Carbonyl sulfide (OCS), the most abundant sulfur gas in the
atmosphere, has a summer minimum associated with uptake by
vegetation and soils, closely correlated with CO 2 . We report the
first direct measurements to our knowledge of the ecosystem flux
of OCS throughout an annual cycle, at a mixed temperate forest.
The forest took up OCS during most of the growing season with
an overall uptake of 1.36 ± 0.01 mol OCS per ha (43.5 ± 0.5 g S per
ha, 95% confidence intervals) for the year. Daytime fluxes accounted
for 72% of total uptake. Both soils and incompletely closed sto-
mata in the canopy contributed to nighttime fluxes. Unexpected
net OCS emission occurred during the warmest weeks in summer.
Many requirements necessary to use fluxes of OCS as a simple
estimate of photosynthesis were not met because OCS fluxes did
not have a constant relationship with photosynthesis throughout
an entire day or over the entire year. However, OCS fluxes provide
a direct measure of ecosystem-scale stomatal conductance and
mesophyll function, without relying on measures of soil evapora-
tion or leaf temperature, and reveal previously unseen heteroge-
neity of forest canopy processes. Observations of OCS flux provide
powerful, independent means to test and refine land surface and
carbon cycle models at the ecosystem scale.
