS mole fractions. The change to a different soil model, on the other hand, had a relatively small effect on the global biosphere COS sink. The small role of the modeled soil component in the COS budget supports the use of COS as a global photosynthesis tracer.;
