Emissions of CO2 from some soils

Chemosphere,Vol. 30, No. 10, pp. 1875-1889, 1995

0045-6535(95)00069-0

Copyright O 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/95 $9.50+0.00

EMISSIONS OF CO 2 FROM SOME SOILS

C. Dueflas, M.C. Fern(mdez, J. Carretero, E. Liger and M. P6mz Department of Applied Physics Faculty of Sciences. University of M~laga. 29071 MALAGA (SPAIN)

(Received in Germany 12 October 1994; accepted 14 February 1995)

ABSTRACT.- Emissions of carbon dioxide were measured from diverse soils in the surroundings of Malaga (Spain). These measurements were carded out by two methods. A direct method using a static open chamber technique and another indirect method obtained from simultaneously measured 2-'2Rnflux and concentration profile measurements of 2nRn and CO2 in the air of soil. The directly measured CO2 fluxes at investigated sites is slightly higher than CO2 fluxes derived from the indirect method. Carbon dioxide emitted by soils, showed a mean direct flux to the atmosphere of 5 mmol m 2 h "~.

1.- INTRODUCTION

Carbon dioxide is produced in the soil by microbial decomposition of soil organic matter and by root respiration. The CO 2 flux at the soil surface and CO2 concentration in soil air depend both on CO2 production and gas transport in the soil air by molecular diffusion in the unsaturated soil zone. To trace gas transport, the radioactive noble gas 222Rn is used (INirr et al., 1983). In this way, we are able to distinguish between pure transport effects and effects on soil gas production or decomposition rates.222Rn is produced in the soil by radioactive decay of =6Ra which is distributed rather uniformly in the soil matrix. The concentration profile of 222Rn and the 2:2Rn flux at soil surface are, therefore, only controlled by individual soil texture and soil parameters and thus provide a measurement of gas transport parameters in the soil at an individual site. The indirect method for evaluating carbon dioxide flux is obtained from the simultaneously 222Rn flux at the soil surface and concentration profile measurements of 222Rn and CO2 in the air of soil. Direct carbon dioxide flux is measured by placing a static open chamber at the soil surface and observing the carbon dioxide concentration change with time. These measurements are disturbed by above-ground vegetation and yield no reliable data for soil CO 2 production except for measurements on bare soil. For this mason, the determination of carbon dioxide flux at the soil surface by indirect method should give more reliable data than direct flux measurements. 1875

1876

In this paper, using both methods we have determined the carbon dioxide flux at the soil surface in different kinds of soils in the surroundings of M~tlaga over different periods on bare and vegetation soils. The results are discussed.

2.- THEORETICAL CONSIDERATIONS Gas transport in the unsaturated soil zone occur through molecular diffusion; then, the CO2 and ~2 - ~-Rn fluxes can be calculated from Fick's first law: IRn = - DRn VCR~

[1]

where 1an is the flux density, DR. is the bulk diffusion coefficient for 222Rn through the volume of the porous medium and C b is the concentration of 222Rn in the interstitial gas. Fick's second law follows from equation [1] by a conservation principle with the added terms due to 222Rn decay and emanation from the solids of the medium, assuming a homogeneous 222Rn source strength and a depth independent diffusion coefficient DR.: OCRn

DRn V2CRn

~'RnCRn + ~R~

[2]

where e is the porosity of the medium defined as the ratio of void volume to total volume, ~ n is the 222Rn decay constant and ~

is the emanation power of the medium into the interstitial volume.

Equation [2] may be solved in the steady state using the boundary conditions that Can -~ 0 at z --~ 0 and CRn = C . ~ at z - ~ oo:

C R n ( Z ) = Ca, R n 1 - e

zR,

131

where:

-

ZRn =

i

Dsn

~ ~Rn

is denominated relaxation depth and C.,~ is the radioactive equilibrium value at great depth. The flux at the soil surface, J0,~, as a function of relaxation depth is therefore:

141

1877

Jo,p..n = - DRnC®,Ra

l'~DR~'Ro n

DRnC®,Rn

=

_

[51

ZRn

According to D t r r and Miinrdch (1987), the source strength of soil CO2 can be decreased exponentially with depth: z

~ c o : ( Z ) = ~0,c02 e

Zco2

[6]

where ~co 2 is the relaxation depth for CO2. The steady state solution of Fick's second law:

OCc°~

-

Dc°2 V2Cco2 + ~co¢(Z)

[7]

Ot

with the boundary conditions: Cco2(Z

=0)

