HR: 09:00h
AN: B41A-05    [PDF]
TI: Free Air Respiratory Carbon Isotope Enrichment Experiment
AU: * Sternberg, L d
EM: l.sternberg@miami.edu
AF: Department of Biology, University of Miami, Coral Gables, FL 33124 United States
AU: Greaver, T
EM: tgreaver@bio.miami.edu
AF: Department of Biology, University of Miami, Coral Gables, FL 33124 United States
AU: Schaffer, B
EM: BAS@mail.ifas.ufl.edu
AF: Tropical Research and Education Center, 18905 S.W. St., Homestead, FL 33031 United States
AU: Moreno, T
EM: TBMoreno@mail.ifas.ufl.edu
AF: Tropical Research and Education Center, 18905 S.W. St., Homestead, FL 33031 United States
AB: Recycling of respiratory carbon in canopies is difficult to measure. Other than a steady state model, there are currently no direct methods of measuring recycling. Respiratory carbon recycling not only will affect the carbon isotope ratio signature of the canopy, but is also important in partitioning gross photosynthesis and respiration from net ecosystem exchange (NEE). In order to better understand recycling, we empirically derived an integrated measure of recycling in a cover crop ({\it Crotalaria juncea} L.) and compared it with that derived by the steady state model. A measured dose of nitrogen gas having CO$_{2}$ with a high Carbon-13 abundance (41%) was applied at ground level ($\sim$10 cm above the soil surface) with a system of hoses in a 10 by 10 m plot embedded in a 30 by 30 m plot. We adjusted the flux rate of this enriched gas weekly to correspond to weekly measurements of soil respiration rates. The gas application rates at ground level were sufficiently low so as to not affect the carbon dynamics of the treatment plot relative to that of a control plot with only nitrogen (no CO$_{2}$) applied. The isotopic composition of the applied gas, however, was high enough to significantly increase the isotopic composition of respired CO$_{2}$. Using the carbon isotopic composition of respiration and biomass from the control and treatment plots and mass balance principles we calculated that 45% of the total respired CO$_{2}$ is recycled by this crop, which compares well with that derived by the steady state model (48%). Partitioning of gross photosynthesis and respiration by isotopic methods usually assumes no recycling. Recycling, however, will have an effect on the isotopic composition of respired CO$_{2}$. After correcting for the recycling effects on the carbon isotope ratios of respired CO$_{2}$ leaving the canopy, we calculated that the average gross photosynthesis for this crop was 40.4$\mu$moles/m$^{2}$s and gross respiration was in the order of 17.8 $\mu$moles/m$^{2}$s. These values are similar to those previously observed in agricultural crops.
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting