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DIVISION S-3—NOTES

A CARBON DIOXIDE FLUX GENERATOR FOR TESTING INFRARED GAS ANALYZER-BASED SOIL RESPIRATION SYSTEMS

^a Dep. of Forest Resources, 115 Green Hall, Univ. of Minnesota, 1530 Cleveland Ave. N., St. Paul, MN 55108
^b Dep. of Soil Science, Univ. of Wisconsin-Madison, 1525 Observatory Dr., Madison, WI 53706

^* Corresponding author (mart0166{at}umn.edu).

	ABSTRACT

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An artificial flux generation device was developed to test the accuracy of a closed-dynamic soil respiration system (LICOR 6400). The device consisted of an enclosed reservoir with a porous top; the reservoir contained a volume of CO₂ enriched air, which was monitored by an infrared gas analyzer (IRGA). When the internal CO₂ concentration within the reservoir was elevated, diffusion rates through the porous medium were measured by recording changes in CO₂ concentration within the reservoir. This diffusion-based artificial flux mimics natural soil respiration, and allows an independent verification of the accuracy of soil respiration measurement systems. We tested a LI-COR 6400 portable photosynthesis system fitted with a 6400-09 soil CO₂ flux chamber. On average, this system overestimated high flux rates by 2 to 4% and underestimated low flux rates by 4 to 20% over five independent trials. Soil respiration appeared to be sensitive to boundary layer mixing, and ambient CO₂ concentrations.

Abbreviations: IRGA, Infrared Gas Analyzer

	INTRODUCTION

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SOIL C FLUXES are of interest because of their impacts on C cycling, global C budgets, and the role of terrestrial systems in biosphere–atmosphere–climate interactions. Approximately 1500 Pg of C are stored in soils globally (IPCC, 2001), and this pool can contribute 50 to 76.5 Pg of C to the atmosphere annually through autotrophic and heterotrophic respiration (Raich and Schlesinger, 1992; Raich and Potter, 1995). Soil C cycles are complex and difficult to quantify; however, our estimates of pool sizes and fluxes have improved greatly due to increased sampling and through advances in monitoring equipment.

One such technological advance has been the portable IRGA equipped with a chamber suited to measuring CO₂ fluxes at the soil surface. Estimates of CO₂ flux are based on a measured increase in CO₂ concentration within a measurement chamber. This change results in CO₂ evolution per unit area per unit time, commonly given in mg CO₂ m^–2 h^–1 or µmol CO₂ m^–2 s^–1. These devices have simplified the measurement of soil respiration, increased measurement accuracy, and allowed increased spatial and temporal sampling frequency when compared with older methods. However, IRGA based systems are not without errors.

Improper use such as poor IRGA calibration, allowing changes in pressure within the measurement chamber (Lund et al., 1999), ignoring uneven concentration gradients, and disturbing the soil medium can affect diffusion and thereby give misleading flux rates (see Davidson et al. [2002] for a review). Infrared gas analyzers may be calibrated with known gasses to ensure that the concentrations measured are reported accurately. Unfortunately, the physics of gas movement that are involved in quantifying a soil CO₂ diffusion rate make testing soil respiration systems very difficult. While an IRGA may give the correct concentration, these readings may not always be translated into a representative surface flux due to misuse or mechanical error.

Previous work has compared different measurement systems and techniques in attempts to validate estimates of soil CO₂ flux. (de Jong et al., 1979; Norman et al., 1997). Specifically, the two differing techniques of alkali absorption and IRGA measurements made with a dynamic chamber have yielded measurement differences of 500% (Jensen et al., 1996). Using the same IRGA technique, two devices from competing manufacturers measured rates differing by 30 to 50% (Le Dantec et al., 1999; Janssens et al., 2000). Even different models of IRGAs from the same manufacturer have been shown to disagree by 10 to 24% (Yim et al., 2002). Unfortunately, these comparisons were among measurement methods with unknown "true" values. Because of this, the accuracy and repeatability of soil flux measurements have not been well documented.

This paper describes a testing device that may be used to estimate the absolute accuracy of IRGA-based soil respiration measurements. These tests provide an independent quantification of the instrument error associated with soil respiration measurements, which in turn will allow field measurements of soil fluxes to be objectively evaluated.

	Materials and Methods

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Flux Generation System Overview
The flux generation system described here is similar to the one created by Widen and Lindroth (2003), and both devices are loosely based on a design proposed by Nay et al. (1994)(see Fig. 1) . The unit built by Widen and Lindroth as well as the device we report work by diffusion and not mass flow. Earlier attempts, by ourselves and Nay et al. (1994), that involved the active pumping of CO₂ may have led to pressure differences and possibly caused underestimations of the surface flux measured. Nay's version might have underestimated flux rates because of the large pore volume and high diffusivity of the diffusion medium (Davidson et al., 2002).

