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Influence of freezing and thawing on the carbon isotope composition in soil CO2
GEODER~ ELSEVIER
Geoderma 69 (1996) 209-216
Influence of freezing and thawing on the carbon isotope composition in soil CO2 Andrzej Dudziak a, Stanislaw Halas b,, a Department of Physics, Technical University of Lublin, 20-618 Lublin, Poland b Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland Received 20 February 1995; accepted 24 August 1995
Abstract The carbon isotopic composition of soil CO2 was measured during the winter of 1990/91 at two sites in the Lublin Upland (Poland). In that season a period with very low air temperature occurred, during which snow covered soils were frozen. We observed variations of 8J3C up to 3 permil. During freezing periods the variations depended on soil physical properties. In sandy soil the 13C/12C ratio during freezing periods increased, while in loess soil it decreased. This appears to be connected with the granulometrical composition of the soil and the size of the pores, which influence the rate of CO2 diffusion into the atmosphere. During thawing periods, when the snow was melting, the CO2 concentration increased and the 13C / ~:C ratios decreased in both types of the soil under investigation due to worse contact of soil horizons with the atmosphere. The major winter variations in 813CO2 of soil may be useful natural markers of infiltrating waters.
I. Introduction Isotope geochemistry can be used to provide information on the processes affecting carbonates in the weathering zone (Salomons and Mook, 1986), on the movement of groundwaters (Deines et al., 1974) as well as on palaeoclimatic and palaeoecologic conditions (Hendy, 1971; Ceding, 1984; Ceding et al., 1989). Rainwater during its infiltration to the soil is saturated by the soil CO2 and may also dissolve carbonates. In these processes a distinction is made between open and closed systems. In an open system carbonates react with water in contact with the gas phase of the fixed COz partial pressure, and in a closed system the water first equilibrates with a CO2 reservoir, then is isolated from the reservoir and reacts with the carbonates (Deines et al., 1974; Salomons and Mook, 1986). To determine which process is more likely, it is important to know the carbon isotope com* Corresponding author. 0016-7061/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI0016-706 1 ( 9 5 ) 0 0 0 6 0 - 7
210
A. Dudziak, S. Halas / Geoderma 69 (1996) 209 216
position of the carbonates and that of soil CO2 in the investigated area. Also the calculations of inflow of modem waters to the aquifers by means of radiocarbon most often requires the knowledge of stable isotopes in soil CO2 (Mook, 1976; Fontes and Gamier, 1979). Carbon dioxide in soil is produced by the decomposition of organic matter and by root respiration. However, ~3C/~2C ratio of soil CO2 is a few permil higher than that in soil respired CO2. This difference is produced by isotope fractionation during gas diffusion to the atmosphere and it depends on the soil respiration rate (D6rr and M~innich, 1980; Cerling, 1984; Cerling et al., 1991). It is observed that the ~3C/~2C ratio in soil CO2 may have significant seasonal variations within a year. As a rule, in the winter period with low plant and microbial activity the CO2 concentration is low, and the J3C/t2C ratio is relatively high, while in spring and summer the CO2 concentration increases and the ~3C/~2C ratio becomes lower (Currie, 1975; Rightmire, 1978; D6rr and Mfinnich, 1980; Hesterberg and Siegenthaler, 1991 ). So far, however, there have been neither comprehensive observations nor convincing explanations of the processes occurring under the winter conditions of a temperate climate. In this period there appear large drops and oscillations of temperature which lead to the freezing and thawing of soil. This brings about changes in the soil respiration rate because such temperature variations stimulate or reduce the microbial destruction of soil organic matter and influence the diffusive transport of gas to the atmosphere (Bleak, 1970; Coxon and Parkinson, 1987). The purpose of this paper is to study the influence of winter conditions on the ~'~C/Jzc ratio in soil CO2 on the basis of field observations which were carried out in natural ecosystems for two different soils in the Lublin Upland.
2. The study area and experimental methods Our investigations were performed in the Lublin Upland (southeastern Poland), near the Lublin city. This area is located at an elevation of 200 m. The climate is temperate with a mean annual precipitation of 600 mm and an average January temperature of - 4°C and a mean July temperature of + 18°C (the mean annual temperature is 7.3°C). The first sampiing site was located in an uncultivated grass-covered field, where the gray brown podzolic soil (Orthic Luvisol) developed from the loess. The second site was located in an organicpoor Cambic Arenosol in a coniferous forest with pine as the major tree species. In each site a set of probes (plastic tubes) were installed in the soil at different depths ranging from 10 to 110 or 90 cm, every 20 cm. The soil gas samples were taken out into 50 ml glass syringes with a Teflon piston and two stop-cocks. Such ampoules enabled us to sweep all dead volumes with soil air. The use of small-size ampoules is particularly advantageous because the sucking time is reasonably short (20 min), whereas the sampling does not greatly disturb the equilibrium in the soil and enables the study of changes versus depth with high resolution. The investigations were carried out more or less regularly (monthly) on full profile (i.e., the samples were taken from the complete set of depths), or at three selected depths below 50 cm, where some stabilisation of 813C values was observed. All sampling hours were restricted to the interval 9-11 am. The soil temperature was measured by mercury thermometer at the depth of 10 cm.
