Photosynthesis and respiration of CO, by forests are two major unknowns in the global carbon cycle. They are the two largest and most variable fluxes, and their difference currently acts as a global net sink. With this project (2017-2020) we aim to provide new insights in the forest carbon balance by separately quantifying photosynthesis and respiration using atmospheric measurements. The method is based on the inverse relationship between oxygen (O,) and CO, which is different for photosynthesis and respiration (e.g. lshidoya et al., 2015). Highly precise measurements of atmospheric 02 and CO2 con centrations will therefore allow to estimate both terms separately. In this poster we present the first results of our field test campaign in a forest in the Netherlands, where we also compared two instru• ments: Oxzilla and Picarro 02 analyzers.