View larger version (73K): [in this window] [in a new window]   Fig. 1. Schematic of the flux generation system. (a) Diffusion medium. (b) Radial hoses/reference IRGA gas inlet. (c) Reference IRGA exhaust. (d) Internal mixing fan. (e) CO2 input port for elevating Ci. (f) Boundary layer mixing fan.

Carbon dioxide efflux rates at the surface of the diffusion medium follow the equation (from LI-COR Inc., 1997):

[1]

where Rs_generated is generated soil respiration (µmol m^–2 s^–1), k is a units conversion = 10/8.314 = 1.2028, P is pressure (kPa), V is the reservoir volume of the flux generation unit (cm³) including all hoses and the approximate air space in the diffusion layer, S is the area of the diffusion opening (cm²), Ci_reservoir is the internal CO₂ concentration (µmol mol^–1), t is the time (s), and W is the concentration of water vapor (µmol mol^–1).

The measurement chamber for the soil respiration system to be tested (test IRGA) is placed upon the diffusion medium and is used as it would be in the field. Care must be taken to note the time at which each test measurement is recorded so that it can be compared with the calculated fluxes from the flux generation system. We were able to produce and ultimately test a wide range of generated efflux rates, approximately 0.5 to 11.0 µmol m^–2 s^–1. This spans the range reported for soils from various biomes (Winston et al., 1997; Davidson et al., 1998).

System Construction
The flux generation reservoir consisted of a 38-L cylinder with a sealed bottom, 69 cm long with a diameter of 27 cm (Fig. 1). A metal screen (5 mm mesh size) was glued to a layer of silk, and then inserted approximately 10 cm from top of the cylinder. We placed 4 cm of 0.65-mm diameter glass beads (Potters Industries Inc., Valley Forge, PA) on top of the screen to act as a constant diffusion medium (Fig. 1a); this layer was carefully leveled to reduce the variation in flux rates across the surface caused by differing depths of the diffusion column. We chose the uniform glass beads to homogenize the diffusion path within the layer and further reduce variations in the surface flux. Smaller particles could be used but this would require either a thinner layer or a higher internal concentration to produce the desired flux rates. We were limited by the concentrations detectable by the LI-COR 6400 (3000 µL L^–1) and we believed that a layer thinner than 4 cm would be too difficult to level and be too vulnerable to differences in depth. Lastly, we measured the surface area of the diffusion layer (557.8 cm²) and the entire reservoir volume (38594.1 cm³) including the hoses, reference IRGA volume, and the approximate pore space of the bead layer.

Input gas for the reference IRGA was drawn from directly below the glass beads through a series of perforated hoses distributed in a radial pattern (Fig. 1b). This inlet consisted of five perforated hoses with 0.32-cm inside diameter (Bev-A-Line IV tubing, Thermoplastic Processes, Millington, NJ). The perforations were spaced approximately 1.5 cm apart and were 1.5 mm in diameter. These hoses were joined to a sixth hose that led to the inlet of the reference IRGA. Exhaust gas from the reference IRGA was fed back into the system at the bottom of the reservoir (Fig. 1c) and was mixed by a slowly turning fan (12 VDC micro fan, 40 by 40cm) (Fig. 1d). The fan was powered by a variable-speed DC power source (BK Precision, Placentia, CA). A valve was fixed on the chamber so CO₂–enriched air could be fed directly into the system before measurement (Fig. 1e). The easiest method of introducing CO₂ into the system was to exhale though the hose until the desired Ci_reservoir was reached. The valve was closed before measurement.

A second fan (12 VDC micro fan, 40 by 40cm) was placed at a 45° angle 75 cm above the bead surface to mix the boundary layer between the beads and the atmosphere (Fig. 1f). This was necessary due to the sensitivity of the generated fluxes to changes in the diffusion gradient between the reservoir and the headspace. To illustrate this, we performed a simple test by using the reference IRGA to sample the ambient concentration outside of the test IRGA chamber. We mounted the reference IRGA inlet to the side of the test IRGA sensor head and attached a foam muffler to dampen measurement noise (an open cell foam cube, 5 cm on a side). As the CO₂ concentration within the flux generation reservoir decreased with time, the ambient CO₂ concentration at the surface of the beads rose and then fell due to the CO₂ volume from the reservoir slowly diffusing into the air space of the laboratory; adding the mixing fan reduced this trend (Fig. 2) . A build up of CO₂ at the surface of the diffusion layer greater than that set inside the test IRGA chamber could result in overestimations of CO₂ flux.