A. Dudziak, S. Halas / Geoderma 69 (1996) 209-216
211
The carbon dioxide was extracted from the air samples cryogenically under reduced pressure at liquid nitrogen temperature (Craig, 1953). Water was separated from the condensate by means of chloroform ice. An admixture of N20 was removed by passing the gas trough hot copper wires, on which the nitrous oxide was totally decomposed (Dudziak and Halas, 1990). We measured the carbon isotopic composition using the modified Nier-type mass spectrometer MI 1305 (Halas, 1979; Halas and Krouse, 1983, 1984). The ~3C/]2C ratios are given as relative deviations from the PDB isotope standard: (~13C= (Rsample/Rstandard-
1)" 1000%~
where R means carbon isotope ratio. The standard error of analysis (including the separation procedure) was 0.25%~. while the internal mass-spectrometer reproducibility was 0.1%~. CO2 concentration in the soil air sample was determined mass-spectrometrically by measuring the amount of carbon dioxide at constant volume of the inlet system with a relative error of 2 percent ( Halas and Dudziak, 1989). Detailed description of experimental methods is given by Dudziak (1993).
3. Results
In the investigated winter there was a long period with frost and snow in January and February (Fig. 1). The results of observations recorded in this time will be described separately for each site because of the differences in their behaviour.
3.1. Sandy forest soil The negative air temperature in January and February caused the freezing of soil to the depth of 30 cm in the middle of February while the thickness of the snow cover was 20 cm. This did not essentially influence the CO2 content, but the 613C value increased by 1%~ (Fig. 2). After the snow, from the 20th of that month there came some wanning and the
i ° -10
o! - • . •
sandyforestsoil loessgrasslandsoil
1990
,~ =
1991
Fig. 1. The mean diurnal air temperature and soil temperature at a depth of 10 cm.
212
A. Dudziak. S. Halas / Geoderma 69 (1996) 209-216
~, 0.2
-16
'~ CO;co . . . . tr. 1
--]
-17 -18 (~ ~0.1
-19 ~
0
0
xt
I x,
I
1990
I
I
1
,
I
-20
m
1991
Fig. 2. Variations of CO2 concentration and its =3C/~2Cratios in soil air in coniferous forest. The points represent average values from the depths of 70 and 90 cm. CO 2 concentr [%]
~'~c [°/~l -20 -18 -16 I '~r/~ ' I
0.0
0.1 --
0.2 0.0
O.Z
0.2
~'0.4
0.4 ~"
.c 0.6 o. 0) a 0.8
0.6 ~ 0.8 a
1.0
1.0
1.2
1.2
~ 15 11 1990
• 25.02 1991 • 13031991 rn~ng)
(snow was
Fig. 3. Vertical distribution of CO2 in soil air and its carbon isotope composition in coniferous forest. ~,
i
0.8 - -
I "1" C02concentr ] - -
-21
0.6
-22 ~"
0.4
-23 '-,~
00~0.2
xt
I x. 1990
[ I
t
I
. I 1991
m
-24
Fig. 4. Variationsof CO2 concentrationand its L3C/~2Cratios in soil air in grass field. The points represent average values from the depths of 70, 90 and 110 era. snow started to thaw intensively. By the end of February the soil was completely thawed and covered with a 10 cm layer of wet snow. The CO2 concentration was then 1.5 times higher and 6~3C was lower by 3%0 than in the preceding period. Simultaneously, for the first time a reversal in the profile was recorded ( 6~3C increased and COz content decreased with depth) (Fig. 3). In the middle of March there were only a few patches of snow in the forest, and the CO2 concentration and 6z3C increased a little. Their gradients with depth were very low, however, which was an exception to the rule at this site. 3.2. L o e s s g r a s s l a n d soil
While the soil temperature fell from + 3°C in January to - 1°C in the middle of February and the soil froze down to 30 cm, CO2 concentration increased by 50% and 6t3C decreased
A. Dudziak, S. Halas / Geoderma 69 (1996) 209-216 C02 concentr. [%]
8'~c [°/®1 -24 0.0
Ii(
-22
I
213
-20
I .
0.2
0.4 0.6
~
.
0.8 0.0
0.2
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~" 0.4 s:~ 0.6 o.
+
,
0.6
ta.
0.8 ¢",
Q 0.8
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• 22.02.1991(snowwall mellmg)
Fig. 5. Verticaldistributionof CO2 in soil air and its carbonisotopecompositionin grass field. by over 1.5%0 (Fig. 4). The subsequent thawing of the soil and the melting of snow caused a further decrease of 6~3C to - 24%0 and an increase of COs content. At this time the ~3C/ J2C ratios were nearly constant in the whole profile (Fig. 5).
4. Discussion
The changes in CO2 concentration and ~3C/~2C ratios during the winter period may be connected to several processes. The freezing of the s a n d y f o r e s t s o i l in January and February did not make it impermeable, because the moisture of soil was low and most of its pores (macropores) remained air-filled. The pores were air permeable for the whole time when the temperature was negative, in spite of the condensing process of water vapour diffusing from deeper soil layer. At the same time the low soil temperature generated low respiratory activity and low C O 2 concentration, which caused relatively high 13CO2 c o n t e n t s (Fig. 2). Increase of t~3C by 1.1%o at the beginning of February may be related to the freezing of soil down to 30 cm and the considerable reduction of respiration in this soil layer. In this way increased the contribution of CO2 generated in deeper soil layers to the common microbial production, as the organic material in those layers was enriched in isotopel3C by about 1%o in comparison to the shallow soil layers (Dudziak, 1993). Moreover, very low respiration activity made the exchange with atmospheric CO2 [ 6~3C--- -8%~ (Mook et al., 1983; Inoue and Sugimura, 1985; Levin, 1987)] more intense, which resulted in the additional shift of 6~3C in soil CO2 towards less negative values. The thawing of soil and the melting of snow cover is associated with several processes which might lead to a large variation of 613C values. The inflow into the soil of fresh water, not saturated with carbon dioxide, causes the dissolution of this gas in the infiltrating water. At 0°C the ratio of CO2 volume dissolved in water to the water volume (at the partial pressure 1.013.105 Pa) is 1.7, so the CO2 content in soil air decreases. The filling of soil pores with water from melting snow decreases air porosity and hence the gas diffusion coefficient. One result of this effect may be an increase of carbon dioxide concentration and decrease of its 13C / ~2C ratio. The observed decrease of CO2 concentration with depth suggests that the gas is transported down, and that the upper soil layer is nearly hermetic. At the same time the shallow layer is the major source of soil COs (it contains
A. Dudziak, S. Halas / Geoderma 69 (1996)20%216
214
-13 --
j
-15 ~'~ -17 -19
f
d e p ~ qo=8mmol/m3h ,''"', ",, ----7depth1m ~ 8 0 ' mmol/m3h
-21 -23 0.00
J
L 0.01
L
I
I
I
~
0.02
L
M
i
[
I
I
0.03
D [cm2/s] Fig. 6. Carbon isotope composition against diffusion coefficient (D) calculated assuming the production rate (q) to be exponentially dependent on depth (x) as follows: q q 4 ) e x p ( - x / 0 . 1 5 m) and 6~3C,~v 27%~. 613C~,,, - - 7%c (from Dudziak. 1993 ).
the majority of decaying organic matter). The downwards direction of C O 2 diffusion agrees with the character of isotope profile (Fig. 3). The thawing of soil may also stimulate respiration activity (Bleak, 1970; Coxon and Parkinson, 1987 ). In the middle of March the upper soil layer was permeable to gas, but the low gradient of CO2 concentration and ~5~3C at low soil temperature suggests that the rate of diffusion at this depth was minimal. Gas properties in the loess grassland soil during freezing behaved differently. Due to the respiration activity decreased together with lowering of soil temperature, a restriction of gas exchange with atmosphere might be a reason for the CO2 concentration increase at this particular time. The loess soil contains a higher traction of micropores and lower fraction of macropores in its structure in comparison with the sandy soil described previously. As such, the loess soil has a larger tendency to seal up the pores during freezing because the freezing of water in micropores causes the breaking of their continuity and condensation of water vapour, decreasing the diffusion transport of soil gases to the atmosphere. This results in the observed increase of CO2 content accompanied by depletion in ~3C (Fig. 4). The same effect is predicted by the model of carbon isotope distribution in CO2 during diffusion through porous media with different porosity and pores tortuosity (Dudziak, 1993). Fig. 6 shows this dependence for a model in which the diffusion coefficient is varying but is the same for the whole soil profile. The production rate (q) is assumed to decrease exponentially with depth: q = qo exp( - x/x()) where qo is the production rate at the surface, Xu is the characteristic depth. As it was stated above, the soil CO2 is enriched in heavy isotope with respect to soilrespired CO2 by faster diffusion of light isotope to the atmosphere. Hence, over a period when this diffusion is diminished the soil CO2 should approximate its isotope ratio to that of soil organic matter. For this reason we recorded such a low ~3CO2 value ( - 2 3 % 0 ) while below the frozen layer of soil 613Corganici s - - 24%0 (Dudziak, 1993). The reasons for the increase of CO2 content and decrease of 6~3C value during the warming and thawing of soil and melting of snow may be the same as those discussed for the sandy forest soil in the same period. When one compares the discussed changes of CO2 properties in both types of soil during a low temperature period, one may state that granulometrical composition and physical
A. Dudziak, S. Halas / Geoderma 69 (1996) 209-216
215
properties of soils play the most important role. Dusty soils with a large fraction of micropores lose the continuity of pores very quickly. Therefore, the diffusive transport of carbon dioxide to the atmosphere is markedly decreased, CO2 concentration increases in the soil air, and the carbon is depleted in ~3C. The situation is different in sandy soils where the fraction of macropores is dominant, hence even the freezing of soil does not cause the loss of pore continuity. The warming and melting of snow in both types of soils causes a distinct shift of soil air parameters, but their patterns are similar.
