HR: 16:30h
AN: B14B-03    [Abstracts]
TI: Tunable Diode Laser Measurements of Leaf-scale Carbon Isotope Discrimination and Ecosystem Respired Carbon and Oxygen Isotope Ratios in a Semi-arid Woodland
AU: * McDowell, N
EM: mcdowell@lanl.gov
AF: Los Alamos National Laboratory, MS-J495, Los Alamos, NM 87545 United States
AU: Chris, B
EM: bickford@unm.edu
AF: University of New Mexico, 167 Castetter Hall, Albuquerque, NM 87131 United States
AU: Hanson, D
EM: dthanson@unm.edu
AF: University of New Mexico, 167 Castetter Hall, Albuquerque, NM 87131 United States
AU: Kern, S
EM: skern@coloradocollege.edu
AF: Los Alamos National Laboratory, MS-J495, Los Alamos, NM 87545 United States
AU: Meyer, C
EM: clifm@lanl.gov
AF: Los Alamos National Laboratory, MS-J495, Los Alamos, NM 87545 United States
AU: Pockman, W
EM: pockman@unm.edu
AF: University of New Mexico, 167 Castetter Hall, Albuquerque, NM 87131 United States
AU: Powers, H
EM: hpowers@lanl.gov
AF: Los Alamos National Laboratory, MS-J495, Los Alamos, NM 87545 United States
AB: We present results and speculative interpretation of leaf-level carbon isotope discrimination and ecosystem respired carbon and oxygen isotope ratios from a semi-arid, C3/C4 woodland located in northern New Mexico, USA. Overstory leaf area index (LAI) is dominated by live juniper (Juniperus monosperma) trees with an LAI value of approximately 1.0 m2 per m2 ground area, and has a seasonally dynamic understory of mixed C3 forbs and C4 grasses and cacti, with a maximum LAI of 0.30 m2 per m2 ground area. Ecosystem respired carbon isotope ratios showed values characteristic of C3 dominated photosynthesis (Keeling plot intercepts of -35 to -22 per mil). Seasonal variation was typical of that found in wetter, C3 dominated forests, as was the dependence on climate (e.g. relationships with vapor pressure deficit, soil water content, and canopy conductance). Leaf-level carbon isotope discrimination of the junipers, measured by coupling a Li-Cor 6400 photosynthesis system to the TDL, provided discrimination-Ci and discrimination-vpd relationships consistent with measured ecosystem respired carbon isotope ratios. The oxygen isotope ratio of ecosystem respiration was dependent on rain water isotope composition, but was correlated with soil water content during rain-free periods. The cumulative effect of vapor pressure deficit after a rain event was tightly correlated with the oxygen isotope ratio of ecosystem respiration, suggesting the primary drivers are evaporative enrichment of soil water and perhaps nocturnal leaf enrichment. Instrument precision for carbon and oxygen isotope ratios of carbon dioxide is 0.06 to 0.18 per mil; however, overall precision is somewhat lower due to pressure and sampling effects.
DE: 0428 Carbon cycling (4806)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0490 Trace gases
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
SC: Biogeosciences [B]
MN: Fall Meeting 2005