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Ecosystem Modeling of 18O fluxes and stocks in H2O and CO2

Scientist: W.J. Riley (

The concentration of 18O in atmospheric CO2 and H2O is a potentially powerful tracer of ecosystem carbon and water fluxes. We have developed an isotope model (called ISOLSM) [Riley et al., 2002, 2003] that simulates the 18O content of canopy water vapor, leaf water, and vertically resolved soil water; leaf photosynthetic 18OC16O (hereafter C18OO) fluxes; CO2 oxygen isotope exchanges with soil and leaf water; soil CO2 and C18OO diffusive fluxes (including abiotic soil exchange); and ecosystem exchange of H218O and C18OO with the atmosphere. The isotope model is integrated into the land surface model LSM1.0, and has also been applied in studies of the isotopic enrichment of CO

We have tested the model in a tallgrass prairie site in the ARM SGP region [Still et al., 2002] and in a Howland, ME forest site. Testing has recently begun in an Oregon forest transect (five sites), at Harvard Forest, and in several Colorado tallgrass steppe sites.

Figure 1 shows simulated and measured leaf and stem water 18O values over three intensive measurement periods in the tallgrass prairie site:

Figure 1

Research Ecosystem Fig1

Figure1. Simulated and measured leaf and stem water 18O

Figure 2 shows predicted and measured depth-resolved soil-water delta18O values over a three-month period:

Figure 2

Research Ecosystem Fig2

Figure2. Predicted and measured depth-resolved soil-water delta 18O

Using ISOLSM, we have also evaluated —

  • simplified methods of predicting the C18OO soil-surface flux
  • the impacts on the C18OO soil-surface flux of the soil-gas diffusion coefficient formulation, soil CO2 source distribution, and rooting distribution
  • the impacts on the C18OO fluxes of carbonic anhydrase (CA) activity in soil and leaves
  • the sensitivity of model predictions to the delta18O value of atmospheric water vapor and CO2

For example, Figure 3 shows the impact of carbonic anhydrase levels on the soil-surface delta18O value of the surface CO2 flux. The relative increase in the hydrolysis rate caused by carbonic anhydrase is given by delta.

Figure 3

Research Ecosystem Fig3

Figure3. Impact of carbonic anhydrase levels on the soil-surface delta 18O value of the surface CO2

We have also, in collaboration with David Noone at the California Institute of Technology, integrated ISOLSM into the Melbourne GCM and the NCAR CCM3 GCM [Noone et al., 2001].

Figure 4 shows an example of predicted soil water delta18O at 50 cm:

Figure 4

Research Ecosystem Fig4

Figure4. Predicted soil water delta 18O at 50 cm.

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Citable References

  • Noone, D.C., C.J. Still, and W.J. Riley, Diagnosing impacts of changes in the biosphere by modeling 18O of atmospheric CO2 with a general circulation model, Sixth International CO2 Conference, Sendei, Japan, 853-856, 2001
  • Riley, W.J., C.J. Still, M.S. Torn, and J.A. Berry, A Mechanistic Model of H218O and C18OO Fluxes between Ecosystems and the Atmosphere: Model Description and Sensitivity Analyses, in press, Global Biogeochemical Cycles, 2002
  • Still, C.S., W.J. Riley, B.R. Helliker, M. Ribas-Carbo, S. Verma, and J.A. Berry, Measured and modeled delta18O in CO2 and H2O above a tallgrass prairie, in preparation, 2002
  • Torn, M.S., S.C. Biraud, C.J. Still, W.J. Riley, J.A. Berry (2010) Seasonal and inter-annual variability in d13C of ecosystem carbon fluxes from 2002-2009 in the U.S. Southern Great Plains, in review Tellus B.
  • Fischer, M. L., D. P. Billesbach, J. A. Berry, W. J. Riley, and M. S. Torn (2007), Spatiotemporal variations in growing season exchanges of CO2, H2O, and sensible heat in agricultural fields of the Southern Great Plains, Earth Interactions, 11,ISI:000251534500001.

    18.      Henderson-Sellers, A., M. Fischer, I. Aleinov, K. McGuffie, W. J. Riley, G. A. Schmidt, K. Sturm, K. Yoshimura, and P. Irannejad (2006), Stable water isotope simulation by current land-surface schemes: Results of iPILPS Phase 1, Global and Planetary Change, 51,ISI:000238041400004, 34-58.

    Lai, C. T., W. Riley, C. Owensby, J. Ham, A. Schauer, and J. R. Ehleringer (2006), Seasonal and interannual variations of carbon and oxygen isotopes of respired CO2 in a tallgrass prairie: Measurements and modeling results from 3 years with contrasting water availability, Journal of Geophysical Research-Atmospheres, 111,ISI:000236730800003.

    22.      Aranibar, J. N., J. A. Berry, W. J. Riley, D. E. Pataki, B. E. Law, and J. R. Ehleringer (2006), Combining meteorology, eddy fluxes, isotope measurements, and modeling to understand environmental controls of carbon isotope discrimination at the canopy scale, Global Change Biology, 12,ISI:000236549600010, 710-730.

    Riley, W. J. (2005), A modeling study of the impact of the delta O-18 value of near-surface soil water on the delta O-18 value of the soil-surface CO2 flux, Geochim Cosmochim Ac, 69,ISI:000228682000003, 1939-1946.


    26.      Cooley, H. S., W. J. Riley, M. S. Torn, and Y. He (2005), Impact of agricultural practice on regional climate in a coupled land surface mesoscale model, Journal of Geophysical Research-Atmospheres, 110,ISI:000227065800002, D03113.

    27.      Still, C. J., W. J. Riley, B. A. Helliker, and J. A. Berry (2005), Simulation of ecosystem oxygen-18 CO2 isotope fluxes in a tallgrass prairie: Biological and physical controls, in Stable Isotopes and Biosphere-Atmosphere Interactions, edited by L. B. Flanagan, Ehleringer, J.R. & D.E. Pataki, Elsevier-Academic Press.

    Riley, W. J., C. J. Still, B. R. Helliker, M. Ribas-Carbo, and J. A. Berry (2003), 18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements, Global Change Biology, 9,740DV-0005 740DV: Document Delivery available, 1567-1581.

    Still, C. J., W. J. Riley, S. C. Biraud, D. C. Noone, N. H. Buenning, J. T. Randerson, M. S. Torn, J. Welker, J. W. C. White, R. Vachon, G. D. Farquhar, and J. A. Berry (2009), Influence of clouds and diffuse radiation on ecosystem-atmosphere CO2 and (COO)-O-18 exchanges, Journal of Geophysical Research-Biogeosciences, 114,ISI:000263950400001.

    McDowell, N., D. Baldocchi, M. Barbour, C. Bickford, M. Cuntz, D. Hanson, A. Knohl, H. Powers, T. Rahn, J. Randerson, W. Riley, C. Still, K. Tu, and A. Walcroft (2008), Understanding the stable isotope composition of biosphere atmosphere CO2 exchange, EOS Transactions American Geophysical Union, 8994-95.