5. Concluding remarks In course of this study we have noticed that during winter the 6~3CO2 in soil may significantly decrease or increase. These major variations in ~3C are not related directly to temperature changes but to a cut-off mechanism of the soil CO2 generator from the atmosphere. The cut-off periods are characterized by a low 6~3CO2 value which is close to the source carbon in the organic matter at deeper unfrozen horizons. The major winter variations in 3~3CO2 of soil may be useful natural markers in the hydrological study of water infiltration rates from subsurface to the aquifer.
Acknowledgements This study was supported by International Atomic Energy Agency, Vienna, Contract No. 5144/RB.
References Bleak, A.T., 1970. Disappearance of plant material under a winter snow cover. Ecology, 51: 915-917. Ceding, T.E., 1984. The stable isotopic composition of modern soil carbonate and its relationship to climate. Earth Planet. Sci. Len., 71: 229-240. Cerling, T.E., Quade, J., Wang, Y. and Bowman, J.R., 1989. Carbon isotopes in soils and palaeosols as ecology and palaeoecology indicators. Nature, 341: 138-139. Cerling, T.E., Solomon, D.K., Quade, J. and Bowman, J.R., 1991. On the isotopic composition of carbon in the soil carbon dioxide. Geochim. Cosmocbim. Acta, 55: 3403-3405. Coxon, D.S. and Parkinson, D., 1987. Winter respiratory activity in aspen woodland forest floor litter and soils. Soil Biol. Biochem., 19: 49-59. Craig, H., 1953. The geochemistry of the stable carbon isotopes. Geochim. Cosmochim. Acta, 3: 53-92. Currie, J.A., 1975. Soil respiration. Techn. Bull. Ministry of Agriculture Fisheries and Food, 29: 461-468. Deines, P., Langmuir, D. and Harmon, R.S., 1974. Stable carbon isotope ratios and the existence of a gas phase in the evolution of carbonate ground waters. Geochim. Cosmochim. Acta, 38:1147-1164. D6rr, H. and Miinnich, K.O., 1980. Carbon-14 and carbon-13 in soil CO2. Radiocarbon, 22: 909-918. Dudziak, A., 1993. Variations of ~3CO2/~2CO2 in soil air. Doctoral dissertation. Maria Curie-Sklodowska Univ., Lublin (in Polish). Dudziak, A. and Halas, S., 1990. Purification of carbon dioxide from N20 for mass-spectrometrical analysis of soil CO2. Appl. Radiat. Isot. Int. J. Radiat. Appl. Instrum., A41:909-911. Fontes, J.C. and Gamier, J.M., 1979. Determination of the initial ~4C activity of the total dissolved carbon - - a rewiev of the existing models and a new approach. Water Resour. Res., 15: 399-413.
