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The oxygen isotope signature of atmospheric carbon dioxide (δ 18 O–CO 2 ) is significantly influenced by terrestrial vegetation through 18 O-exchange between CO 2 and leaf water. However, the impact of short-term variations of environmental conditions on this 18 O-exchange has not been sufficiently characterized yet for different plant functional types. In the present study, δ 18 O of CO 2 and water vapor were measured online in chamber-based experiments with spruce, wheat, poplar and maize using infrared laser spectroscopy. The impact of the plants on ambient δ 18 O–CO 2 was inferred from the chamber-based CO 18 O isoforcing (CO 18 O-Iso), i.e., the product of the net CO 2 flux through the chamber and the δ 18 O–CO 2 of this flux obtained from differential measurements at the chamber inlet and outlet. The measured CO 18 O-Iso was compared to the CO 18 O isoforcing (CO 18 O-Iso sim ) calculated as a function of the δ 18 O of leaf water at the evaporation site (δ 18 O–H 2 O ev ) and the degree of oxygen isotope equilibration between CO 2 and leaf water ( θ ). Plants were exposed to elevated air temperature (35 °C) and cessation of water supply. CO 18 O-Iso was reduced at 35 °C due to the reduction of stomatal conductance ( g s ) in all plant species except for maize, and at decreasing water availability in all four plant species due to a reduction of θ , assimilation rate ( A r ) and g s , while leaf water became progressively 18 O-enriched. The combination of θ , g s , A r and δ 18 O–H 2 O ev accounted for up to 98% of the variations in CO 18 O-Iso, which were well represented by CO 18 O-Iso sim , whereas the relationship between individual determinants and CO 18 O-Iso was weaker. The degree of isotopic CO 2 –H 2 O equilibration calculated from isotopic gas exchange reached maximum values of 0.51 and 0.53 in maize and spruce, and 0.67 and 0.74 in wheat and poplar, respectively. Although θ was highly sensitive to the parameterization of mesophyll conductance ( g m ), most of the g m literature values for each species yielded values for θ significantly lower than previously reported for the respective plant species. This finding, as well as the observed temporal variations in the oxygen isotopic exchange introduced by varying environmental conditions, should be considered for the parameterization of δ 18 O–CO 2 models. |
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