= Catm,CO 2

and lira C c o l ( Z ) = C,..coz + Cairn,co 1

g ...¢¢¢

is: _ _z

Cco2(Z) : Ca,re,co 2 + C®,co 2 1 - e

Zco,

[8]

The diffusion coefficient in the unsaturated soil zone is given by: D:

Do

[91

K

where Do is the molecular diffusion coefficient in air and x is the tortuosity, a characteristic parameter of each soil type that describes the higher diffusion resistance due to variation of diffusion cross-section in the soil by soil grains and water fdled capillary regions (Num'off et al., 19~). The CO 2 flux at the soil surface is obtained from equations [1], [8] and [9] as:

1878

Jo,c%

Do.co2 C~,co2 g

IlOl

ZCO~

and the 222Rnflux is obtained from equations [5] and [9] as:

Jo, Rn =

DO,Rn C~,Rn K ZRn

Ill]

The CO.,/222Rn flux ratio obtained from equations [10] and [11] do not depend on soil parameters:

Jo.co:

D0.co: C®.co2 ZRn Jo,Rn D0,R n C®,Rn ZCO:

[121

The equation [12] permits us to evaluate the indirect C02flux at the soil surface using measured CO, and 222Rn concentration profile data (C co2,C.R ., ZCO: and Zan) and the direct measured -'22Rnflux at the soil surface (J0.~)- The direct 222Rn flux measurement is not disturbed by above-ground vegetation or by changes in the natural conditions induced by sampling. The ratio of the molecular diffusion coefficients for CO 2 and 2-'2Rn at a given temperature is constant (Jost, 1960). All the equations are derived under the assumption that the gas transport in the soil water pha~ can be neglected. This assumption is true for 222Rn and CO2 because the molecular diffusion coefficient of gases is 104 times smaller in water than in air. In Eq.[1] we have paid no attention to the transport due to macroscopic flow because assuming Darcy's law, the macroscopic flow (Jm,c) is:

Jmac ~: C . v ~ C . [

K0 dP)dz

where K is the permeability, rl is the dynamic viscosity and p is the absolute pressure. In the study soils, the K values have been small. The soil properties are described below this paragraph.

3.- MATERIAL AND METHODS

3.1.- The chamber. The chamber used consists of a cylindrical container 0.55 m in diameter and 0.32 m high. The chamber was made of stainless steel with a sampling tube. A small electric fan inside the chamber was used to maintain uniform mixing of emitted gases. The fan is operated a few minutes before the accumulation period is finished.

1879 Equal pressure between the environment and the interior of the chamber was maintained through an orifice of 3 mm diameter. The chamber is placed 1-2 cm deep in the soil.

3.2.- Carbon dioxide. Carbon dioxide was measured by injecting 10 cm 3 samples into a flame ionization detector gas chromatograph, heated at 240 °C and equipped with 80/100 mesh, 6 feet long, ¼" diameter Chromosorb 102 packed column. Calibration was obtained using the standard samples. The accuracy of the measurements, including the uncertainty on the standard concentration (1%) and the reproducibility of the analysis (1.2%), is estimated to be better than 3.4%.

3.3.- Radon. For the 222Rn analysis, air was sampled into a 1 L or 0.5 L lucite walled cylindrical cell, previously evacuated, as a modified design of the original Lucas cell 0Lucas, 1957) developed by Quind6s el al. (1991). The walls of the cell were coated internally with SZn(Ag) coated mylar, but the main difference from the traditional one is the ability to open the cell after use by removing the bottom. The background of this kind of cell ranged from 0.7 to 1.3 c.p.m., with counting times of 30 minutes, resulting in a lower detection limit of around 10 Bq/m 3. Radon was measured by counting the alpha particles emitted by Radon and its daughters products, 2~spo and "-14Bi, when they reach radioactive equifibrium. The precision of the measurements was about 5% taking only the statistical error into account (Carretero, 1994).

3.4.- Interstitial air. Measurements of carbon dioxide and 222Rn in soil air concentrations were made inserting some stainless ~teel sampling tubes into the soil. The 10 mm diameter tubes terminated in a 3.5 cm diameter, 6 cm long, named as filtration chamber which was designed to permit the aspiration of interstitial air from the soil. The chamber was

rifled with glass fibres to prevent the aspiration of soft or clay particles. The upper ends of the tubes were sealed except for the time that samples were being tako.n. A small purging took place before the sampling. First, we measured the Z~Rn concentration and after the carbon dioxide concentration.

