Seasonal and interannual variations in the isotopic composition of carbon fluxes in a tallgrass prairie ecosystem.
STILL, C.* 1,2, J.BERRY 1, M.RIBAS-CARBO 1, B.HELLIKER 3 and S.VERMA 4
Carnegie Institution of Washington, Stanford, CA 94305 USA 1
Stanford University, Stanford, CA 94305 USA 2
University of Utah, Salt Lake City, UT 84112 USA 3
University of Nebraska, Lincoln, NE 68588 USA 4
The photosynthetic composition of an ecosystem is an important controller of carbon fluxes and surface energy balance partitioning. Many Great Plains grasslands contain seasonally changing mixtures of C3 and C4 plants. We present data on the isotopic composition of carbon fluxes collected at a mixed C3-C4 tallgrass prairie site in Oklahoma. This ecosystem is typically characterized by C3 grass and forb cover in the cooler and wetter spring, while the hotter and drier summer favors C4 grasses. In order to assess seasonal variations in these photosynthetic mixtures, we collected nighttime respiration samples from a small tower above the canopy and analyzed them for their CO2 concentration and carbon isotope composition. The C3:C4 mixture was calculated with a simple mixing analysis. Collections throughout the 1999 growing season indicate the C4 contribution to ecosystem respiration varied from 40% in April to 54% in June to 88% in September. We also collected daytime gradient samples from the highest and lowest levels and calculated the canopy discrimination. Again, the C3:C4 mixture was determined with a simple mixing analysis. The C4 percentage calculated from this approach doesn't agree with the percentage determined from the nighttime respiration for sampling periods in June and July. This implies a sampling problem (i.e. different sampling 'footprints' between the day and night) and/or a seasonal imbalance between the isotopic composition of photosynthesis and respiration. Finally, we demonstrate the potential use of the oxygen isotope composition of CO2 as a new constraint on C3 and C4 photosynthetic fluxes.
Keywords: carbon and oxygen isotopes; carbon dioxide; C3 and C4 plants; biogeochemistry; tallgrass prairie
This abstract is being presented at: 9:45 AM in session:
Oral Session #9: Respiration and Isotopes.