Net photosynthesis is the largest single flux in the global carbon cycle, but controls over its variability are poorly understood because there is no direct way of measuring it at the ecosystem scale. We report observations of ecosystem carbonyl sulfide (COS) and CO 2 fluxes that resolve key gaps in an emerging framework for using concurrent COS and CO 2 measurements to quantify terrestrial gross primary productivity. At a wheat field in Oklahoma we found that in the peak growing season the flux-weighted leaf relative uptake of COS and CO 2 during photosynthesis was 1.3, at the lower end of values from laboratory studies, and varied systematically with light. Due to nocturnal stomatal conductance, COS uptake by vegetation continued at night, contributing a large fraction (29%) of daily net ecosystem COS fluxes. In comparison, the contribution of soil fluxes was small (1-6%) during the peak growing season. Upland soils are usually considered sinks of COS. In contrast, the well-aerated soil at the site switched from COS uptake to emissions at a soil temperature of around 15°C. We observed COS production from the roots of wheat and other species and COS uptake by root-free soil up to a soil temperature of around 25°C. Our dataset demonstrates that vegetation uptake is the dominant ecosystem COS flux in the peak growing season, providing support of COS as an independent tracer of terrestrial photosynthesis. However, the observation that ecosystems may become a COS source at high temperature needs to be considered in global modeling studies.carbonic anhydrase | LRU | ERU | flux partitioning | soil metabolism C arbonyl sulfide (COS) is an atmospheric trace gas that holds great promise for studies of carbon cycle processes at regional to continental scales (1, 2). The drawdown of atmospheric CO 2 over the continents reflects the difference between terrestrial photosynthesis and respiration fluxes that are both substantially larger than the net CO 2 fluxes. This limits our ability to obtain information on gross fluxes from measurements of atmospheric CO 2 alone. On the other hand, the drawdown of atmospheric COS over the continents is thought to largely reflect photosynthetic fluxes (1,(3)(4)(5)(6). At the global scale, the largest source of COS is the ocean, and uptake by leaves and soil are its largest sinks at 62% and 30% of the total sink, respectively (1).The uptake of COS in leaves is due to hydrolysis catalyzed by the enzyme carbonic anhydrase (CA), resulting in production of H 2 S and CO 2 (7). During leaf uptake, COS and CO 2 share the same diffusional pathway. The resulting close coupling of vegetation COS and CO 2 fluxes during photosynthesis (8-10) makes COS a promising tracer for gross carbon uptake where concurrent respiration precludes direct measurements of photosynthesis. For example, eddy covariance (EC) measurements of COS and CO 2 can be used to obtain independent estimates of gross primary productivity (GPP) at the ecosystem scale.COS-based estimates of GPP are derived from ecosystem COS fluxes and ...