HR: 09:15h
AN: B11E-06    [Abstracts]
TI: Simulating Stable Carbon Isotopic Signatures and Exchange of a C4 Canopy: Comparison with a 'Big-Leaf' Model
AU: * Zhang, J
EM: jzhang@umn.edu
AF: University of Minnesota, Department of Soil, Water, and Climate, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Griffis, T J
EM: tgriffis@umn.edu
AF: University of Minnesota, Department of Soil, Water, and Climate, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Baker, J M
EM: jbaker@umn.edu
AF: USDA-ARS, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AB: Considerable uncertainties remain in using stable isotope techniques to partition net ecosystem CO₂ exchange into its component fluxes. Many of these uncertainties arise from the determination of isotopic signatures (i.e., ecosystem isotope discrimination and isotope ratio of ecosystem respiration) and the simplifying assumptions associated with the 'big-leaf' analogy in the isotopic approach. It is necessary to rigorously test these assumptions in order to improve the isotopic partitioning. In this study, a multilayer canopy model was adapted to simulate the dynamics of the isotopic signatures of a C₄ corn canopy to test several assumptions in a 'big-leaf' isotopic partitioning model for the growing season of 2003. Results showed that: 1) The bulk canopy conductance inverted from the Penman-Monteith equation departed significantly from the modeled canopy stomatal conductance when the canopy was highly decoupled from the atmosphere. For such conditions, the use of bulk canopy conductance as a proxy for the canopy stomatal conductance presents a potential limitation for the isotopic approach. 2) Sunlit and shaded leaves showed different isotope discriminations in response to different CO₂ leakiness of the bundle sheath cells. Since the estimation of isotope discrimination is highly sensitive to the leakiness factor, care needs to be taken to determine an appropriate leakiness factor for the entire canopy. 3) The assumption on the equivalence between the daytime and nighttime isotope ratio of ecosystem respiration (δ_R) is problematic, because daytime δ_R may differ by up to 2 per mil over the diurnal period. Variations in δ_R were caused by changes in the contributions of component respiration. Daytime ecosystem respiration was more enriched as a result of the increased contribution from the relatively enriched foliar and rhizosphere respiration. Our modeling of δ_R provided opportunities for investigating the quantitative relationship between δ_R and environmental factors, which can subsequently be used to estimate daytime δ_R to improve the isotopic partitioning. Future research involving automated chambers and isotope laser spectroscopy will be used to help validate our modeled values of daytime δ_R.
DE: 0400 BIOGEOSCIENCES
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0416 Biogeophysics
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting