Gross Primary Productivity of Four European Ecosystems Constrained by Joint CO 2 and COS Flux Measurements

Gross primary productivity (GPP), the gross uptake of carbon dioxide (CO ) by plant photosynthesis, is the primary driver of the land carbon sink, which presently removes around one quarter of the anthropogenic CO emissions each year. GPP, however, cannot be measured directly and the resulting uncertainty undermines our ability to project the magnitude of the future land carbon sink. Carbonyl sulfide (COS) has been proposed as an independent proxy for GPP as it diffuses into leaves in a fashion very similar to CO , but in contrast to the latter is generally not emitted. Here we use concurrent ecosystem-scale flux measurements of CO and COS at four European biomes for a joint constraint on CO flux partitioning. The resulting GPP estimates generally agree with classical approaches relying exclusively on CO fluxes but indicate a systematic underestimation under low light conditions, demonstrating the importance of using multiple approaches for constraining present-day GPP.