3.5.- Location and properties of soils. All the samples were taken in four sites in an area located outside of M(daga. Fig. 1 shows the location of study soils and Table 1 exhibits the characteristics of mentioned soils. We studied some soil properties like these: granulometric analyses, density, porosity and permeability. Table 2.a shows the grain size distribution and Table 2.b the different properties.

1880

TABLE 1.- Characteristics of the sampling sites in Malaga. Charactedstics

M1

M2

M3

M4

LOCATED SITES

Semiurban

Semiurban

Urban

Semiurban

Bare

Pine

VEGETATION TYPE HOURS

Ficus and Bare

OF

Eucalyptus

All day

2 h at Sunset

All day

~ero

OF

JUN 91

JAN 92

FEB 92

JUN 92

MEASUREMENTS

JAN 93

JUN 93

OCT 92

JUN 93

(19 + 6) °C

(17 :t: 4) °C

(23 ± 7) °C

(17 + 5) °C

(7 + 6) %

(12 ± 6) %

(8 -t- 6) %

(7 ± 5) %

2.85 %

3.92 %

1.82 %

1.58 %

SUNSHINE PERIOD

AVERAGE SOIL TEMPERATURE AT 15 cm DEPTH AVERAGE HUMIDITYOVER THE FIRST em ORGANIC MATTER

ON

THE FIRST cm

TABLE 2.a.- Grain size distributions. GRAIN SIZE M1

M2

M3

M4

CLAY AND SLIME

32%

88%

15%

21%

FINE SAND

13%

4%

24%

26%

COARSE SAND

55%

8%

61%

53%

DISTRIBUTION

1881

Table 2.b.- Properties of the soils. PROPERTIES

M1

M2

M3

M4

1.76 ± 0.02

1.58 ± 0.02

1.75 + 0.02

1.52 + 0.02

25.9 + 0.3

42.9 ± 0.1

36.4 ± 0.4

43.4 ± 0.3

5.88.10 .7

< 10.7

2.32.10 .6

3.72.10 .6

DENSITY (~cm 3) POROSITY (%) PERMEABILITY

(m/s)

Compactly Soft clay. TYPE OF SOIL

Slight permeable.

Loose sandy.

Resistant clay.

sandy.

Impermeable.

Moderate permeability.

Moderate permeability.

Granulometric soil analyses were carded out by the sieve method, Jim6nez and de Justo (1975). Porosity was determined in the laboratory from real and apparent density measurements of a soil sample obtained by perforation of the soil with a core sampler which is a hollow cylimler whose volume is known. Permeability was determined in situ assuming Darcy's law and measuring the flow for a fixed pressure drop.

Cmadalmedina river

M4 •

Highway

Mlo

%,

Guadalhorce river

t

1Kra

Mediterranean Sea

FIGURE 1.- The location of sampling points.

1882

4.- RESULTS AND DISCUSSIONS

4.1.- The carbon dioxide by the direct method. Soil gas flux is measured directly by the accumulation method (Wilkening et al,, 1972) using the chamber described in Section 3.1. The concentration change was measured after an accumulation period of 3(I min to 1 hour. The flux density is calculated from: ACco 2

Jco: = h A-----~

where A C c o 2 is the concentration increase during At and h is the height of the chamber. The average accuracy of carbon dioxide flux by direct method has been 19%. The minimum detectable flux (MDF) has been 0.6 mmol m -2 h ~ for an accumulation period of 30 min. Table 3 shows the average flux of carbon dioxide accompanied by an indication of the standard deviation found and the total number of measurements for each type of soils.

TABLE 3.- Average flux carbon dioxide and number of measurements for each type of soil. LOCATION Jco

2

(mmol m 2 h -~)

Number of measurements

M1

M2

M3

M4

2.2 + 1.1

8+ 3

3.3 + 1.4

6+ 3

36

36

15

32

Altogether, we obtained 119 individual flux measurements higher than the MDF for the study soils and 6 lower. The temporal variability of the flux at an individual site is large. That variability may be to due in that the period of measurement has been wide with situations very different, although the time sampling was always 16 h (U.T.) There are, however, rather large differences in the average values at the different sites. Taking into account that soil emissions of CO 2 reflect detrital decompositions involving transitory populations of animals and microorganisms as well as root respiration, it is very important to remember here the situation of sampling points. At sampling sites M2 and M4 (abundant vegetation) we observe the highest flux values with a mean value of 7 mmol m-'- h-~; at sampling sites M1 and M3 (bare soils), the mean flux is only 2.75 mmol m 2 h ~. The average value of carbon dioxide for all study soils is 5 mmol m -2 h -1. This value agrees well with measurements on other soils. Do'rr and Miinnich (1987) have found a value of 6 mmol m 2 h -1 for vegetation soils and 3.76 mmol m 2 h q for bare soils (1990). G a u d r y et al, (1992) show an average flux of 5 mmol m -2 h ~ in South Africa and 5.8 mmol m 2 h q on vegetation soils in Brittany (France). Keller et ai. (1986) obtained a mean

1883 flux of carbon dioxide of 9 mmol

m 2 hq

in tropical forests. Schlednger (1977) found a large range of values for

carbon dioxide flux depending on the latitude of the tropical forests.

