Exploring the potential of Δ17O in CO2 for determining mesophyll conductance.

Mesophyll conductance to CO2 from the intercellular air space to the CO2-H2O exchange site has been estimated using δ18O measurements (gm18). However, the gm18 estimates are affected by the uncertainties in the δ18O of leaf water where the CO2-H2O exchange takes place and the degree of equilibration between CO2 and H2O. We show that measurements of Δ17O (i.e.Δ17O=δ17O-0.528×δ18O) can provide independent constraints on gm (gmΔ17) and that these gm estimates are less affected by fractionation processes during gas exchange. The gm calculations are applied to combined measurements of δ18O and Δ17O, and gas exchange in two C3 species, sunflower (Helianthus annuus L. cv. 'sunny') and ivy (Hedera hibernica L.), and the C4 species maize (Zea mays). The gm18 and gmΔ17 estimates agree within the combined errors (P-value, 0.876). Both approaches are associated with large errors when the isotopic composition in the intercellular air space becomes close to the CO2-H2O exchange site. Although variations in Δ17O are low, it can be measured with much higher precision compared with δ18O. Measuring gmΔ17 has a few advantages compared with gm18: (i) it is less sensitive to uncertainty in the isotopic composition of leaf water at the isotope exchange site and (ii) the relative change in the gm due to an assumed error in the equilibration fraction θeq is lower for gmΔ17 compared with gm18. Thus, using Δ17O can complement and improve the gm estimates in settings where the δ18O of leaf water varies strongly, affecting the δ18O (CO2) difference between the intercellular air space and the CO2-H2O exchange site.

Advancing understanding of land-atmosphere interactions by breaking discipline and scale barriers.

Vegetation and atmosphere processes are coupled through a myriad of interactions linking plant transpiration, carbon dioxide assimilation, turbulent transport of moisture, heat and atmospheric constituents, aerosol formation, moist convection, and precipitation. Advances in our understanding are hampered by discipline barriers and challenges in understanding the role of small spatiotemporal scales. In this perspective, we propose to study the atmosphere-ecosystem interaction as a continuum by integrating leaf to regional scales (multiscale) and integrating biochemical and physical processes (multiprocesses). The challenges ahead are (1) How do clouds and canopies affect the transferring and in-canopy penetration of radiation, thereby impacting photosynthesis and biogenic chemical transformations? (2) How is the radiative energy spatially distributed and converted into turbulent fluxes of heat, moisture, carbon, and reactive compounds? (3) How do local (leaf-canopy-clouds, 1 m to kilometers) biochemical and physical processes interact with regional meteorology and atmospheric composition (kilometers to 100 km)? (4) How can we integrate the feedbacks between cloud radiative effects and plant physiology to reduce uncertainties in our climate projections driven by regional warming and enhanced carbon dioxide levels? Our methodology integrates fine-scale explicit simulations with new observational techniques to determine the role of unresolved small-scale spatiotemporal processes in weather and climate models.

Temperature dependence of isotopic fractionation in the CO2 -O2 isotope exchange reaction.

RATIONALE: Oxygen isotope exchange between O2 and CO2 in the presence of heated platinum (Pt) is an established technique for determining the δ17 O value of CO2 . However, there is not yet a consensus on the associated fractionation factors at the steady state. METHODS: We determined experimentally the steady-state α17 and α18 fractionation factors for Pt-catalyzed CO2 -O2 oxygen isotope exchange at temperatures ranging from 500 to 1200°C. For comparison, the theoretical α18 equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO2 and the corresponding α17 values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO2 and O2 differing in δ18 O value from -66 ‰ to +4 ‰. RESULTS: The experimentally determined steady-state fractionation factors α17 and α18 are lower than those obtained from the updated theoretical calculations (of CO2 -O2 isotope exchange under equilibrium conditions) by 0.0024 ± 0.0001 and 0.0048 ± 0.0002, respectively. The offset is not due to scale incompatibilities between isotope measurements of O2 and CO2 nor to the neglect of non-Born-Oppenheimer effects in the calculations. There is a crossover temperature at which enrichment in the minor isotopes switches from CO2 to O2 . The direct sum evaluation yields a θ value of ~0.54, i.e. higher than the canonical range maximum for a mass-dependent fractionation process. CONCLUSIONS: Updated theoretical values of α18 for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO2 yields θ values higher than the canonical range maximum for mass-dependent fractionation processes. This demonstrates the need to include anharmonic effects in the calculation and definition of mass-dependent fractionation processes for poly-atomic molecules. The discrepancy between the theory and the experimental α17 and α18 values may be due to thermal diffusion associated with the temperature gradient in the reactor.

Leaf scale quantification of the effect of photosynthetic gas exchange on Δ47 of CO2.

The clumped isotope composition (Δ47, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO2 has been suggested as an additional tracer for gross CO2 fluxes. However, the effect of photosynthetic gas exchange on Δ47 has not been directly determined and two indirect/conceptual studies reported contradicting results. In this study, we quantify the effect of photosynthetic gas exchange on Δ47 of CO2 using leaf cuvette experiments with one C4 and two C3 plants. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate the important roles of the Δ47 value of CO2 entering the leaf, kinetic fractionation as CO2 diffuses into, and out of the leaf and CO2-H2O isotope exchange with leaf water. We experimentally confirm the previously suggested dependence of Δ47 of CO2 in the air surrounding a leaf on the stomatal conductance and back-diffusion flux. Gas exchange can enrich or deplete the Δ47 of CO2 depending on the Δ47 of CO2 entering the leaf and the fraction of CO2 exchanged with leaf water and diffused back to the atmosphere, but under typical ambient conditions, it will lead to a decrease in Δ47.