View larger version (20K): [in this window] [in a new window]   Fig. 2. Ambient concentrations approximately 15 cm above the diffusion layer measured with and without the boundary layer mixing fan.

It should also be noted that the airflow within the chamber itself disturbs the boundary layer and can influence measurement values (Le Dantec et al., 1999). The fan we used as an external mixing fan (Radio Shack model 273-240, Radio Shack, Fort Worth, TX) claims airflow of 3.6 mL s^–1. This would result in a wind speed at the bead surface of approximately 0.66 m s^–1 (calculations from Olander, 1994). This is only slightly faster than both the 0.4 m s^–1 reported for the inside of the LICOR 6409 (Le Dantec et al., 1999), and the mean wind speed of 0.5 m s^–1 measured at 0.75 m above the ground for the Willow Creek eddy flux tower (Cook and Davis, Penn State, personal communication, 2003, Mean wind speed was calculated using May–August data during 1000–1400 h local time. http://cheas.psu.edu [verified 25 Nov. 2003]).

Leak Test
To verify that CO₂ was not diffusing through unplanned openings, we conducted a simple leak test by capping the cylinder with a sheet of plastic sealed with silicon grease. The reference IRGA was attached to the reservoir, the internal CO₂ concentration of the flux generation reservoir was increased to roughly 2800 µmol mol^–1, and the seal was set in place. This closed volume was monitored for 1.3 h. The Ci_reservoir dropped 9.6 µmol mol^–1, which resulted in a flux error of 0.06 µmol m^–2 s^–1 or 0.6% of the corresponding generated flux (with Ci_reservoir 2800, Rs_generated 10.5 µmol m^–2 s^–1). This was well below the sensitivity of the test IRGA so this error was ignored.

Diffusion Lag Time
After the diffusion layer was installed, we measured the lag time for gas movement across the diffusion medium. We set the reference IRGA to log Ci_reservoir every second while the test IRGA, resting on the bead surface, was configured to do the same. Pulses of CO₂ were introduced into the reservoir and were observed within seconds in the test IRGA. From this we could assume that a drop in Ci_reservoir corresponded to a similar diffusion rate at the bead surface, so no time correction for the test IRGA measurements was needed.

Test Measurements and Flux Comparison
The test IRGA was a second LI-COR 6400 fitted with a 6400-09 soil CO₂ flux chamber. The internal CO₂ of the reservoir was elevated to approximately 3000 µmol mol^–1, and the reference IRGA was programmed to log Ci_reservoir every minute. The maximum accurately detectable concentration listed in the specifications of the LI-COR 6400 is approximately 3000 µmol mol^–1, so care was taken to ensure Ci_reservoir remained less than this. The test IRGA was then readied as specified by LI-COR Inc. (LI-COR Inc., 1997) and placed on a special 10-cm diameter PVC collar inserted into the diffusion layer. This collar was similar to the ones provided by LI-COR for use in field soil respiration measurements, but was 6 cm tall, rather than 4.5 cm, to ensure the top was well above the beads of the diffusion layer. Also, holes were drilled in this collar, well below the surface of the beads, to aid in lateral gas movement within the diffusion layer. Test measurements were then taken and the time carefully noted.

Once the testing cycle was completed (i.e., the Ci_reservoir was near the ambient concentration), we used Eq. [1] to calculate fluxes from the diffusion layer for the 1-min increments between consecutive Ci_reservoir measurements (Fig. 3a) . Then, using nonlinear regression [Rs_generated = aexp(–btime), JMP version 3.2.5 (SAS Institute, Cary, NC)], we predicted Rs_generated for each measurement from the test IRGA. The test fluxes and the generated fluxes were then compared (Fig. 3b). This cycle of measurements was done five times at different locations on the diffusion surface. The five sets of comparisons were pooled and a t test was used to determine if the slope of the line between test and generated fluxes was significantly different from 1. We then used orthogonal regression (JMP version 3.2.5) to predict the measured test fluxes for each of the five trials at generated fluxes of 0.5, 2.5, 4.5, 6.5, 8.5, and 10.5 µmol m^–2 s^–1.

View larger version (23K): [in this window] [in a new window]   Fig. 3. (a) Generated flux rates produced by the testing apparatus for one of the five trials. Rates were calculated by Eq. [1]. The nonlinear equation was used to predict generated fluxes for the times test measurements were made. (b) Correlation between generated and measured fluxes with all five trials combined. (c) A comparison of flux rates when the internal ambient set point of the LI-COR 6400 soil respiration system was set above and below room ambient (500µL L–1).