4.2.- The effect of temperature and soil moisture on C02 flux. The carbon dioxide flux has simultaneously been measured with the soil humidity on the first centimetres and the soil temperature at 15 and 80 cm depth. The mean values of soil humidity and temperature at 15 cm have been shown in Table 1 with the standard deviation found for each soil type. Edwards, 1975; D6rr and Miinnich, 1987 and Born et ai., 1990 have been found a correlation between CO2 fluxes and the soil temperature in the first cm of depth. In this paper, we have not found a correlation between the carbon dioxide flux and soil temperature at 15 cm depth. The influence of soil temperature may be masked by the effect of precipitation or soil moisture. Precipitation, however, is linked with soil moisture via the hydraulic conductivity of the soil (Eagleson, 1978) and then the influence of precipitation changes with soil type. In a sandy soil with high hydraulic conductivity, soil moisture is only slightly influenced during periods with high precipitation while on clay soil water content is induced for a longer period. At our specific sampling sites, M3 and M4 were sandy soils and the others clay, but the period of measurements has been characterised by low precipitation and this may be the reason why we have not found correlation between CO2 fluxes and soil temperature. Finally, Table 4 shows the average fluxes for days when

T l 5 c m - Tso m

<'1) and T]sc,, - Tso.. > 0; the difference Tt5 m - Ts0~m < 0 may be typical for cold days and

T15¢m- Ts0 ~m> 0 for hot and dry days. We may observe in Table 4 that there is no appreciable difference between the CO2 fluxes for Tt5 - 1".0 < 0 and T15 - Tso > 0. The little differences are masked by the standard deviation.

TABLE 4.- CO fluxes for positive and negative thermic gradient soils. THERMIC GRADIENT

MI

M2

M3

M4

T~5 - Tso < 0 Jco2 (retool m 2 h q)

1.9:1:0.8

8"1-4

3.2 :t: 1.8

5.8 + 2.4

T~5 - Tso > 0 Jco2 (retool m "2 h "~)

2.4:1:1.4

7.15:1.4

3.5:1:1.6

7+4

4.3.- The carbon dioxide by the indirect method. In soil M1, measurements of 222Rn and CO 2 air soil concentrations were made inserting tubes into the soil at 10, 18, 47, 75, 150 and 222 cm depth; in M2, at 10, 15, 30, 45 and 60 cm depth; in M3, at 5, 10, 15, 30, 45, 60, 85 and 105 cm depth and in M4 at 5, 10, 15, 30, 45, 60, 120, 180, 240 and 325 cm depth.

1884

The mean concentration profiles of 222Rnin the study soils are given in Fig.2.a and Fig.2.b (upper part). 2~--~Rnconcentration profiles increases with depth to a constant value, C.R,, that varies according to the soil types. The solid curves indicate the 222Rnconcentration profiles fitted to exponential function by least squares, equation [3]. The mean carbon dioxide concentration profiles in the four soil types are given in Fig.2.a and Fig.2.b (lower part). The concentration in carbon dioxide increases with depth and can be observed; this increase with depth is due to microbial decomposition of soil organic material and root respiration. The continuous curve indicates the carbon dioxide profiles fitted to exponential function, equation [8], by least squares.

Table 5 shows the characteristic parameters of the studied adjustments, C. and ~ for the 2"~2Rnand carbon dioxide; C. for 222Rnis expressed in Bq/m 3 and in p.p.m.v, for the carbon dioxide respectively. TABLE 5.- Fitting results of 222Rn and carbon dioxide concentration profiles in air soil. SOIL TYPE

GAS

N

C.(Rn) or Co(COo)

~ (cm)

X2

P (%)