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A. Dudziak, S. Halas / Geoderma 69 (1996) 209-216
Halas, S., 1979. Automatic inlet system with pneumatic changeover valves for isotope mass spectrometer. J. Phys. E: Sci. Iustrum., 12: 418--420. Halas, S. and Dudziak, A., 1989. Precise determination of ~3C/~2C and CO2 concentration in minute samples of soil air by mass-spectrometry. Isotopenpraxis, 25:349-351. Halas, S. and Krouse, H.R., 1983. Isotopic analysis of nanomole gas samples by means of dynamic flow mass spectrometry. Rev. Sci. Instrum., 54: 437~-43. Halas, S. and Krouse, H.R., 1984. Device for rapid transfer of condensable gases into a capillary. Rev. Sci. lnstrum., 55:1160-1161. Hendy, C.H., 1971. The isotopic geochemistry of speleothems. 1- The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators. Geochim. Cosmochim. Acta, 35: 801-824. Hesterberg, R. and Siegenthaler, U., 1991. Production and stable isotopic composition of CO2 in a soil near Bern, Switzerland. Tellus, 43B: 197-205. Inoue, H. and Sugimura, Y., 1985. The carbon isotopic ratio of atmospheric carbon dioxide at Tsukuba, Japan. J. Atmos. Chem., 2: 331-344. Levin, 1., 1987. Atmospheric CO2 in continental Europe - - an alternative approach to clean air CO2 data. Tellus, 39B: 21-28. Mook, W.G., 1976. The dissolution-exchange model for dating groundwater with ~4C. In: Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrol. IAEA, Vienna, pp. 212-225. Mook, W,G., Koopmans, M., Carter, A.F. and Keeling, C.D., 1983. Seasonal, latitudinal and secular variations in the abundance and isotopic ratios of atmospheric carbon dioxide. 1. Results from land stations. J. Geophys. Res., 88: 10.915-10.933. Rightmire, C.T., 1978. Seasonal variation in P(CO2) and ~3C content of soil atmosphere. Water Resour. Res., 14: 691-692. Salomons, W. and Mook, W.G., 1986. Isotope geochemistry of carbonates in the weathering zone. In: P. Fritz and J. Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry. Part 2. The Terrestrial Environment, B. Elsevier, Amsterdam, pp. 239-269.
Geoderma 69 (1996) 209-216
Influence of freezing and thawing on the carbon isotope composition in soil CO2 Andrzej Dudziak a, Stanislaw Halas b,, a Department of Physics, Technical University of Lublin, 20-618 Lublin, Poland b Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland Received 20 February 1995; accepted 24 August 1995
Abstract The carbon isotopic composition of soil CO2 was measured during the winter of 1990/91 at two sites in the Lublin Upland (Poland). In that season a period with very low air temperature occurred, during which snow covered soils were frozen. We observed variations of 8J3C up to 3 permil. During freezing periods the variations depended on soil physical properties. In sandy soil the 13C/12C ratio during freezing periods increased, while in loess soil it decreased. This appears to be connected with the granulometrical composition of the soil and the size of the pores, which influence the rate of CO2 diffusion into the atmosphere. During thawing periods, when the snow was melting, the CO2 concentration increased and the 13C / ~:C ratios decreased in both types of the soil under investigation due to worse contact of soil horizons with the atmosphere. The major winter variations in 813CO2 of soil may be useful natural markers of infiltrating waters.
I. Introduction Isotope geochemistry can be used to provide information on the processes affecting carbonates in the weathering zone (Salomons and Mook, 1986), on the movement of groundwaters (Deines et al., 1974) as well as on palaeoclimatic and palaeoecologic conditions (Hendy, 1971; Ceding, 1984; Ceding et al., 1989). Rainwater during its infiltration to the soil is saturated by the soil CO2 and may also dissolve carbonates. In these processes a distinction is made between open and closed systems. In an open system carbonates react with water in contact with the gas phase of the fixed COz partial pressure, and in a closed system the water first equilibrates with a CO2 reservoir, then is isolated from the reservoir and reacts with the carbonates (Deines et al., 1974; Salomons and Mook, 1986). To determine which process is more likely, it is important to know the carbon isotope com* Corresponding author. 0016-7061/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI0016-706 1 ( 9 5 ) 0 0 0 6 0 - 7
210
A. Dudziak, S. Halas / Geoderma 69 (1996) 209 216
position of the carbonates and that of soil CO2 in the investigated area. Also the calculations of inflow of modem waters to the aquifers by means of radiocarbon most often requires the knowledge of stable isotopes in soil CO2 (Mook, 1976; Fontes and Gamier, 1979). Carbon dioxide in soil is produced by the decomposition of organic matter and by root respiration. However, ~3C/~2C ratio of soil CO2 is a few permil higher than that in soil respired CO2. This difference is produced by isotope fractionation during gas diffusion to the atmosphere and it depends on the soil respiration rate (D6rr and M~innich, 1980; Cerling, 1984; Cerling et al., 1991). It is observed that the ~3C/~2C ratio in soil CO2 may have significant seasonal variations within a year. As a rule, in the winter period with low plant and microbial activity the CO2 concentration is low, and the J3C/t2C ratio is relatively high, while in spring and summer the CO2 concentration increases and the ~3C/~2C ratio becomes lower (Currie, 1975; Rightmire, 1978; D6rr and Mfinnich, 1980; Hesterberg and Siegenthaler, 1991 ). So far, however, there have been neither comprehensive observations nor convincing explanations of the processes occurring under the winter conditions of a temperate climate. In this period there appear large drops and oscillations of temperature which lead to the freezing and thawing of soil. This brings about changes in the soil respiration rate because such temperature variations stimulate or reduce the microbial destruction of soil organic matter and influence the diffusive transport of gas to the atmosphere (Bleak, 1970; Coxon and Parkinson, 1987). The purpose of this paper is to study the influence of winter conditions on the ~'~C/Jzc ratio in soil CO2 on the basis of field observations which were carried out in natural ecosystems for two different soils in the Lublin Upland.
