Carbon isotopic studies of assimilated and ecosystem respired CO2 in a SE pine forest 
Maureen H. Conte, Ecosystems Center, Marine Biology Laboratory
Co-P.I.s: Jeffrey Chanton and Behzad Mortazavi, Dept. of Oceanography, Florida State University
Abstract
A major uncertainty in the application of isotopic studies to resolve variations in the relative importance of the terrestrial and oceanic carbon sinks is our ignorance about the extent and causes of variability in isotopic fractionation by terrestrial photosynthesis and respiration. Two important parameters in the mass balance equations that are used to assess relative magnitudes of carbon sinks are photosynthetic discrimination (Δ) and the 13C of ecosystem-respired CO2 (δCr), yet no satisfactory method exists to measure either on large spatial scales. The extent to which these two parameters vary alters conclusions about the timing and nature of the terrestrial carbon sink. The proposed research will determine how climatic variables, operating by affecting Δ, and the balance between autotrophic and heterotrophic respiration affect the isotopic composition of ecosystem respired carbon dioxide (δCr). The research will also employ a novel technique, based upon the isotopic composition of ablated leaf waxes in canopy aerosols, to link ecosystem level measurements of Δ with the atmospheric signal. This method can provide crucial data on the large regional scale variations in Δ that are needed to accurately model the magnitude and geographical pattern of the terrestrial carbon sink using variations in [CO2] and δ13CO2.
This project will be located at the Ameriflux site in the Austin-Cary Experimental Forest, Gainesville Florida.
Hypotheses: (1) Foliage, ecosystem- and soil-respiration respond to moisture deficits by exhibiting greater 13C enrichment. (2) The isotopic signal of vascular plantderived leaf wax biomarkers in aerosols reflects Δ (3) Wax isotopic variations co-vary with those of leaf soluble sugars and leaf respiration with a temporal delay and damping which is quantitatively related to turnover times of different pools.
We will measure the δ13C respired CO2 from foliage organic matter (δCp), individual leaf wax compounds and sugars, from which leaf level carbon isotopic discrimination will be determined. We will determine the response of the δ13C signature of soil- (δCs), foliage-(δCf), and ecosystem-respired (δCr) CO2 and by proxy photosynthetic discrimination via leaf waxes and sugars to soil moisture, canopy vapor pressure deficit and temperature.
We address questions about the effects of climate change on the carbon dioxide source sink relationships of terrestrial ecosystems in the SE. Results will improve understanding of interaction between the terrestrial and oceanic biospheres. Data will contribute to models to predict Δ and the δ13C of ecosystem respired CO2 from standard data collected at Ameriflux sites and across forest ecosystems in general.
Publications
None yet
Presentations
Conte, M. H and J. C. Weber (2005) Quantification of Isotopic Fractionation of Atmospheric Carbon Dioxide by Terrestrial Photosynthesis Using the Carbon Isotopic Composition of Plant Wax Aerosols, Eos Trans. AGU 86(52), Fall Meet. Suppl., Abstract B13B-02
Weber, J. C. and M. H. Conte (2006) Spatial and Temporal Patterns in the Carbon Isotopic Signal of Leaf Wax Aerosols in Continental Air Masses: Linkages with Ecosystem Discrimination, Eos Trans. AGU 87(52), Fall Meet. Suppl., Abstract B21E-06
Crumsey, J., M. H. Conte, J. C. Weber, B. Mortazavi, M. Smith and J Chanton (2006), Turnover of Leaf Waxes in Florida Slash Pine: Results of an Isotopic Labeling Experiment, Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract B22D-05 (awarded best student paper)
Mortazavi, B., J. Chanton, M. Conte and T. Martin (2007), Influence of Tree Height on the Carbon Isotopic Discrimination of Canopy Photosynthesis in Southeastern Pine Forest Ecosystems, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract B13B-1197