222Rn

63

21840 + 160

68.4 + 1.4

1.44

> 99.5

CO2

36

14750 + 30

55.0 + 0.3

2.2

> 99.5

222Rn

45

11800 + 600

152 + 14

9.84

97.5

CO2

36

15200 + 700

108 + 9

0.72

> 99.5

~'22Rn

16

2880 + 100

72 5- 4

20.34

99

CO,

15

10000 5- 400

74 5- 4

4.32

> 99.5

222Rn

32

8430 5- 70

105.6 5- 2.2

3.36

M1

M2

M3

M4

> 99.5 I

CO.,

32

10200 + 90

70.4 + 1.5

1.84

]

> 99.5

The values of C, vary according to the type of soil. The average relaxation depth is in good keeping with the theoretical value of 100 cm for 222Rn(Nazaroff et al., 1988). Also, the number of profiles carried out (N), the chi squared (•2) and the degree of confidence (p) are given in this table. The results of adjustments are very acceptable because the majority exhibits a degree of confidence greater than 99.5%; this behaviour confirms the supposed hypothesis: a) The transport in the unsaturated zone is really due to molecular diffusion, b) The source strength of soil CO 2 decreases exponentially with depth.

4.4.- Comparison between carbon dioxide obtained fluxes for direct and indirect methods.

Table 6 shows the average carbon dioxide fluxes by indirect and direct methods for the studied soils. In general, the direct method is slightly higher than indirectly measured fluxes. The greatest difference between direct and indirect flux is exhibited by M3 soil; the location of this soil was very peculiar: the water table was situated

1885

222Rrl ( . I 0 3 B q / m 3)

0

4

8

12

~

16

Il1l

222Rn (.I0 3 Bq/m 3) 2O I

24 r

I

28

0

2

4

6

r [ i I i

i

g

0.2

0,4

0.6

0.8

L [ I ~

I l

® CO2 ('103 p . p . m . v . ) 0

4

8

12

-----16

C02 ('103 p.p.m.v.) 20

0

4

8

12

0.2

0.4

0.6

\

0.8

F I G U R E 2.a.- Upper part: Concentration profiles of Rn in the soil air in M1 and M2. Lower part:

Corresponding CO 2 profiles in the soil air at same locations.

1886

222Rn ('10 3

o

Bq/m3)

1

222R1-1 (.10 3 Bq/m 3)

2

0

3

2

4

10

6 I

0.4

0.8

1.2

1.6

t

t

t

i

L I

8

J

I

I1

~

CO 2 (.10 3 p.p.m.v.)

CO 2 (.103 p.p.m.v.) 4

t

12

16

0

4

12

8 ,

,

I

il

16

0.4

0.8

•

\

1.2

1.6

I

i

I

I

FIGURE 2.b.- Upper part: Concentration profiles of Rn in the soil air in M3 and M4. Lower part: Corresponding CO2 profiles in the soil air at same locations.

1887 at 120 cm depth in dry periods impeding the migration of gases from the soil to the atmospheric air. The location of this soil was on the coast (30 m from the sea shore) and there were always breezes. In the M2 soil, the carbon dioxide flux is equal by both methods. This behaviour shows that when the vegetation is forest type, the direct method gives correct results. According to Keller et ~ . (1986), t r a m

et al. (1970), Sehlesinger (1977) and

Hutdfinson and M m i e r (19ill), the static open chamber is appropriate to obtain representative results in the forest

environment where air motions are usually very weak at the soil surface. The little difference between the average carbon dioxide flux by the direct and indirect method may indicate that the convective processes operating in addition to diffusion are of little importance in this study. In general, the diffusion theory prediction of surface flux is in good agreement with the average observed values of flux. TABLE 6.- The average carbon dioxide fluxes by indirect and direct methods. TYPE SOIL

J c o 2 (mmol m "2 ha) INDIRECT METHOD

JCO 2 (mmol m "2 hl ) DIRECT METHOD

M1

1.8 ____0.8

2.2 ::1:1.1

M2

85:5

8__.3

M3

1.8 + 1.4

3.3:1:1.4

M4

4.0 + 1.4

6+3

CONCLUSIONS

- 222Rn

proves to be a useful tracer for soil gas transport. Our measurements confirm the assumption of steady state

conditions and depth constant 2~Rn source strength.

- The little difference between carbon dioxide flux by indirect and direct methods given that the transport in the soil air is mainly controlled by molecular diffusion rather than as production or decomposition rates.

- Further investigation of carbon dioxide flux by both methods are needed to obtain representative values.

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2-8 (1990).

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1888

DORR, H. and K.O. MONNICH. ~22Rn flux and soil air concentration profiles in West Germany. Soil -~2-'Rnas tracer for gas transport in the unsaturated soil zone. Telius, 42B, pp. 20-28 (1990).

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