Document: DAV-3-34-67

Stable isotope composition of water vapor and CO₂ of air in mesquite-C₄ grass vegetation.

WILLIAMS, D.G.* ¹, G.LIN ² and J.BRUNEL ³

University of Arizona, Tucson, AZ 85748 USA ¹
Columbia University USA ²
Laboratoire d'Hydrologie, IRD, France ³

Abstract:
Increases in C₃ woody plant cover in former C₄-dominated grasslands have significantly altered patterns of CO₂ and water vapor exchanges with the atmosphere. How grass, soil, and woody plant components contribute to CO₂ and water vapor fluxes as water availability during growth periods changes is unclear. We estimated the proportional contribution of these ecosystem components to water vapor and CO₂ fluxes using stable isotope analyses. Stable isotope ratios and concentrations of water vapor and CO₂ of air from different heights in the canopy were measured over 24-h cycles at two sites in southeastern Arizona during the annual 'monsoon' rainy period and post-monsoon drought. The 'savanna' site had an open overstory canopy of the woody C₃ species mesquite (Prosopis velutina) and abundant cover of perennial C₄ grass (Sporobolus wrightii) in the understory. The 'woodland' site had a continuous cover of mesquite with a very sparse herbaceous understory. ¹³C of respired CO₂ during the monsoon period was -17.9 and -20.1 at the savanna site in 1998 and 1999, respectively, indicating approximately an equal contribution of C₃ and C₄ carbon to these fluxes. ¹³C of respired CO₂ at the woodland site during the rainy period in 1999 ranged from -28.3 to -21.7 . Respiration from residual C₄ carbon in the SOM may have contributed to the relatively positive ¹³C values measured at this site that is currently devoid of C₄ grasses. ²H and ¹⁸O values of water evaporated from mesquite, C₄ grass, and soil differed substantially, allowing these to be distinguished in Keeling plots of water vapor. Mesquite, which had access to deeper soil moisture, had the greatest proportional contribution to evapotranspiration fluxes at both sites, especially as water availability declined during the post-monsoon drought. These results illustrate the utility of the Keeling plot approach for charcterizing ecosystem-atmosphere interactions in semi-arid environments.

Keywords: mesquite-C₄ ecosystem, stable isotopes, water vapor, respired CO₂, ecosystem-atmosphere interaction

This abstract is being presented at: 9:00 AM in session:
Oral Session #9: Respiration and Isotopes.