PARENT SESSION
Poster Session #72: Water Relations II.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Partitioning of CO₂ fluxes in a semi-arid ecosystem using stable isotopes: Effects of monsoon rains.

Pierce, Danielle^*,1, van Haren, Joost¹, Lin, Guanghui¹, Yepez, Enrico^*,2, Scott, Russel³, Martins, Dean³, Williams, Dave², ¹ Columbia Biosphere 2 Center, Oracle, AZ² University of Arizona, Tucson, AZ³ United States Department of Agriculture (USDA), Tucson, AZ

ABSTRACT- Partitioning ecosystem CO₂ fluxes can reveal biogeochemical response mechanisms of terrestrial ecosystems to climate or land-use changes. In this study, we used the "Keeling plot" approach to partition net ecosystem CO₂ exchange (NEE) into relative contributions from various components in a mesquite bosque before, during and after the monsoon season. The study site was located in the Upper San Pedro River Basin in southeastern Arizona. This woodland ecosystem is dominated by C₃ mesquite trees (Prosopis velutina) in the canopy and C₄ sacaton grass (Sporobolus wrightii) in the understory. Carbon dioxide was collected along a vertical profile of 10 heights in mid June (pre-monsoon), late July (peak monsoon) and late September (post monsoon) of 2001 and then analyzed for ¹³C of CO₂. In addition, soil respiration rates were measured along four transects up to 100 m from the profile tower. Comparisons of data sampled during the pre- and peak-monsoon period suggests that precipitation input from monsoon events significantly increased nighttime NEE and soil respiration during the peak monsoon period. The nighttime NEE (or ecosystem respiration) showed a significant contribution from the understory components (higher ¹³C for respired CO₂) during the peak monsoon season, probably resulting from an increase in respiration of C₄ understory plants and/or an increase in soil respiration. Furthermore, extensive profile as well as understory sampling proved important to partitioning CO₂ fluxes between the understory and the canopy.

KEY WORDS: respiration, monsoon, partitioning