2. The study area and experimental methods Our investigations were performed in the Lublin Upland (southeastern Poland), near the Lublin city. This area is located at an elevation of 200 m. The climate is temperate with a mean annual precipitation of 600 mm and an average January temperature of - 4°C and a mean July temperature of + 18°C (the mean annual temperature is 7.3°C). The first sampiing site was located in an uncultivated grass-covered field, where the gray brown podzolic soil (Orthic Luvisol) developed from the loess. The second site was located in an organicpoor Cambic Arenosol in a coniferous forest with pine as the major tree species. In each site a set of probes (plastic tubes) were installed in the soil at different depths ranging from 10 to 110 or 90 cm, every 20 cm. The soil gas samples were taken out into 50 ml glass syringes with a Teflon piston and two stop-cocks. Such ampoules enabled us to sweep all dead volumes with soil air. The use of small-size ampoules is particularly advantageous because the sucking time is reasonably short (20 min), whereas the sampling does not greatly disturb the equilibrium in the soil and enables the study of changes versus depth with high resolution. The investigations were carried out more or less regularly (monthly) on full profile (i.e., the samples were taken from the complete set of depths), or at three selected depths below 50 cm, where some stabilisation of 813C values was observed. All sampling hours were restricted to the interval 9-11 am. The soil temperature was measured by mercury thermometer at the depth of 10 cm.
A. Dudziak, S. Halas / Geoderma 69 (1996) 209-216
211
The carbon dioxide was extracted from the air samples cryogenically under reduced pressure at liquid nitrogen temperature (Craig, 1953). Water was separated from the condensate by means of chloroform ice. An admixture of N20 was removed by passing the gas trough hot copper wires, on which the nitrous oxide was totally decomposed (Dudziak and Halas, 1990). We measured the carbon isotopic composition using the modified Nier-type mass spectrometer MI 1305 (Halas, 1979; Halas and Krouse, 1983, 1984). The ~3C/]2C ratios are given as relative deviations from the PDB isotope standard: (~13C= (Rsample/Rstandard-
1)" 1000%~
where R means carbon isotope ratio. The standard error of analysis (including the separation procedure) was 0.25%~. while the internal mass-spectrometer reproducibility was 0.1%~. CO2 concentration in the soil air sample was determined mass-spectrometrically by measuring the amount of carbon dioxide at constant volume of the inlet system with a relative error of 2 percent ( Halas and Dudziak, 1989). Detailed description of experimental methods is given by Dudziak (1993).
3. Results
In the investigated winter there was a long period with frost and snow in January and February (Fig. 1). The results of observations recorded in this time will be described separately for each site because of the differences in their behaviour.
3.1. Sandy forest soil The negative air temperature in January and February caused the freezing of soil to the depth of 30 cm in the middle of February while the thickness of the snow cover was 20 cm. This did not essentially influence the CO2 content, but the 613C value increased by 1%~ (Fig. 2). After the snow, from the 20th of that month there came some wanning and the
i ° -10
o! - • . •
sandyforestsoil loessgrasslandsoil
1990
,~ =
1991
Fig. 1. The mean diurnal air temperature and soil temperature at a depth of 10 cm.
