Forest fluxes of carbonyl sulfide (OCS): the balance between photosynthetic and soil uptake

The net ecosystem exchange of CO2 (NEE) is measured at the canopy scale at many flux sites around the world. NEE is the difference between the photosynthetic uptake (gross primary productivity, GPP) of CO2 by leaves and the respiration (R) of CO2 by leaves, roots, soils, etc. There are a number of methods used to partition GPP and R. The most common method is to estimate the daytime respiration from the observed night-time temperature dependence of respiration and use this to calculate GPP as the difference between NEE and R.

Studies have suggested that the measurement of carbonyl sulfide (OCS) fluxes could provide a more direct method to partition GPP, as OCS is irreversibly hydrolyzed by plants during photosynthesis but it is not respired. OCS is the most abundant sulfur containing gas in the atmosphere (~500 pptv), with a dominant sink in the terrestrial biosphere - through leaf and soil uptake. However, the magnitude of leaf uptake vs. soil uptake in forest ecosystems is not well resolved.

We present measurements of OCS using an infrared laser absorption based instrument operated continuously at Harvard Forest in Western Massachusetts since October 2010. Direct eddy flux measurements of OCS have been made since August 2011. Measurements of OCS mixing ratios at various heights (October 2010 - August 2011) are used to estimate the OCS flux using the modified Bowen ratio. We investigate the diurnal and seasonal relationship of OCS with photosynthetic and soil processes through comparison with CO2 and H2 fluxes: the flux of CO2 is a combination of photosynthetic uptake by leaves and respiration from soils, while H2 is lost to microbes in the soil.