On the variability of the leaf relative uptake rate of carbonyl sulfide compared to carbon dioxide: Insights from a paired field study with two soybean varieties

Carbonyl sulfide (COS) has been proposed as a promising tracer for the estimation of the gross primary productivity (GPP) from ecosystem to global scale in recent years. Despite substantial work at spatial scales from leaf to regions, the uncertainty of COS-based GPP estimates are poorly known compared to widely used GPP estimates derived from the net ecosystem CO2 exchange. One key uncertainty in this context is the leaf relative uptake (LRU) of the COS with respect to the GPP, which must be known a priori. To investigate the influence of environmental factors, like drought, on the variability of the LRU, we conducted an experiment using ecosystem flux measurements of COS, CO2 and H2O from two eddy covariance towers above a soybean field, growing a commercial cultivar and a chlorophyll deficient mutant variety, in two separate plots. Our findings suggest that the LRU does not only differ between plant varieties due to differences in the ratio of the internal to ambient CO2 mole fraction and the internal resistance to COS, but also changes in response to drought. We also found the internal resistance to COS uptake to be a significant factor in controlling the total COS flux for both varieties, but more so for the commercial cultivar. Our study indicates that species-specific differences in the LRU need to be investigated further, and that environmental stress might complicate the usage of COS as a tracer for predicting GPP at ecosystem and global scale.