Understanding the processes that control the terrestrial exchange of carbon is critical for
assessing atmospheric CO₂ budgets. Carbonyl sulfide (COS) is taken up by vegetation during
photosynthesis following a pathway that mirrors CO₂ but has a small or nonexistent emission component,
providing a possible tracer for gross primary production. Field measurements of COS and CO₂ mixing ratios
were made in forest, senescent grassland, and riparian ecosystems using a laser absorption spectrometer
installed in a mobile trailer. Measurements of leaf fluxes with a branch-bag gas-exchange system were made
across species from 10 genera of trees, and soil fluxes were measured with a flow-through chamber. These
data show (1) the existence of a narrow normalized daytime uptake ratio of COS to CO₂ across vascular
plant species of 1.7, providing critical information for the application of COS to estimate photosynthetic
CO₂ fluxes and (2) a temperature-dependent normalized uptake ratio of COS to CO₂ from soils. Significant
nighttime uptake of COS was observed in broad-leafed species and revealed active stomatal opening prior
to sunrise. Continuous high-resolution joint measurements of COS and CO₂ concentrations in the boundary
layer are used here alongside the flux measurements to partition the influence that leaf and soil fluxes
and entrainment of air from above have on the surface carbon budget. The results provide a number of
critical constraints on the processes that control surface COS exchange, which can be used to diagnose the
robustness of global models that are beginning to use COS to constrain terrestrial carbon exchange.Keywords: surface fluxes, carbonyl sulfide, laser absorption spectrometry, carbon budget, instrument developmen