Towards a better understanding of the oxygen isotope signature of atmospheric CO$_{2}$: Determining the 18O-exchange between CO$_{2}$ and H$_{2}$O in leaves and soil on-line with laser-based spectroscopy

The oxygen isotope signature of carbon dioxide (d18O-CO2) is a powerful tool to disentangle CO2 fluxes in terrestrial ecosystems, as CO2 attains a contrasting 18O signature by the interaction with isotopically different soil and leaf water pools during soil respiration and photosynthesis, respectively. However, using the d18O-CO2 signal to quantify plant-soil-atmosphere CO2 fluxes is still challenging due to a lack of knowledge concerning the magnitude and effect of individual fractionation processes during CO2 and H2O diffusion and during CO2–H2O isotopic exchange in soils and leaves, especially related to short-term changes in environmental conditions (non-steady state). This study addresses this research gap by combined on-line monitoring of the oxygen isotopic signature of CO2 and water vapor during gas exchange in soil and plant leaves with laser-based spectroscopy, using soil columns and plant chambers. In both experimental setups, the measured d18O of water vapor was used to infer the d18O of liquid water, and the d18O of CO2 and H2O for calculation of the degree of oxygen isotopic equilibrium between the two species (?). Gas exchange experiments with different functional plant types (C3 coniferous, C3 monocotyledonous, C3 dicotyledonous, C4) revealed that ? and the influence of the plant on the ambient d18O-CO2 (CO18O-isoforcing) not only varied on a diurnal timescale but also when plants were exposed to limited water availability, elevated air temperature, and abrupt changes in light intensity (sunflecks). Multiple regression analysis showed that up to 97 % of temporal dynamics in CO18O-isoforcing could be explained by variations in stomatal conductance, ?, and d18O of H2O at the evaporation site. The determined in vivo activity of carbonic anhydrase, the enzyme which catalyzes the CO2–H2O oxygen isotope exchange inside leaves, corresponded to differences in the amplitude of ? between the different plant species and was a limiting factor for isotopic equilibration. Preliminary experiments with soil columns filled with sand demonstrated that gas-permeable microporous polypropylene tubing, which was installed at different depths in the soil columns, was appropriate for determining d18O-H2O and d18O-CO2 simultaneously without fractionation and, thus, to be promising for further studies on the oxygen isotopic exchange between CO2 and H2O in soils. Altogether, this study highlights that the d18O-CO2 exchange in the soil-plant-atmosphere continuum is highly dynamic in response to short-term variations in environmental conditions, and emphasizes the need for an improved parameterization of models simulating d18O-CO2