Carbonyl Sulfide Sulfur Isotope Fractionation During Uptake by C3 and C4 Plants

Carbonyl sulfide (COS) is a major long-lived sulfur-containing gas in the atmosphere. Similar to CO₂, COS main sink is the uptake by terrestrial plants. Therefore, COS is used as a proxy for CO₂ uptake by terrestrial plants (gross primary production [GPP]). Currently, COS budget estimates have large uncertainties. Sulfur isotopes measurements are used in an isotopic mass-balance to constrain the COS budget. Here we conducted plant-chamber experiments for determining the isotopic fractionation during COS uptake by C3 and C4 plants, and that associated with enzymatic fractionation inside plant leaves. We also conducted COS diffusion in nitrogen experiment to determine the fractionation associated with COS diffusion through plant stomata. We then measured the δ³⁴S values of COS from these experiments by a gas chromatograph/multicollector inductively coupled plasma mass spectrometer. Applying these measured δ³⁴S values, we calculated fractionations (ε_plant) of −1.6 ± 0.1‰ and −5.4 ± 0.5‰ during COS uptake for C3 plants and C4 plants respectively. A fractionation of −5.2 ± 0.5‰ for COS diffusion through plant stomata was calculated as well. We estimate the total diffusive component of plant uptake fractionation (including boundary layer, stomata, and gas-liquid interface diffusions) as −1.6 ± 0.1‰. Using plant chamber measurements under high CO₂ and COS concentrations we calculated the enzymatic component of the plant uptake fractionation as −15 ± 2‰. Based on the diffusive and enzymatic components of plant uptake fractionation, we calculate COS concentrations inside plant leaves. Using the C3 and C4 fractionation results, a value of −2.3 ± 0.5‰ is estimated as the global plant uptake fractionation, which can be incorporated into improved COS-based-GPP estimates in the future.