Field research was conducted at the University of Minnesota Rosemount Research and Outreach Center. The experiment was conducted in a 17 ha agricultural field, which is relatively homogeneous, flat, and with adequate fetch. The field was in corn (zea maize) production for 4 years previous to the spring, 2002 planting of soybean (glycine max). The isotopic discrimination of C 4 plants has been shown to vary from -9 to -17‰ and C 3 plants -20 to -34‰ (Pate, 2001). The experiment began a few hours following soybean harvest on October 25 (day of year 298) and continued to November 19 (day of year 323). Measurements were interrupted on November 7 (day of year 311) while the field was tilled with a combination chisel plow/tandem disk. Climatic conditions were cloudy, cold, and dry, resulting in relatively small fluxes during the experiment. Fig. 5. Variability in the isotopic ratio of respired (  13 C R ) carbon using the flux ratio technique.  13 C R became relatively more depleted in 13 CO 2 with time, indicating a greater contribution from soybean decomposition. The shift in source contribution was correlated with increased precipitation and surface soil water content. Fig. 4. The isotopic ratio of respired carbon (  13 C R ) derived from the Keeling Plot (top panel) and a direct estimate using the flux ratio (bottom panel) with the slope expressed as a relative isotopic ratio (  13 C R ). Trace Gas System A new development in micrometeorology and trace gas research is the ability to measure high frequency (10 Hz) 12 CO 2, 13 CO 2 and C 18 O 16 O mixing ratios directly and continuously using TDLAS. The Trace Gas Analyzer (TGA 100, Campbell Scientific Inc., Logan Utah) has recently been developed for making these measurements and is commercially available. Introduction The combination of micrometeorological and stable isotope techniques offers a relatively new approach for improving the description of ecosystem scale processes (Yakir and Wang, 1996; Bowling et al., 1999). The stable isotopes, 12 CO 2 and 13 CO 2, can be used as natural tracers to study biophysical processes because photosynthesis discriminates against 13 CO 2 and fixes proportionally more 12 CO 2 (Farquhar et al., 1989). The distinct difference between C 3 and C 4 species in the degree of discrimination means that temporal changes in the isotopic ratio of respired carbon and changes in the flux ratio of 13 CO 2 / 12 CO 2 can offer potential insight into the relative source contribution, mechanisms, and biophysical description of respiration in systems that have experienced known changes in species composition. This poster describes the results of a 26 day experiment using a micrometeorological gradient and tunable diode laser absorption (TDLAS) technique to measure continuous 12 CO 2 and 13 CO 2 mole mixing ratios and fluxes over a recently harvested soybean field. Measuring Field-Scale Isotopic CO 2 Exchange with Tunable Diode Laser Absorption Spectroscopy and Micrometeorological Techniques T. J. Griffis (1), J. M. Baker (2), S. D. Sargent (3), B. D. Tanner (3) and J. Zhang (1) (1) Department of Soil, Water, and Climate, University of Minnesota-Twin Cities, Minnesota, USA (2) USDA-ARS, University of Minnesota-Twin Cities, Minnesota, USA (3) Campbell Scientific, Inc. Logan, Utah, USA Fig. 1. The Trace Gas Analyzer was used to measure 12 CO 2 and 13 CO 2 mixing ratios at wavenumber frequencies of and cm -1, respectively. Results Conclusions The gradients of 12 CO 2 and 13 CO 2 mixing ratios were measured above the roughness sublayer at two sampling heights (1.65 and 2.35 m). Our sampling routine consisted of: 1) calibration using ~350  mol mol -1 CO 2 with known isotopic ratio; 2) calibration using ~600  mol mol -1 CO 2 with known isotopic ratio; 3) measurement of CO 2 mixing ratio at height z 1, and 4) measurement of CO 2 mixing ratio at height z 2. A 3D sonic anemometer-thermometer (model CSAT3, Campbell Scientific Inc., Utah, USA) was used to obtain the eddy diffusivity (K) of the sensible heat flux. Similarity was assumed for the diffusivity of the CO 2 isotopes. Micrometeorological Technique Fig. 2. Fluctuations of 12 CO 2 and 13 CO 2 mixing ratios (top panels), relative isotopic ratio (  13 CO 2 ) and friction velocity (u * ) (bottom panels). Large increases in 12 CO 2 and 13 CO 2 were observed for u * < 0.1 m s -1. During these periods a significant decrease in the  13 CO 2 was observed as relatively 13 CO 2 depleted carbon was respired into the surface layer. Fig. 3. Half-hour flux estimates of CO 2 (top panel), 12 CO 2 (middle panel) and 13 CO 2 (bottom panel). The CO 2 flux was measured with the 3D sonic anemometer and an open-path infrared gas analyzer (LI-7500, LI- COR Inc., Lincoln, Nebraska, USA). The largest fluxes were observed following tillage (DOY 311). 1. Combination of TDLAS and micrometeorological techniques provides a robust method for measuring long-term isotopic CO 2 mixing ratios and fluxes; 2. The flux ratio approach provides a direct means of evaluating the isotopic ratio of respired carbon and supports the Keeling Plot technique; 3. Large variability was observed in  13 C R indicating that source contribution can vary significantly on short timescales. This may have major implications for micrometeorological flux partitioning approaches that use isotope techniques. Objectives 1.Measure the temporal variation in 12 CO 2 and 13 CO 2 mole mixing ratios and gradients; 2.Estimate 12 CO 2 and 13 CO 2 fluxes using a gradient technique combined with eddy covariance estimates of eddy diffusivity; 3. Examine the variability in the isotopic ratio of respired carbon; 4. Determine the suitability of the system for long- term, unattended measurements. Research Site Hypotheses 1. The isotopic ratio of respired carbon (  13 C R ) will vary significantly due to changes in source (soybean/corn) contribution to the flux; 2. The isotopic ratio of respired carbon will become heavier (  13 C R less negative) with increasing time as the flux contribution decreases from the soybean residue. Tillage Bowling et al., (2003) demonstrated that the TGA could be used to make continuous measurements under field conditions and tested the TDLAS technique against flask measurements analyzed with a mass spectrometer. The high frequency of isotopic CO 2 measurements using the TDLAS approach is unprecedented and the increase in temporal resolution could lead to greater insight into the biophysical controls on CO 2 exchange.