212
A. Dudziak. S. Halas / Geoderma 69 (1996) 209-216
~, 0.2
-16
'~ CO;co . . . . tr. 1
--]
-17 -18 (~ ~0.1
-19 ~
0
0
xt
I x,
I
1990
I
I
1
,
I
-20
m
1991
Fig. 2. Variations of CO2 concentration and its =3C/~2Cratios in soil air in coniferous forest. The points represent average values from the depths of 70 and 90 cm. CO 2 concentr [%]
~'~c [°/~l -20 -18 -16 I '~r/~ ' I
0.0
0.1 --
0.2 0.0
O.Z
0.2
~'0.4
0.4 ~"
.c 0.6 o. 0) a 0.8
0.6 ~ 0.8 a
1.0
1.0
1.2
1.2
~ 15 11 1990
• 25.02 1991 • 13031991 rn~ng)
(snow was
Fig. 3. Vertical distribution of CO2 in soil air and its carbon isotope composition in coniferous forest. ~,
i
0.8 - -
I "1" C02concentr ] - -
-21
0.6
-22 ~"
0.4
-23 '-,~
00~0.2
xt
I x. 1990
[ I
t
I
. I 1991
m
-24
Fig. 4. Variationsof CO2 concentrationand its L3C/~2Cratios in soil air in grass field. The points represent average values from the depths of 70, 90 and 110 era. snow started to thaw intensively. By the end of February the soil was completely thawed and covered with a 10 cm layer of wet snow. The CO2 concentration was then 1.5 times higher and 6~3C was lower by 3%0 than in the preceding period. Simultaneously, for the first time a reversal in the profile was recorded ( 6~3C increased and COz content decreased with depth) (Fig. 3). In the middle of March there were only a few patches of snow in the forest, and the CO2 concentration and 6z3C increased a little. Their gradients with depth were very low, however, which was an exception to the rule at this site. 3.2. L o e s s g r a s s l a n d soil
While the soil temperature fell from + 3°C in January to - 1°C in the middle of February and the soil froze down to 30 cm, CO2 concentration increased by 50% and 6t3C decreased
A. Dudziak, S. Halas / Geoderma 69 (1996) 209-216 C02 concentr. [%]
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Fig. 5. Verticaldistributionof CO2 in soil air and its carbonisotopecompositionin grass field. by over 1.5%0 (Fig. 4). The subsequent thawing of the soil and the melting of snow caused a further decrease of 6~3C to - 24%0 and an increase of COs content. At this time the ~3C/ J2C ratios were nearly constant in the whole profile (Fig. 5).
4. Discussion
The changes in CO2 concentration and ~3C/~2C ratios during the winter period may be connected to several processes. The freezing of the s a n d y f o r e s t s o i l in January and February did not make it impermeable, because the moisture of soil was low and most of its pores (macropores) remained air-filled. The pores were air permeable for the whole time when the temperature was negative, in spite of the condensing process of water vapour diffusing from deeper soil layer. At the same time the low soil temperature generated low respiratory activity and low C O 2 concentration, which caused relatively high 13CO2 c o n t e n t s (Fig. 2). Increase of t~3C by 1.1%o at the beginning of February may be related to the freezing of soil down to 30 cm and the considerable reduction of respiration in this soil layer. In this way increased the contribution of CO2 generated in deeper soil layers to the common microbial production, as the organic material in those layers was enriched in isotopel3C by about 1%o in comparison to the shallow soil layers (Dudziak, 1993). Moreover, very low respiration activity made the exchange with atmospheric CO2 [ 6~3C--- -8%~ (Mook et al., 1983; Inoue and Sugimura, 1985; Levin, 1987)] more intense, which resulted in the additional shift of 6~3C in soil CO2 towards less negative values. The thawing of soil and the melting of snow cover is associated with several processes which might lead to a large variation of 613C values. The inflow into the soil of fresh water, not saturated with carbon dioxide, causes the dissolution of this gas in the infiltrating water. At 0°C the ratio of CO2 volume dissolved in water to the water volume (at the partial pressure 1.013.105 Pa) is 1.7, so the CO2 content in soil air decreases. The filling of soil pores with water from melting snow decreases air porosity and hence the gas diffusion coefficient. One result of this effect may be an increase of carbon dioxide concentration and decrease of its 13C / ~2C ratio. The observed decrease of CO2 concentration with depth suggests that the gas is transported down, and that the upper soil layer is nearly hermetic. At the same time the shallow layer is the major source of soil COs (it contains
A. Dudziak, S. Halas / Geoderma 69 (1996)20%216
