HR: 16:50h
AN: B44B-04    [Abstracts]
TI: Seasonal and interannual variations in carbon and oxygen isotopes of atmospheric CO$_{2}$ observed over a C$_{4}$-dominated tallgrass prairie in central Kansas, USA
AU: * Lai, C
EM: lai@biology.utah.edu
AF: Department of Biology, University of Utah, 257 South, 1400 East, Salt Lake City, UT 84112-0840 United States
AU: Owensby, C
EM: owensby@ksu.edu
AF: Department of Agronomy,, Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506 United States
AU: Ham, J
AF: Department of Agronomy,, Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506 United States
AU: Ehleringer, J
EM: ehleringer@biology.utah.edu
AF: Department of Biology, University of Utah, 257 South, 1400 East, Salt Lake City, UT 84112-0840 United States
AB: We conducted weekly measurements of carbon (\delta$^{13}$C) and oxygen (\delta$^{18}$O) isotopes in atmospheric CO$_{2}$ over a C$_{4}$-dominated tallgrass prairie in 2002, 2003 and 2004. Air samples above and within canopies were collected using 100-ml flasks for both day- and nighttime periods. A two-source mixing line approach estimated isotope ratios of ecosystem respired CO$_{2}$ for both carbon (\delta$^{13}$C$_{R}$) and oxygen (\delta$^{18}$O$_{R}$). In general, values of \delta$^{13}$C$_{R}$ showed a significant shift from $\sim$ -20 $\permil$ in early spring to $\sim$ -12 $\permil$ in mid-summer for all 3 years, reflecting the dominance of C$_{4}$ photosynthesis in the $\it{wet}$ and warm environment. Precipitation in the spring has a profound impact on the seasonal variations in \delta$^{13}$C$_{R}$ values and net ecosystem exchange (NEE) CO$_{2}$ fluxes. Variations in \delta$^{13}$C$_{R}$ corresponded with NEE fluxes on both weekly and interannual time scales; more positive \delta$^{13}$C$_{R}$ values (C$_{4}$ dominance) were observed with greater NEE fluxes under well-watered conditions. When C$_{4}$ photosynthetic uptake of atmospheric CO$_{2}$ decreased, values of \delta$^{13}$C$_{R}$ reflected an increased impact of C$_{3}$ forbs and nearby C$_{3}$ cropland. The coupling between photosynthetic fluxes and respired \delta$^{13}$C suggests that a significant portion of recently fixed carbon was returned to the atmosphere through autotrophic respiration within days. Measuring oxygen isotopes of ecosystem CO$_{2}$ provides a means to further separate total ecosystem respiration into contributions from above- and belowground components. Our measurements showed that values of \delta$^{18}$O$_{R}$ ranged from $\sim$22 to $\sim$35 $\permil$ (VSMOW scale) within a season. These variations were a result of respired CO$_{2}$ equilibrated with two isotopically distinct ecosystem water pools: \delta$^{18}$O values in leaf water are more positive relative to soil water owing to the evaporative enrichment during the day. \delta$^{18}$O values of leaf and soil water will be modeled to constrain \delta$^{18}$O$_{R}$ measurements in order to partition respiratory fluxes in this tallgrass prairie.
UR: http://ecophys.biology.utah.edu/Research/DOE_TCP/index.html
DE: 3322 Land/atmosphere interactions
DE: 1815 Erosion and sedimentation
DE: 1600 GLOBAL CHANGE (New category)
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting