United States

Department of

Agriculture

Forest Service

Southern

Research Station

Research Paper

SRS-28

Quantitative Comparison of In Situ

Soil CO2 Flux Measurement Methods

Jennifer D. Knoepp and James M. Vose
Jennifer D. Knoepp, Research Soil Scientist, and

James M. Vose, Project Leader, U.S. Department of

Agriculture, Forest Service, Southern Research Station,

Coweeta Hydrologic Laboratory, 3160 Coweeta Lab Road,

Otto, NC 28763.

The Authors

June 2002

Southern Research Station

P.O.Box 2680

Asheville, NC 28802

Chemical terms used in this paper

BACl2 barium chloride

BaCO3 barium carbonate

C carbon

CO2 carbon dioxide

HCl hydrogen chloride

KOH potassium hydroxide

M molar

NaOH sodium hydroxide

OH- hydroxide ion

μmol m-2 sec-1 micromole per meter square per second

The use of trade or firm names in this publication is for reader information and does not

imply endorsement by the U.S. Department of Agriculture of any product or service.
1

Quantitative Comparison of In Situ Soil CO2

Flux Measurement Methods

Jennifer D. Knoepp and James M Vose

Abstract

Development of reliable regional or global carbon budgets requires

accurate measurement of soil CO2 flux. We conducted laboratory

and field studies to determine the accuracy and comparability of

methods commonly used to measure in situ soil CO2 fluxes. Methods

compared included two static techniques, sodium hydroxide (NaOH)

and soda lime as CO2 traps; a multichamber open dynamic system

(OC) with an infrared gas analyzer; and a headspace analysis

system, with gas chromatograph with circulating fan (GCF) and gas

chromatograph without circulating fan (GC), measuring changes in

headspace CO2

 concentration using a gas chromatograph. In the

laboratory, we compared NaOH, OC, GCF, and GC using sand-filled

cylinders to simulate a soil system. Three concentrations of

standard CO2

 gas (representing low, medium, and high soil-CO2 flux

rates) entered the cylinders through individually monitored flow

meters. Flux rates measured using the OC method equaled the actual

CO2 flux at all three CO2 concentrations. Flux rates determined with

GC and GCF were < 55 percent of the actual flux rate. The percent

of actual CO2 flux measured increased with increasing CO2

concentrations. At all concentrations, NaOH collected between 40

and 47 percent of CO2

 entering the system. A field study was

conducted to verify laboratory results and allow comparison with

the soda lime trap (SODA) method. In laboratory and field studies,

all methods detected significant differences in flux rates among the

standard CO2

 concentrations and field sites. Regression analyses

showed good relationships between NaOH, SODA, and GC methods

with flux rates measured using the OC methods (r2 ≥ 0.78). Slope

values for these regression equations ranged from 0.34 for NaOH to

0.54 for GC and SODA. These results suggest that data collected

using the other methods could be standardized to OC flux rates.

However, because methodological differences significantly affect

CO2 flux measurements, care should be used in applying these

relationships.

Keywords: CO2

 traps, flux rates, gas chromatograph, infrared gas

analyzer, NaOH base trap, open dynamic chamber, soda lime trap,

soil respiration.

Introduction

Carbon dioxide fluxes from soil and organic matter on the

soil surface are a major component of the global terrestrial C

cycle (Raich and Schlesinger 1992). To develop accurate C

budgets for terrestrial ecosystems, accurate quantification

of soil, humus, and leaf litter fluxes (termed “forest floor CO2

flux”) are required. Net forest floor CO2 efflux reflects the

combined contributions of respiration from organic matter

decomposition, micro and macro soil organisms, and root

growth and maintenance. This combination of contributing

sources often results in large spatial and temporal variability

in forest floor CO2

 flux. The significance of forest floor CO2

flux in terrestrial C budgets and its spatial and temporal

variability require accurate and precise techniques that are

simple enough to allow multiple site measurements.

Several techniques are available for measuring forest floor

CO2 flux, including static chambers containing soda lime or

alkali base, traps (Cropper and others 1985, Edwards 1982,

Raich and others 1990), soil CO2

 concentration profiles (de

Jong and Schappert 1972), open dynamic chamber methods

(OC) (Edwards and Sollins 1973, Garret and Cox 1973,

Hanson and others 1993, Vose and others 1995), and head

space analysis via gas chromatography (GC), often used

when additional trace gas fluxes are of interest (Castro and

others 1994). Techniques have been compared since the

early 1930s (Smith and Brown 1932), and more recent

comparisons have often revealed wide disparity among

measured fluxes (Cropper and others 1985, Edwards and

Sollins 1973, Norman and others 1992, Raich and others

1990, Rochette and others 1992). For example, static

chamber techniques generally provide lower (10 to 30

percent) CO2

 flux estimates than dynamic chamber methods

(Ewel and others 1987, Rochette and others 1992), and soil

CO2 concentration profile approaches are generally higher

than dynamic chamber techniques (de Jong and others

1979). Variations in CO2 flux estimates among techniques

have been attributed to differing protocols, the interaction

of certain techniques with environmental conditions

(Edwards 1982, Naganawa and others 1989), and the

magnitude of CO2 flux (Jensen and others 1996, Rochette

and others 1992). For example, the effectiveness of NaOH as

a trap for CO2 varies with base normality (Minderman and

Vulto 1973), incubation time (Kirita and Hozumi 1966), soil

moisture (Edwards 1982), and soil temperature (Naganawa

and others 1989). Soil CO2 fluxes measured with a base trap

and an open dynamic chamber method are similar at low flux

rates but differ greatly at high fluxes (Jensen and others

1996).

While the results of previous comparisons demonstrate an

often predictable disparity among techniques, few studies

have evaluated the accuracy of the various approaches

(Hiroki and Watanabe 1997, Nay and others 1994) under

controlled conditions. Our objective was to test the

accuracy and comparability of common methods of

measuring CO2

 flux from soil. Method accuracy was

examined using a simulated soil system in the laboratory.
2

We tested the static base trap (NaOH), open dynamic

chamber (OC), headspace analysis with gas chromatograph

with a circulating fan (GCF) and without a circulating fan

(GC). Field tests were conducted to verify results and

included two static methods [NaOH and soda lime trap

(SODA)] and two dynamic methods, OC and GC. Combining

the laboratory results with field measurements allowed us to

assess the comparability of methods.

Materials and Methods

We tested both static and dynamic methods of measuring

soil CO2 flux. First we mimicked a soil system and made

laboratory comparisons of flux rates using sand-filled

polyvinyl chloride (PVC) cylinders and a range of standard

CO2

 gases. Second, we conducted field comparisons on a

number of sites with a range of CO2 flux rates. The

laboratory comparisons, with known CO2 flux rates, allowed

us to test the accuracy of each method. The field

comparisons allowed us both to verify the laboratory

results and to compare the methods.

Open Dynamic Chamber

The OC method measures the difference in CO2

concentration of inlet air (ambient) as it enters the chamber

and outlet air as it leaves the chamber. The OC system

consists of monitored air delivery and return systems and

an ADC LCA4 infrared gas analyzer (IRGA). The OC system

is automated, battery powered, and designed to measure

soil CO2 flux from 10 chambers, at 100-minute intervals, over

a 24-hour period. Inlet air was delivered to the chamber from

a 20-L bottle, which was located 20 m away from the sample

collection site to prevent concentration fluctuations. Inlet

air CO2 concentration was measured (after drying with

Drierite®) from the buffer using a second sample line

connected to the IRGA. A perforated tube (diffuser) in the

headspace of the chamber was used to mix inlet and

headspace air (fig. 1A). Headspace air (outlet air) and inlet

air were sampled simultaneously using a separate pump at a

flow rate equal (+/- 5 percent) to inlet flow. Outlet air was

dried and delivered to the IRGA for determination of CO2

concentration. Inlet and outlet airflow rates resulted in at

least one turnover of the chamber headspace per minute.

Inlet and outlet CO2 concentrations were measured for 10

minutes (differences between the inlet and outlet air

stabilized after 6 to 7 minutes), and CO2

 flux rates were

determined by the difference in inlet and outlet CO2

concentration during the last minute of sampling [flux =

(inlet CO2 - outlet CO2) x inlet flow rate].

Gas Chromatograph

The GC method measures change in headspace CO2

concentration over time using a gas chromatograph. The

area to be sampled was covered with a chamber top, and a

gas-tight syringe was immediately inserted through a

septum-covered access port to collect the initial gas sample.

Successive samples were collected to determine change in

chamber CO2 concentration. Filling one 50-mL gas syringe

with chamber air and rapidly expelling it into the chamber

immediately before sample collection mixed headspace

gases. Needles were sealed with septa until analysis. Five

samples were collected from each chamber at 3- to 5-minute

intervals, and exact times were recorded. Samples were

analyzed for CO2

 concentration using a Varion 3700 GC. Flux

rates were determined by calculating the slope of change in

concentration over time using only the linear portion of the

curve. The data were graphed to determine linearity, and

linear regression analyses of the data were conducted (r2 ≥

0.95). If nonlinearity was observed (i.e., an asymptote

reached at the end of the measurement period), the

nonlinear portion of the data was deleted and regression

analysis conducted on the linear portion.

NaOH Base Trap

The base trap method (NaOH) is a static measurement

method in which CO2 entering the sample chamber is

trapped by a chemical reaction with the base. NaOH (2.0 M)

was put in an open jar and placed on or above the soil

surface beneath a closed chamber. NaOH blanks (zero CO2

flux) are necessary to calculate net CO2 flux. In laboratory

tests, NaOH in open jars was sealed in PVC cylinders at

equilibrium with the standard test gas, and in field tests in

3.1-L cans at equilibrium with the atmosphere. NaOH traps

were removed after 24 hours. Excess BaCl2 was added to the

NaOH to precipitate the carbonate as BaCO3, and samples

were titrated with standardized 1 M HCl to determine the

amount of unreacted NaOH. Singh and Gupta (1977) found

that the surface area of the NaOH solution in proportion to

the total surface area covered by the chamber may affect the

adsorption of CO2. Anderson (1982) recommends that base

solution surface area should be about 25 percent for the

most efficient trapping of CO2. Kirita and Hozumi (1966)

found that the rate of CO2 absorption was limited unless 80

percent of the base solution remained unreacted. The

surface area of NaOH solution in our study was 4 percent in

the field and 5 percent in the lab, suggesting that CO2

 flux

measurements could be biased downward. We conducted a

field test to calculate the effect of base trap surface area on

CO2 adsorption by 2.0 M NaOH. Base trap containers

covering 5, 12, and 37 percent of the total collection area
3

Cap

Headspace 8cm

15 cm

Sand

10cm

Inlet

Port

Outlet Port

w/ Septum

15.3 c m

Vent

Dif f us er

B

Cap

Headspace 8cm

15 cm

Sand

10cm

Inlet

Port

Outlet Port

w/ Septum

Fan

15.3 cm

Vent

Diffuser

C

Cap

8cm

Headspace

15 cm

Sand

10cm

Inlet

Port Diff user

Outlet Port

w/ Septum

Stand

Base Trap

15.3 cm

Vent

D

B

Figure 1— Diagram of cylinders used to simulate a soil system in the laboratory to compare in situ CO2 flux methods. All cylinders contain 10 cm of

Ottawa-density sand and an inlet port with diffuser in the base. Cylinders show modifications necessary for each method; (A) inlet port with diffuser

in the cylinder headspace, no septum on outlet port (OC method); (B) no modification (GC method); (C) 4 cm circulating fan in cylinder headspace

(GCF method); (D) stand and base trap container (NaOH method).

Cap

8cm

Headspace

8.0 cm

Sand

10cm

Inlet

Port

Outlet Port

to IRGA

15.3 cm

Inlet Port

w/ Diff user

Vent

Diffuser

Inlet port

w/ diffuser Outlet port

to IRGA

Diffuser

Inlet

port

Headspace

15 cm

Vent

Outlet port

w/ septum

Cap

8cm

Sand

10 cm

Diffuser

Inlet

port

Headspace

15 cm

Vent

Outlet port

w/ septum

Cap

8cm

Sand

10 cm

Diffuser

Inlet

port

Headspace

15 cm

Vent

Outlet port

w/ septum

Cap

8cm

Sand

10 cm

Diffuser

Fan

Base trap

Stand

A

Inlet

port
4

were used. There was no significant difference between CO2

flux measured using the 5 and 12 percent trap size; however,

the measured flux rate of the trap with a surface area 37

percent of the collection area was 15 percent greater than

the other two sizes. Based on this finding, we corrected all

NaOH measurements for base trap size prior to statistical

analyses.

Soda Lime Trap

Another static measurement method used was the SODA

method. In this method, soda lime is used to trap CO2

entering the sample chamber. Ovendried (105 oC) soda lime

(1.5-2.0 mesh) was put in an open jar and placed on the soil

surface beneath a closed chamber. Blanks necessary for CO2

flux calculation were sealed in cylinders as described for the

base trap method. Soda lime traps were removed after 24

hours, ovendried, and reweighed to determine CO2

absorbed.

Laboratory Comparisons

Laboratory comparisons were conducted using PVC

cylinders to simulate a soil/gas system (fig. 1A-D) and

generate known CO2 fluxes. Within the cylinders, which were

15.3 cm inside diameter and 30 cm long, we placed a diffuser

in the bottom chamber, 10 cm of Ottawa density sand, and

PVC caps at the top and bottom with access ports. Standard

CO2 gases entered the PVC cylinders through the bottom

access port. Headspace in the cylinders was 1470 cm3 for the

OC measurements but 2756 cm3 for the other methods to

allow space for any modifications. An airflow regulator

maintained a constant pressure to a manifold that distributed

the gas to each test cylinder at approximately 50 mL per

minute. Flow meters linked to a data logger continuously

monitored the actual flow rate of standard CO2 into each

cylinder. Before making any measurements, we flushed

standardized gases through all cylinders until airflow in and

airflow out reached equilibrium (about 2 hours). Blanks were

set up for all methods tested. Blank cylinders also were

equilibrated with the standard gases; then, gas flow was

turned off and the bottom port of the cylinder was closed.

We did not test the SODA method in the laboratory because

dry CO2 was delivered to the chambers, and soda lime

requires the presence of water to react with CO2.

Over about 2 months we measured CO2

 flux through the

cylinders using each method. All measurements were

compared with the actual CO2 flux, which we calculated from

recorded flow rates (approximately 50 mL per minute) and

standard CO2 concentrations. Standards used included 604

parts per million, 1,023 parts per million, and 3,039 parts per

million CO2

, producing a range of CO2 fluxes from 1.1 to 5.9

μmol m-2 s-1, similar to those measured in most field studies.

We took lab measurements using four ports (three test

cylinders, one blank) of the 10-port OC system (fig. 1A).

Inlet and outlet airflow rates were approximately 1900 mL per

minute allowing greater than one headspace turnover per

minute. The balanced flow (inlet and outlet) and low-flow

rates are shown to reduce or eliminate potential effects of

pressure differences on flux estimates (Fang and Moncrieff

1996). Flux rates were calculated, and blank cylinder flux

rates (no CO2 flow) were subtracted.

We examined two variants of the GC method in the

laboratory—the first using a standard PVC cap alone (fig

1B). In the second, we placed a 4-cm computer fan attached

to a 9-V battery (GCF) inside the cylinder cap and used

headspace analysis to determine the effect of air mixing on

CO2

 flux measurements (fig. 1C). We collected five 20-mL

gas samples for flux determination, at time zero and at 3-

minute intervals. Actual sample collection times were

recorded. We calculated flux rates and subtracted blank

cylinder flux values.

For the NaOH method, we placed 20 mL of 2.0 M NaOH in a

jar 3.6 cm in diameter on a stand on the sand surface and

capped the PVC cylinder (fig. 1D). The NaOH traps and

blank were removed after 24 hours, and total CO2 absorbed

determined. The blank-cylinder CO2 value was subtracted

from the measured flux rate.

Field Comparisons

Field measurements were conducted using NaOH, SODA,

OC, and GC methods to determine soil CO2 flux within 0.0625

m2 aluminum bases inserted 2 cm into the soil. Plexiglass

chamber tops (6.8-L volume) were used for measurements in

three of the methods—NaOH, SODA, and GC. We located

the bases in two groups of four about 30 m apart on three

forest sites and in one fescue pasture (eight bases per site).

Forest types included two aggrading mixed oak-hardwood

communities with rhododendron in the understory and one

aggrading cove hardwood community. Bases on the

forested sites had been in place for 3 years as part of a

separate study. The fescue pasture site was well maintained,

fertilized, and limed regularly and harvested (hay) twice a

year. Aluminum bases also were used on this site but were

installed just before flux measurements. All sites were

located on coarse or fine loamy-textured soils with high

permeability in the Dystrochrepts or Hapludults great

groups.
5

The four field sites were sampled between August 20 and

October 2, 1997. Carbon dioxide flux measurements using all

methods were made over a 30-hour period on each site.

Weather patterns were stable at the time of measurement, and

no precipitation events occurred during the flux-measurement

period. The same measurement sequence was followed on all

sites. First, NaOH and soda lime flux rates were measured

coincidentally with two chambers of each group of four (n = 4

per site per method). Thirty-six mL of 2.0 M NaOH or 36 g of

ovendried soda lime were placed in a glass jar 5.5 cm in

diameter. Jars were placed on the soil surface and covered

with the chamber top for 24 hours. Blank values were

obtained by placing identical jars of NaOH and soda lime in

3.1-L steel cans and sealing the lid for 24 hours.

Next, GC flux was measured after chamber tops were

removed and bases left uncovered for 2 hours to allow soil

gas flow to re-equilibrate. Measurements were made on the

two sets of four bases sequentially (n = 8 per site). A timezero gas sample (40 mL) was collected using a 50-mL gastight syringe after the plexiglass chamber tops were placed

on bases and secured. Chambers were sampled again in the

same order, approximately every 5 minutes, until five

samples were collected from the headspace of each of the

four chambers. Total time elapsed averaged 25 minutes.

Sealed syringes were returned to the laboratory where

samples were analyzed for CO2 within 24 hours. Fluxes of

CO2 were calculated as previously described.

Finally, OC fluxes were measured after GC sample collection

was complete. Chamber tops were removed and soil gas

fluxes allowed to re-equilibrate for about 30 minutes (less

time was necessary because chamber tops were in place for

a shorter time for GC flux measurements than for static

measurements). Measurements were made on the two sets

of four bases sequentially (n = 8 per site). Four 1-L PVC

chambers were placed in the center of the bases and pushed

through the forest floor into the mineral soil to ensure that

they were sealed from the atmosphere. Each chamber was

sampled for 10 minutes to allow the CO2

 concentration

within the chamber to equilibrate. Inlet and outlet airflow

rates were 1000 mL per minute. These values were used to

calculate CO2 flux [flux = (outlet concentration - inlet

concentration) x flow].

Statistical Analyses

Data were analyzed using the General Linear Models

Procedure of SAS Institute Inc. (SAS 1985) and Tukey’s

mean separation test. For each method, accuracy was

determined using laboratory data by calculating the

difference between actual and measured CO2

 flux rates

through the PVC cylinder. Examining method differences

among flux rates and sites for both laboratory and field

measurements tested the comparability of methods. We

determined method effectiveness by examining differences

among sites for each method and conducted regression

analyses using the mean value for each group of bases and

each standard gas to establish the relationships among

methods.

Results and Discussion

Laboratory Measurements—Accuracy

The OC system was the most accurate method; measured

flux rates did not differ significantly from the actual rates

calculated for each standard CO2 gas (fig. 2). In contrast, the

other methods were less accurate, and their measured flux

rates were lower than the actual. For some, the magnitude of

these differences varied with the actual flux rate. NaOH

consistently measured 40 to 47 percent of the actual flux

rate, whereas GC measurement accuracy increased with

increasing actual flux rate (fig. 2). At the lowest flux rate (1.1

μmol m-2 s-1), GCF measured a negative rate, and GC

measured 19 percent of actual. Both GCF and GC methods

measured approximately 30 percent of the actual flux for the

medium rate (2.0 μmol m-2 s-1) and 42 to 54 percent of the

high flux rate (5.9 μmol m-2 s-1) (fig. 2).

To assess the precision of measurement techniques, we

compared coefficients of variation (CV’s). Actual flux rates

had CV’s < 6 percent, which were caused by variation in

flow rates among replicate PVC cylinders and represent the

experimental error (fig. 3). Hence, the CV’s of the four

measurement techniques reflected the methodological error

plus the experimental error. Considerable variation in CV’s

was observed among measurement techniques and across

actual flux rates (fig. 3). In general, the OC method had the

lowest CV’s (5 to 12 percent), followed by NaOH (8 to 21

percent), GC (10 to 24 percent), and GCF (8 to 70 percent).

With the exception of the NaOH method, CV’s for all

methods were lowest at the medium rate and greatest at the

low rate.

A major limitation in evaluating techniques for measuring

soil CO2 flux in situ is the lack of an actual flux rate. We

are aware of only two other attempts to generate a known

CO2

 flux rate in the laboratory. Nay and others (1994)

diffused CO2 beneath a foam column and used Fick’s law

of diffusion to calculate the actual CO2 flux rate from the

foam surface. They found good agreement between a

closed flow-through dynamic chamber system
6

[CDC-headspace air is recirculated to detect changes in CO2

concentration (Norman and others 1992)] and actual flux,

except at high actual flux rates (~5 μmol m-2 s-1) where

measured flux was 20 percent lower than actual. Soda lime

traps overestimated (25 percent) actual flux at low rates (~1

μmol m-2 s-1) and underestimated (57 percent) actual flux at

high rates. Bekku and others (1997) measured glucose

consumption and CO2 flux by fungal respiration (maximum

rates ~10 to 20 μmol m-2 s-1). They found good agreement

(95 percent) among OC, CDC, and GC methods compared to

calculated flux rates, whereas an alkali absorption method (1

M KOH) overestimated actual flux by 30 percent. Our

laboratory results indicated that only the OC method

provided accurate estimates of actual flux, whereas all other

methods (GC, GCF, and NaOH) underestimated the CO2

 flux.

Differences in these laboratory approaches to generating

actual flux, the range of actual fluxes, and the choice of

methods and protocols complicates any attempt to compare

them. Like previous laboratory studies (Bekku and others

1997, Nay and others 1994), our method for generating

actual flux differed from natural processes. The efflux of CO2

from soil occurs through the process of diffusion based on

concentration gradients, whereas our technique used a

continuous flow of CO2 to generate actual flux. Hence, we

also conducted field comparisons to determine if the

patterns observed under controlled/artificial conditions

were reproduced in the field.

Field Measurements—Comparability

All four methods—SODA, NaOH, OC, and GC—measured

significant differences among the four study sites (tables 1,

2). In all methods, the pasture site had the greatest CO2

 flux

rate (table 2). In general, field results were similar to those in

the laboratory; CO2 flux rates measured using the OC

method were greater than those measured using all other

Figure 2—Results of laboratory experiment showing measured flux of CO2 minus actual CO2 flux

(μmol m-2 sec-1). Solid bars represent mean values of three PVC cylinders used for each method and

⊥ represents standard errors.

1.0

0.0

-1.0

-2.0

-3.0

-4.0

1.15 1.99 5.99

Actual flux (μmol m-2 sec-1)

Actual flux - measured (

μmol m-2 sec-1)

OC

GC

GCF

NAOH
7

Table 1—Results of analysis of variance examining the

effect of site for each method

Method df F-value Prob > F CV

SODA 14 49.8 < 0.01 29

NaOH 15 7.0 < 0.01 33

OC 31 9.4 < 0.01 47

GC 31 6.2 < 0.01 44

df = degrees of freedom; Prob > F = probability of a value greater

than F; CV = coefficient of variation; SODA = soda lime static

trap; NaOH = 2.0 M NaOH static trap; OC = open dynamic

chamber; and GC = gas chromatograph.

Figure 3—Coefficient of variation versus CO2

 flux rate (μmol m-2 sec-1) in the laboratory showing the

mean of all fluxes measured at low (1.1), mid (2.0), and high (5.9) flux rates. Values represent calculated

actual fluxes (Actual, n = 12) and CV’s for four methods tested (n = 3).

methods. The GC method measured 51 to 71 percent of the

OC field flux rates, compared to 54 percent of OC at high

CO2 flux rates in the laboratory. Similarly, the NaOH field

measurements were consistent with laboratory results,

measuring 38 to 55 percent of the CO2 flux measured by the

OC. The SODA method gave inconsistent measurements

when compared with OC. The percent of OC flux measured

increased with increasing flux rates ranging from 11 to 40

percent.

Previous method comparisons have shown inconsistent

responses using static trap methods, and results often vary

with CO2 flux rates (Cropper and others 1985, Edwards and

Sollins 1973, Jensen and others 1996, Rochette and others

1992). Static chamber methods may overestimate or equal

fluxes as determined with OC, CDC, or GC at low rates (e.g.,

80

70

60

50

40

30

20

10

0

1.1 2.0 5.9

Flux (μmol m-2 sec-1)

Coefficient of variation (percent)

Actual

OC

GC

NaOH

GCF
8

when soils are cool), and underestimate fluxes when rates

are high (Jensen and others 1996, Nakadai and others 1993,

Nay and others 1994, Rochette and others 1997). In this

study, both static methods yielded flux rates lower than the

OC method in field and laboratory comparisons. Somewhat

lower flux values in the static trap field measurements were

expected due to the 24-hour sample period because this

cumulative measure incorporates the diurnal variation in

bulk soil CO2

 flux rates. We conducted the OC and GC

measurements between 10 a.m. and 2 p.m., when CO2 flux

rates are increasing due to increasing soil temperature.1

Diurnal measurements showed that midday flux rates range

from 8 to 16 percent greater than the 24-hour mean.

However, diurnal sampling does not appear to have affected

the NaOH measurements, which were consistently 38 to 55

percent of the OC flux rate in both field and laboratory.

Some of the variation in results of NaOH method

comparisons may be due to procedural differences.

Minderman and Vulto (1973) found that measured CO2

fluxes increase with increasing base trap molarity, with 2.0

M the maximum tested. Kirita and Hozumi (1966) found that

when the amount of OH- available for reaction falls below

80 percent, CO2 absorption is limited. The lack of reactive

base available may explain the decrease in measured CO2

flux with increasing rates found in many studies. We used

2.0 M NaOH for our static trap method, assuring

nonlimiting reactive base during the 24-hour measurement

period, which could explain why we observed consistent

measurements using NaOH across all flux rates in

laboratory and field. The surface area of the base solution

as a proportion of the collection chamber also affects the

absorption of CO2

 (Singh and Gupta 1977). Studies have

shown that the surface area should be ≥ 6 percent of the

soil surface covered by the chamber, and that maximum

absorption may occur at about 25 percent coverage

(Anderson 1982, Raich and Nadelhoffer 1989). The

normality of bases used in previous methods comparisons

ranges from 0.2 M to 1.0 M, and base trap surface areas

covered between 16 and 25 percent of the total collection

area (Cropper and others 1985, Edwards and Sollins 1973,

Rochette and others 1992). Few studies have tested for the

effect of collection solution surface area on flux rates with

nonlimiting reactive base available, but our study indicates

that for 2.0 M NaOH there was a 15-percent difference

between 5 to 12 percent versus 37-percent base surface

area.

The reliability of SODA as a CO2 trap is dependent both on

procedural differences, such as the amount and mesh size of

the soda lime used (Minderman and Vulto 1973) and on

environmental conditions during measurements. Because

water is necessary for the adsorption of CO2 by soda lime,

the moisture content both of the soda lime and the soil air

being sampled may affect adsorption (Edwards 1982). The

variability in our results—11 to 40 percent when compared

to OC—could result from site differences in soil moisture

(cove versus midslope hardwood forest), a variable that was

not noted in this study.

The few studies that compare OC and GC techniques

indicate very good agreement between methods (Bekku and

others 1995, Bekku and others 1997). In our study, the GC

method considerably underestimated flux relative to the OC

method, both in the laboratory and the field. One possible

explanation for differences obtained in the field is the lack of

a circulating fan in the GC chambers in the field comparison

(6,670 cm3 1 volume). In laboratory tests, the circulating fan Knoepp, Jennifer D. 2001. Unpublished data on file with the author.

Table 2—Site means of in situ soil CO2 flux (μmol m-2 sec-1) measurements for

common methods

 In situ soil CO2

 flux (μmol m-2 sec-1)

b

Methoda Forest 1 Forest 2 Forest 3 Pasture

SODA(4) 0.68 (0.06) 0.22 (0.07) 0.24 (0.03) 2.13 (0.22)

NaOH (4) 1.06 (0.23) 0.75 (0.11) 1.23 (0.21) 2.04 (0.25)

OC (8) 2.66 (0.37) 1.96 (0.55) 2.24 (0.20) 5.32 (0.73)

GC (8) 1.90 (0.26) 1.26 (0.11) 1.28 (0.13) 2.74 (0.47)

SODA = soda lime static trap; NaOH = 2.0 M NaOH static trap; OC = open dynamic chamber;

and GC = gas chromatograph.

a

 Values in parentheses next to each method are the number of flux measurements in the

mean.

b

 Values in parentheses following each mean are standard errors of the mean.
9

actually decreased measured CO2

 flux. However, the large

chamber in the field test may require better mixing of gases

in the headspace than was obtained by sample collection

procedures.

Based on the similarity of field and laboratory results, we

concluded that the artificial approach (i.e., continuous

supply of CO2

 beneath a sand column) used in the

laboratory to simulate actual soil CO2 flux yielded valid flux

rate measurements. Hence, we combined field and

laboratory data for regression analyses between NaOH,

SODA, and GC, and the OC method (table 3). All methods

were significantly correlated with OC (p-values < 0.01).

Slopes were ≤ 0.53, indicating that all methods

underestimated CO2

 flux rates compared with the OC method

(fig. 4). The strength of the relationships between all

methods and the OC method suggests that these equations

Table 3—Results of regression analysis showing

relationship between in situ soil CO2 flux methods, soda

lime trap static chamber, NaOH trap static chamber,

closed dynamic chamber—gas chromatograph method,

and the open dynamic chamber–IRGA system

Method df Intercepta Slopea r

2

SODA 7 -0.79 (0.30) 0.53 (0.09) 0.85

NaOH 10 0.22 (0.21) 0.34 (0.06) 0.78

GC 10 0.05 (0.25) 0.53 (0.07) 0.86

df = degrees of freedom; SODA = soda lime static trap; NaOH =

NaOH static trap; and GC = gas chromatograph method.

a

 Values in parentheses following each mean are standard errors of

the mean.

Figure 4—Regression of CO2 flux measured with closed chamber system using gas

chromatograph analysis of changes in headspace CO2 concentration (GC), static

2.0 M NaOH base trap (NaOH), and static soda lime trap (SODA) with the open

dynamic chamber infrared gas analyzer system (OC).

SODA r2 = 0.85

GC r2 = 0.86

NaOH r2 = 0.78

OC CO2 flux (μmol m-2 sec-1)

0123456

6

5

4

3

2

1

0

CO2 flux (

μmol m-2 sec-1)

GC-field

GC-lab

NaOH-field

NaOH-lab

SODA-field

1:1 line
10

could be used to standardize flux measurements and allow

comparisons of data in different studies. However, because

method procedures may affect results, standardizing CO2

flux rate measurements using different methods must be

done carefully.

Laboratory studies showed that the OC system gave the

most accurate CO2 flux measurements. Field methods

comparison showed that OC measurements yielded the

greatest flux rates on all sampled sites. However, all

methods tested—SODA, NaOH, GC, and OC—measured

significant differences in flux rates among sites. This

suggests that all four methods may be useful for detecting

treatment effects and site differences, but only OC is

suitable for estimating ecosystem carbon budgets.

Acknowledgments

Project funds from the U.S. Department of Agriculture,

Forest Service, Southern Research Station, Coweeta

Hydrologic Laboratory, supported this research. We thank

Paul Hanson, Helga Van Miegroet, and Robert Mitchell for

helpful reviews of the manuscript and Chris Sobek for

technical assistance.

Literature Cited

Anderson, J.P.E. 1982. Soil respiration. In: Page, A.L.; Miller,

R.H.; Kenney, D.R., eds. Methods of soil analysis, Part 2

Chemical and Microbiological Properties. Madison, WI: ASA

and SSSA: 831–871.

Bekku, Y.; Koizumi, H.; Nakadai, T.; Iwaki, H. 1995.

Measurement of soil respiration using closed chamber method:

an IRGA technique. Ecological Research. 10: 369–373.

Bekku, Y.; Koizumi, H.; Oikawa, T.; Iwaki, H. 1997.

Examination of four methods for measuring soil respiration.

Applied Soil Ecology. 5: 247–254.

Castro, M.S.; Peterjohn, W.T.; Melillo, J.M.; Steudler, P.A.

1994. Effects of nitrogen fertilization on the fluxes of N2

O,

CH4, and CO2 from soils in a Florida slash pine plantation.

Canadian Journal of Forest Research. 24: 9–13.

Cropper, W.P.; Ewel, K.C.; Raich, J.W. 1985. The measurement

of soil CO2 evolution in situ. Pedobiologia. 28: 35–40.

de Jong, E.; Redmann, R.E.; Ripley, E.A. 1979. A comparison of

methods to measure soil respiration. Soil Science. 127(5):

300–306.

de Jong, E.; Schappert, H.J.V. 1972. Calculations of soil

respiration and activity from CO2

 profiles in the soil. Soil

Science. 113: 328–333.

Edwards, N.T. 1982. The use of soda-lime for measuring respiration

rates in terrestrial ecosystems. Pedobiologia. 23: 321–330.

Edwards, N.T.; Sollins, P. 1973. Continuous measurement of

carbon dioxide evolution from partitioned forest floor

components. Ecology. 54: 406–412.

Ewel, K.C.; Cropper, W.P. Jr.; Gholz, H.L. 1987. Soil CO2

evolution in Florida slash pine plantations. I. Importance of

root respiration. Canadian Journal of Forest Research. 17:

325–329.

Fang, C.; Moncrieff, J.B. 1996. An improved dynamic chamber

technique for measuring CO2 efflux from the soil surface.

Functional Ecology. 10: 297–305.

Garrett, H.E.; Cox, G.S. 1973. Carbon dioxide evolution from the

forest floor of an oak-hickory forest. Soil Science Society of

America Journal. 37: 641–644.

Hanson, P.J.; Wullschleger, S.D.; Bohlman, S.A.; Todd, D.E.

1993. Seasonal and topographic patterns of forest floor CO2

efflux from an upland oak forest. Tree Physiology. 13: 1–15.

Hiroki, M.; Watanabe, M.M. 1997. Field measurement of carbon

dioxide evolution from soil by a flow-through chamber method

using a portable photosynthesis meter. Soil Science and Plant

Nutrition. 43: 255–260.

Jensen, L.S.; Mueller, T.; Tate, K.R. [and others]. 1996. Soil

surface CO2 flux as an index of soil respiration in situ: a

comparison of two chamber methods. Soil Biology and

Biochemistry. 28: 1297–1306.