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the majority of decaying organic matter). The downwards direction of C O 2 diffusion agrees with the character of isotope profile (Fig. 3). The thawing of soil may also stimulate respiration activity (Bleak, 1970; Coxon and Parkinson, 1987 ). In the middle of March the upper soil layer was permeable to gas, but the low gradient of CO2 concentration and ~5~3C at low soil temperature suggests that the rate of diffusion at this depth was minimal. Gas properties in the loess grassland soil during freezing behaved differently. Due to the respiration activity decreased together with lowering of soil temperature, a restriction of gas exchange with atmosphere might be a reason for the CO2 concentration increase at this particular time. The loess soil contains a higher traction of micropores and lower fraction of macropores in its structure in comparison with the sandy soil described previously. As such, the loess soil has a larger tendency to seal up the pores during freezing because the freezing of water in micropores causes the breaking of their continuity and condensation of water vapour, decreasing the diffusion transport of soil gases to the atmosphere. This results in the observed increase of CO2 content accompanied by depletion in ~3C (Fig. 4). The same effect is predicted by the model of carbon isotope distribution in CO2 during diffusion through porous media with different porosity and pores tortuosity (Dudziak, 1993). Fig. 6 shows this dependence for a model in which the diffusion coefficient is varying but is the same for the whole soil profile. The production rate (q) is assumed to decrease exponentially with depth: q = qo exp( - x/x()) where qo is the production rate at the surface, Xu is the characteristic depth. As it was stated above, the soil CO2 is enriched in heavy isotope with respect to soilrespired CO2 by faster diffusion of light isotope to the atmosphere. Hence, over a period when this diffusion is diminished the soil CO2 should approximate its isotope ratio to that of soil organic matter. For this reason we recorded such a low ~3CO2 value ( - 2 3 % 0 ) while below the frozen layer of soil 613Corganici s - - 24%0 (Dudziak, 1993). The reasons for the increase of CO2 content and decrease of 6~3C value during the warming and thawing of soil and melting of snow may be the same as those discussed for the sandy forest soil in the same period. When one compares the discussed changes of CO2 properties in both types of soil during a low temperature period, one may state that granulometrical composition and physical
A. Dudziak, S. Halas / Geoderma 69 (1996) 209-216
215
properties of soils play the most important role. Dusty soils with a large fraction of micropores lose the continuity of pores very quickly. Therefore, the diffusive transport of carbon dioxide to the atmosphere is markedly decreased, CO2 concentration increases in the soil air, and the carbon is depleted in ~3C. The situation is different in sandy soils where the fraction of macropores is dominant, hence even the freezing of soil does not cause the loss of pore continuity. The warming and melting of snow in both types of soils causes a distinct shift of soil air parameters, but their patterns are similar.
5. Concluding remarks In course of this study we have noticed that during winter the 6~3CO2 in soil may significantly decrease or increase. These major variations in ~3C are not related directly to temperature changes but to a cut-off mechanism of the soil CO2 generator from the atmosphere. The cut-off periods are characterized by a low 6~3CO2 value which is close to the source carbon in the organic matter at deeper unfrozen horizons. The major winter variations in 3~3CO2 of soil may be useful natural markers in the hydrological study of water infiltration rates from subsurface to the aquifer.
Acknowledgements This study was supported by International Atomic Energy Agency, Vienna, Contract No. 5144/RB.
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Halas, S., 1979. Automatic inlet system with pneumatic changeover valves for isotope mass spectrometer. J. Phys. E: Sci. Iustrum., 12: 418--420. Halas, S. and Dudziak, A., 1989. Precise determination of ~3C/~2C and CO2 concentration in minute samples of soil air by mass-spectrometry. Isotopenpraxis, 25:349-351. Halas, S. and Krouse, H.R., 1983. Isotopic analysis of nanomole gas samples by means of dynamic flow mass spectrometry. Rev. Sci. Instrum., 54: 437~-43. Halas, S. and Krouse, H.R., 1984. Device for rapid transfer of condensable gases into a capillary. Rev. Sci. lnstrum., 55:1160-1161. Hendy, C.H., 1971. The isotopic geochemistry of speleothems. 1- The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators. Geochim. Cosmochim. Acta, 35: 801-824. Hesterberg, R. and Siegenthaler, U., 1991. Production and stable isotopic composition of CO2 in a soil near Bern, Switzerland. Tellus, 43B: 197-205. Inoue, H. and Sugimura, Y., 1985. The carbon isotopic ratio of atmospheric carbon dioxide at Tsukuba, Japan. J. Atmos. Chem., 2: 331-344. Levin, 1., 1987. Atmospheric CO2 in continental Europe - - an alternative approach to clean air CO2 data. Tellus, 39B: 21-28. Mook, W.G., 1976. The dissolution-exchange model for dating groundwater with ~4C. In: Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrol. IAEA, Vienna, pp. 212-225. Mook, W,G., Koopmans, M., Carter, A.F. and Keeling, C.D., 1983. Seasonal, latitudinal and secular variations in the abundance and isotopic ratios of atmospheric carbon dioxide. 1. Results from land stations. J. Geophys. Res., 88: 10.915-10.933. Rightmire, C.T., 1978. Seasonal variation in P(CO2) and ~3C content of soil atmosphere. Water Resour. Res., 14: 691-692. Salomons, W. and Mook, W.G., 1986. Isotope geochemistry of carbonates in the weathering zone. In: P. Fritz and J. Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry. Part 2. The Terrestrial Environment, B. Elsevier, Amsterdam, pp. 239-269.