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

Scientist: W.J. Riley (wjriley@lbl.gov)

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] 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, but coupling with other models should be straightforward.

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

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.

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.

References

1. 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.
2. 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.
3. 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.

 

 

 

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