Kirita, H.; Hozumi, K. 1966. Re-examination of the absorption

method of measuring soil respiration under field conditions. I.

Effect of the amount of KOH on observed values. Seiri, Seitai.

14: 23–31.

Minderman, G.; Vulto, J.C. 1973. Comparison of techniques for

the measurement of carbon dioxide evolution form soil.

Pedobiologia. 13: 73–80.

Naganawa, T.; Kyuma, K.; Yamamoto, H. [and others]. 1989.

Measurement of soil respiration in the field: influence of

temperature, moisture level, and application of sewage sludge

compost and agro-chemicals. Soil Science and Plant Nutrition.

35: 509–516.

Nakadai, T.; Koizumi, H.; Usami, Y. [and others]. 1993.

Examination of the method for measuring soil respiration in

cultivated land: effect of carbon dioxide concentration on soil

respiration. Ecological Research. 8: 65–71.

Nay, S.M.; Mattson, K.G.; Bormann, B.T. 1994. Biases of

chamber methods for measuring soil CO2

 efflux demonstrated

with a laboratory apparatus. Ecology. 75: 2460–2463.

Norman, J.M.; Garcia, R.; Verma. S.B. 1992. Soil surface CO2

fluxes and the carbon budget of a grassland. Journal of

Geophysical Research. 97: 18,845–18,853.

Raich, J.W.; Bowden, R.D.; Steudler, P.A. 1990. Comparison of

two static chamber techniques for determining carbon dioxide

efflux from forest soils. Soil Science Society of America Journal.

54: 1754-1757
11

Raich, J.W.; Nadelhoffer, K.J. 1989. Belowgraound carbon

allocation in forest ecosystems: global trends. Ecology. 70:

1346–1354.

Raich, J.W.; Schlesinger, W.S. 1992. The global carbon dioxide

flux in soil respiration and its relationship to vegetation and

climate. Tellus. 44D: 81–99.

Rochette, P.; Ellert, B.; Gregorich, E.G. [and others]. 1997.

Description of a dynamic closed chamber for measuring soil

respiration and its comparison with other techniques. Canadian

Journal of Soil Science. 77: 195–203.

Rochette, P.; Gregorich, E.G.; Desjardins, R.L. 1992.

Comparison of static and dynamic closed chambers of

measurement of soil respiration under field conditions. Canadian

Journal of Soil Science. 72: 605–609.

SAS Institute Inc. 1985. SAS user’s guide: statistics. Vers. 5. Cary,

NC: SAS Institute Inc.

Singh, J.S.; Gupta, S.R. 1977. Plant decomposition and soil

respiration in terrestrial ecosystems. The Botanical Review. 43:

449–528.

Smith, F.B.; Brown, P.E. 1932. Further studies on soil respiration.

Journal of the American Society of Agronomy. 24: 577–583.

Vose, J.M.; Elliott, K.J.; Johnson, D.W. [and others]. 1995.

Effects of elevated CO2

 and N fertilization on soil respiration

from ponderosa pine (Pinus ponderosa Doug. Ex. Laws.).

Canadian Journal of Forest Research. 25: 1243–1251.
12
Knoepp, Jennifer D.; Vose, James M. 2002. Quantitative comparison of in situ soil CO2 flux measurement

methods. Resch. Pap. SRS–28. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern

Research Station. 11 p.

Development of reliable regional or global carbon budgets requires accurate measurement of soil CO2

 flux. We

conducted laboratory and field studies to determine the accuracy and comparability of methods commonly used to

measure in situ soil CO2 fluxes. Methods compared included two static techniques, sodium hydroxide (NaOH) and

soda lime as CO2 traps; a multichamber open dynamic system (OC) with an infrared gas analyzer; and a headspace

analysis system, with gas chromatograph with circulating fan (GCF) and gas chromatograph without circulating

fan (GC), measuring changes in headspace CO2 concentration using a gas chromatograph. In the laboratory, we

compared NaOH, OC, GCF, and GC using sand-filled cylinders to simulate a soil system. Three concentrations of

standard CO2 gas (representing low, medium, and high soil-CO2 flux rates) entered the cylinders through

individually monitored flow meters. Flux rates measured using the OC method equaled the actual CO2 flux at all

three CO2 concentrations. Flux rates determined with GC and GCF were < 55 percent of the actual flux

rate. The percent of actual CO2 flux measured increased with increasing CO2 concentrations. At all

concentrations, NaOH collected between 40 and 47 percent of CO2 entering the system. A field study was

conducted to verify laboratory results and allow comparison with the soda lime trap (SODA) method. In

laboratory and field studies, all methods detected significant differences in flux rates among the standard

CO2 concentrations and field sites. Regression analyses showed good relationships between NaOH, SODA,

and GC methods with flux rates measured using the OC methods (r2 ≥ 0.78). Slope values for these

regression equations ranged from 0.34 for NaOH to 0.54 for GC and SODA. These results suggest that data

collected using the other methods could be standardized to OC flux rates. However, because methodological

differences significantly affect CO2 flux measurements, care should be used in applying these relationships.

Keywords: CO2 traps, flux rates, gas chromatograph, infrared gas analyzer, NaOH base trap, open dynamic

chamber, soda lime trap, soil respiration.
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