Title: Field research was conducted at the University of Minnesota Rosemount Research and Outreach Center'
1Measuring 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_at_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 lt 0.1 m s-1. During these periods
a significant decrease in the d13CO2 was 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.