HR: 14:20h
AN: B23B-03    [Abstracts]
TI: Soil Organic Matter \delta $^{13}$C across the Great Plains grasslands: both rainfall and temperature control C3 vs. C4 productivity
AU: * von Fischer, J C
EM: jcvf@lamar.colostate.edu
AF: Colorado State University, Dept. of Biology & Natural Resource Ecology Lab, Ft. Collins, CO 80523 United States
AU: Tieszen, L L
EM: tieszen@usgs.gov
AF: USGS - EROS Data Center, International Program Mundt Federal Building, Sioux Falls, SD 57198
AB: Grass species that dominate the Great Plains grasslands assimilate CO2 by one of two photosynthetic systems, C3 or C4, with the proportions of these types largely controlled by climate. Because C3 and C4 plants differ in their magnitudes of carbon isotope fractionation, regional and global scale biogeochemical analyses (e.g., isotopic inversion studies of CO2) depend on knowing the relative activity of these groups and how they vary with climate. However, regional analyses of C3/C4 proportions, such as Teeri and Stowe (1979), have measured species composition but not relative production. To quantify the relationship between climate and relative production, we have analyzed the carbon isotope composition of soil organic matter (SOM) from 75 native prairie relicts across the Great Plains and compared these values to long-term (30-year mean) climate data. Although temperature has long been recognized as a key determinant of C3 vs. C4 success because of its effects on photorespiration, we find that the timing of rainfall is an additional important predictor; increased rainfall in summer months leads to increased relative production of C4 species. This finding adds complexity to the dominant view that temperature alone controls C3/C4 balance, and suggests that regional scale biogeochemical studies should evaluate the degree to which rainfall is included as a predictor of C3 vs. C4 productivity.
DE: 1851 Plant ecology
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting