Continuous Carbonyl Sulfide Fluxes in a Tallgrass Prairie Using the Flux-Gradient Method

An ecosystem’s carbonyl sulfide (OCS or COS) flux is a powerful proxy for plant-controlled carbon and water exchange. This study is the first to apply the OCS flux-gradient method to constrain gross primary productivity (GPP) and nighttime transpiration. It is also one of only a few OCS studies in grassland ecosystems, which are characterized by complex species distributions that vary temporally. A laser absorption spectrometer measured OCS fluxes during the growing season at the Fermi tallgrass prairie eddy covariance site in Batavia, Illinois, USA. Ambient atmospheric concentrations of OCS, carbon monoxide (CO), carbon dioxide (CO2), and water vapor (H2O) were sampled at 1 Hz frequency at four different heights within and above the vegetation canopy from May to October, 2016. We observed a well-defined seasonal cycle of OCS concentrations that tracked the Northern Hemisphere growing season. The data also show a strong diel cycle in concentrations, above-canopy gradients, and OCS fluxes associated with changes in radiation and atmospheric stability. Nighttime OCS in the canopy dropped to a minimum of 17.6 ± 9.5 ppt, which, to our knowledge, is the lowest tropospheric OCS concentration ever observed in the free atmosphere. Close coupling of OCS and CO2 concentrations was demonstrated (R2 = 0.654, p < 0.001) even as the season and species makeup evolved. We converted OCS concentration gradients to ecosystem OCS fluxes using the net ecosystem exchange of carbon dioxide from existing eddy covariance data on site. OCS fluxes reached a maximum uptake of -102 ± 9.5 pmol m-2 s-1 on June 18. After correcting for soil OCS fluxes using a published empirical model, we compared GPP derived from OCS flux-gradients (mean = -23.3 µmol m-2 s-1) against GPP derived from ecosystem respiration (mean = -19.2 µmol m-2 s-1), and the two methods were well-correlated (R2 = 0.566, p < 0.001, RMSE = 8.81, n = 150). Ratios of OCS flux night:day indicate nighttime canopy conductance was ~8% of daytime conductance. In this first field study of its kind, we demonstrated that the OCS flux-gradient method to constrain GPP has great potential to improve ecosystem carbon budget closure.