How best do we use carbonyl sulfide (OCS) to link the carbon and water cycles in forest ecosystems?
For the past decade, observations of carbonyl sulfide (OCS or COS) have been investigated as a proxy for carbon uptake by plants. OCS is destroyed by enzymes that interact with CO2 during photosynthesis, namely carbonic anhydrase (CA) and RuBisCO (CA is the dominant sink). The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink, whereas the sources and sinks of CO2 are co-located in ecosystems. The drawdown of OCS can therefore be related to the uptake of CO2 without the added complication of co-located emissions comparable in magnitude.
From years of measurements of OCS fluxes above Harvard Forest (a mixed forest east of Boston, MA) to the measurement of the OCS flux on a cruise to the North Atlantic (WACS) and OCS mixing ratios into the San Francisco Bay area (CalNex), we have learned a great deal about carbonyl sulfide in the past few years.
ONGOING RESEARCH
We have recently been funded by NSF to measure mercury and OCS fluxes at Harvard Forest in 2019. More details to follow.
We also measured OCS fluxes on a cruise to the North Atlantic (WACS) and interpretation of those results are continuing.
RECENT RESULTS
As part of a recent overview of carbonyl sulfide and its role in terrestrial ecosystems (Whelan et al., 2018), reviewed the state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. They found that OCS uptake is correlated well to plant carbon uptake, especially at the regional scale and that OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. This review highlighted a number of important gaps in our understanding of the total OCS budget and more work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain.
Measurements of OCS fluxes above Harvard Forest (a mixed forest east of Boston, MA) for three years allow us to compile an extensive dataset to investigate the role of stomatal conductance in the ecosystem carbon cycle in a mixed mid-latitude forest. Recent publications: Commane et al., PNAS, 2015, Wehr et al., Biogeosciences, 2017.
We measured OCS mixing ratios along the coast of California, through San Francisco Bay into the Sacramento as part of the CalNex project. We sampled outflow from the Redwood forests of California and measured high OCS emissions from some unexpected anthropogenic sources: Commane et al., 2013. Campbell et al., JGR-Biogeosciences, 2018 also used this data to estimate the impact of fog on stomatal conductance on the regional scale.