Trace Gas System

1. Measuring Field-Scale Isotopic CO2 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 (tgriffis@soils.umn.edu) 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, 12CO2 and 13CO2, can be used as natural tracers to study biophysical processes because photosynthesis discriminates against 13CO2 and fixes proportionally more 12CO2 (Farquhar et al., 1989). The distinct difference between C3 and C4 species in the degree of discrimination means that temporal changes in the isotopic ratio of respired carbon and changes in the flux ratio of 13CO2/12CO2 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 12CO2 and 13CO2 mole mixing ratios and fluxes over a recently harvested soybean field. Tillage Micrometeorological Technique The gradients of 12CO2 and 13CO2 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 mmol mol-1 CO2 with known isotopic ratio; 2) calibration using ~600 mmol mol-1 CO2 with known isotopic ratio; 3) measurement of CO2 mixing ratio at height z1, and 4) measurement of CO2 mixing ratio at height z2. 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 CO2 isotopes. Fig. 3. Half-hour flux estimates of CO2 (top panel), 12CO2 (middle panel) and 13CO2 (bottom panel). The CO2 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). • Objectives • Measure the temporal variation in 12CO2 and 13CO2 mole mixing ratios and gradients; • Estimate 12CO2 and 13CO2 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 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 C4 plants has been shown to vary from -9 to -17‰ and C3 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. Trace Gas System A new development in micrometeorology and trace gas research is the ability to measure high frequency (10 Hz) 12CO2, 13CO2 and C18O16O 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. Hypotheses 1. The isotopic ratio of respired carbon (d13CR) will vary significantly due to changes in source (soybean/corn) contribution to the flux; 2. The isotopic ratio of respired carbon will become heavier (d13CR less negative) with increasing time as the flux contribution decreases from the soybean residue. 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 CO2 measurements using the TDLAS approach is unprecedented and the increase in temporal resolution could lead to greater insight into the biophysical controls on CO2 exchange. Fig. 4. The isotopic ratio of respired carbon (d13CR) 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 (d13CR). Results Fig. 5. Variability in the isotopic ratio of respired (d13CR) carbon using the flux ratio technique. d13CR became relatively more depleted in 13CO2 with time, indicating a greater contribution from soybean decomposition. The shift in source contribution was correlated with increased precipitation and surface soil water content. Conclusions 1. Combination of TDLAS and micrometeorological techniques provides a robust method for measuring long-term isotopic CO2 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 d13CR indicating that source contribution can vary significantly on short timescales. This may have major implications for micrometeorological flux partitioning approaches that use isotope techniques. Fig. 2. Fluctuations of 12CO2 and 13CO2 mixing ratios (top panels), relative isotopic ratio (d13CO2) and friction velocity (u*) (bottom panels). Large increases in 12CO2 and 13CO2 were observed for u* < 0.1 m s-1. During these periods a significant decrease in the d13CO2was observed as relatively 13CO2 depleted carbon was respired into the surface layer. Fig. 1. The Trace Gas Analyzer was used to measure 12CO2 and 13CO2 mixing ratios at wavenumber frequencies of 2308.225 and 2308.171 cm-1, respectively.