	Results and Discussion

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Our measured error is generally much smaller than the 15% underestimation reported by Nay et al. (1994), and the 19 and 21% errors reported by Widen and Lindroth (2003). Nay et al. (1994) reported on a different type of soil respiration system, further indicating that manufacturers and designs may differ. Widen and Lindroth (2003) used a similar soil respiration system but failed to include an external fan to match the disturbance and homogenization of the internal fan of the IRGA chamber. From our results and those of Widen and Lindroth (2003) and Le Dantec et al. (1999), it's possible to see the impact of external air movement on the fluxes measured within a chamber. Infrared gas analyzer measurements may approach true values when internal and external wind speeds are closely matched, but appear to respond proportionally to wind speeds above and below that within the chamber (Le Dantec et al., 1999).

We believe that the small discrepancies between test and reference measurements we observed may stem from the heterogeneity of the particles in the diffusion layer (both in depth and in distribution), and from the still present changes in boundary layer CO₂ concentration. Although the addition of the boundary layer mixing fan helped to alleviate this problem, at high internal reservoir concentrations (e.g., 2000–3000 µmol mol^–1) the ambient set point may have been lower than surface layer concentration, resulting in a stronger diffusion gradient inside the test chamber. The opposite is true at low Ci_reservoir (e.g., 600–700 µmol mol^–1) where the ambient set point may be too high. Attempts to increase the mixing of the boundary layer through additional fans created new problems, possibly due to pressure differentials, and lead to overestimations; an effect similar to error associated with concentration differentials. Careful monitoring of the ambient CO₂ concentration and continually adjusting the ambient set point of the IRGA could possibly reduce these measurement errors.

Ambient Test
A test was conducted to quantify the effects of the internal ambient set point on the observed flux rates. We measured surface CO₂ flux at three random locations on the bead surface, each at three different ambient set points: 300, 500, and 700 µL L^–1. We found that when the external ambient was approximately 500 µL L^–1, setting the ambient of the test IRGA to 300 µL L^–1 resulted in an overestimation of 80, 34, and 28% at low, med, and high generated fluxes (Fig. 3c). Setting the ambient to 700 µL L^–1 resulted in underestimations of 70, 26, and 0.3%; interestingly, a setting of 700 µL L^–1 performs better at high rates than did matching the ambient. When the ambient set point was set to approximately match the 500 µL L^–1 of the laboratory, the test IRGA only underestimated the low rates by 5% and overestimated the medium rates by 2%. At high rates, however, the test IRGA overestimated fluxes by 12%.

These results lead us to believe that the flux measured by the test IRGA is very sensitive to the internal ambient set point, although this effect would be muted in the field because of smaller soil particle sizes. The smaller particles of natural soils would increase the strength of the diffusion barrier; this would lead to a larger diffusion gradient and most likely reduce the importance of the small discrepancies in concentration as observed here.

We believe that an apparatus like the one described here could be used to test such problems as differing ambient concentrations, pressure changes, and disturbance effects, but there is substantial need for improvements in the design of these types of testing devices. While the internal concentration (3000 µmol mol^–1) did mimic some systems (Billings et al., 1998), increasing Ci to 5000 or 10000 µmol mol^–1 would more closely resemble a typical soil and would decrease the sensitivity to the ambient concentration gradients. This could be accomplished by using smaller beads that were more representative of native soil particles. Also, we would like to mention that, while the five trials completed with the utmost care resulted in a slight overestimation (Fig. 3b), the results from Fig. 3c indicate that the error could be much higher if changes in ambient concentrations are ignored.

In conclusion, these results demonstrate the potential accuracy of the LI-COR 6400 soil respiration system. Measurement errors may occur; but with careful calibration, proper use, and independent verification, accurate flux measurements may be obtained. A testing system, such as the one described, may be used to determine the absolute accuracy of IRGA based soil respiration systems, for calibrating IRGA-based systems, and for detecting potential sources of measurement error.

	ACKNOWLEDGMENTS

 
This work was supported by the National Institute for Global Environmental Change (NIGEC), Midwest Center; and by the University of Minnesota, Department of Forest Resources. We would like to thank Charlie Paulson for his help and good humor in the lab.

Received for publication May 26, 2003.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Martin, J. G. Articles by Norman, J. M. Search for Related Content PubMed Articles by Martin, J. G. Articles by Norman, J. M. Agricola Articles by Martin, J. G. Articles by Norman, J. M. Related Collections Soil Methods/Instrumentation Biogeochemical Processes Carbon Sequestration Global Change Soil Biology