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Stable Isotope Analyses of Carbon Dioxide Exchange in Forest and Pasture Ecosystems

Stable Isotope Analyses of Carbon Dioxide Exchange in Forest and Pasture Ecosystems. L. Flanagan, J. Ometto, T. Domingues, L. Martinelli, J. Ehleringer Atlanta LBA Ecology, February 12-14, 2001. Research Objectives: To study effects of:.

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Stable Isotope Analyses of Carbon Dioxide Exchange in Forest and Pasture Ecosystems

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  1. Stable Isotope Analyses of Carbon Dioxide Exchange in Forest and Pasture Ecosystems L. Flanagan, J. Ometto, T. Domingues, L. Martinelli, J. Ehleringer Atlanta LBA Ecology, February 12-14, 2001

  2. Research Objectives: To study effects of: • Environmental variation on forest carbon dioxide and water vapor exchange (Using C stable isotope measurements) • Land-use change on ecosystem stable isotope discrimination (Forest [C3] conversion to Pasture [C4])

  3. Rationale for Expected Environmental Effects on Forest Physiology: 1. Large seasonal changes in precipitation and associated seasonal drought

  4. Rationale for Expected Environmental Effects on Forest Physiology: 2. El Nino/La Nina can cause substantial interannual variation in precipitation

  5. Stable Isotopes Provide Integrated Eco-physiological Measurements 13C measurements represent changes in the ratio of stomatal conductance to photosynthetic capacity Spatial and temporal integration depends on the nature of the measurements: • Single leaves • Tree rings • Atmospheric CO2

  6. The carbon isotope composition of plant tissues depends on • d13Ca, atmospheric source • a, 13CO2 diffusion rates relative to 12CO2 • b, enzymatic discrimination during carboxylation • ci/ca, ratio of internal to ambientCO2 d13Cleaf = d13Ca - a - (b - a)•ci/ca -8 ‰ 4.4 ‰ 27 ‰ 0.4 - 0.9

  7. d13Cleaf = d13Ca - a - (b - a)•ci/ca This carbon isotope discrimination occurs continuously during photosynthesis and the resulting organic carbon integrates over the entire photosynthetic period. ci ca

  8. Precipitation Soil Moisture Stomatal Conductance Photosynthetic Capacity Leaf Ci/Ca Carbon Isotope Discrimination

  9. -25 Leaf d13C, per mil -35 Low High Water Availability

  10. Sampling Atmospheric CO2 Stable Isotope Ratios • Increases the spatial integration of Eco-Physiological information obtained

  11. A Keeling Plot

  12. Keeling Plot Technique Provides an estimate of: Spatially integrated changes in the ratio of stomatal conductance to photosynthetic capacity • Spatial integration similar to E.C. footprint • Temporal integration: Days – Week (primarily represents recently fixed carbon)

  13. Land Use Change Effects C3 C4

  14. 18O in CO2 could be an important signal for C3-C4 vegetation conversions

  15. The 18O Content of Atmospheric CO2 in terrestrial ecosystems is controlled by: • Discrimination during CO2 Assimilation (equilibration with chloroplast water) • Release of Respiratory CO2 from Soils (equilibration with soil water)

  16. We expect differences between C3 and C4 plants for discrimination against C18O16O because: • Leaf Water O-18 values • Ci/Ca differences • Carbonic Anhydrase Activity

  17. C3 and C4 plants contribute different DC18O16O signals

  18. Conclusions: • Significant temporal variation occurs in d13C of forest respired carbon dioxide • Associated with seasonal and interannual variation in precipitation??

  19. Conclusions: • A shift occurs in the d13C of respired CO2 caused by forest-pasture conversion • Pastures do not have a pure C4 signal • Temporal variation is caused by C3 encroachment and pasture burning

  20. Conclusions: • 18O in CO2 could be an important signal for forest-pasture conversions • Tropical pasture respired CO2 is higher in 18O than that from tropical forest • DC18O16O is different in C3 and C4 ecosystems

  21. Discrimination against CO2 containing 18O

  22. Predicted d18OLW and ∆C18O16O values for forests and pastures in Amazonia d18OLW ∆C18O16O CA eq. C3 forest -5.6 ‰ 2.8 ‰ 100 % C4 grassland +2.3 ‰ 6.7 ‰ 38 %

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