Plant and Soil 183: 291-299, 1996. 291 (~ 1996 Kluwer Academic Publishers. Printed in the Netherlands.

Diurnal cycle of carbon isotope ratio in soil CO2 in various ecosystems

A n d r z e j D u d z i a k and S t a n i s l a w H a l a s 1 Department of Physics, Technical University of Lublin, 20-618 Lublin, Poland and Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland*. J Corresponding attthor

Received 31 October 1995. Accepted in revised torm 30 May 1996

Key words: 13C/12C ratio, CO 2 concentration, diurnal cycle, microbial respiration, root respiration, soil CO2

Abstract

Our investigations of diurnal variations of the 13C/12C ratio and CO2 content in soil air were carried out in three environments during periods of high biosphere activity. It has been observed that diurnal variation of CO2 concentration is negatively correlated 613C. Particularly great variations occurred at shallow soil depths (10-30 cm) when the plant cover activity was high while the soil temperature was rather low. Under such conditions the ~13C variations had the magnitude of 4%o, while the CO2 concentration varied more than doubly. The maximum of the 13C/J2C ratlo and the minimum of the CO2 concentration in a cultivated field with winter wheat took place in the afternoon, whereas in deciduous forest similar patterns were observed at dawn. In these cases soil temperatures at 10 cm depths varied less than 2°C. Hence, under wheat the variation in root respiration rate seem to be the main reason of the recorded varations. In an uncultivated grass-field during the hottest period in summer we did not measure any distinct variations of CO2 properties in spite of the fact that soil temperature varied up to 5°C. This might be due to dominant microbial respiration at the high soil temperature, which exceeded 20 °C.

Introduction

Changes in solar energy within the 24-hour-cycle reaching a given point on Earth, result in variations of temperature and plant activity. Consequently, some changes in root respiration and in microbial decom- position rate of soil organic matter - the two major sources of carbon dioxide in the soil - may take place. Very marked daily variations in the root respiration of intact tobacco, corn and cotton plants were observed by Harris and van Bavel (1957). The maximum occurred at 4 p.m., while the minimum between 2 and 10 a.m. and was about twice lower than the maximum. These changes were regular although solar radiation varied considerably on the particular days of the investiga- tion. The same type of root respiration variations was recorded by Huck et al. (1962) for Derris, corn and soybean, but daytime respiratory rates were only 25 to 50% higher than those at night. The measurements were taken under the conditions of constant tempera- ture and humidity, so it has been concluded that it is

* FAX No: +4881376191

the shoot that is responsible for increased metabolic activity in the root in daylight hours. Small differences between the day and night respiration rates in corn roots were observed also by Veen (1981 ).

Field investigation of the soil respiration rate in the diurnal cycle have shown that its maximum occurs in the afternoon and may twice exceed the minimum value, occurring before dawn (Brown and Rosenberg, 1971; Currie, 1975; Nobel and Palta, 1989). The respiration is strongly correlated with soil tempera- ture, because an increase in temperature stimulates soil microbial activity. The range of variation depends on soil moisture. A reverse situation, i.e. the maxi- mum flux of CO2 at night has been recorded during a dry period in forest soils (Edwards and Sollins, 1973; Witkamp, 1969). The reason for changes of this type may be connected with the convection of CO2 from soil or with air moisture variations, but no convincing explanation has been presented.

In spite of the occurrence of the described variat- Ions in the soil respiration rate, there was no infor- mation about the diurnal variations of carbon isotope

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composition in soil CO2, although this kind of change was observed in the surrounding environment. The reported amplitude of changes of the 13C/12C ratio in the atmospheric CO2 in summer has been from 2% in a turf field to 6% in pine forest (Inoue and Sugimura, 1984; Keeling, 1961; Szaran, 1990). The highest con- tent of the isotope 13C occurred in the early afternoon, and the lowest just before dawn. Also the 13C/12C ratio of ZCO2 in paddy-field water varied by 8%0 in daytime (Nakamura et al., 1990).

The purpose of this study is to observe diurnal vari- ations of the carbon isotope composition in the soil CO2 in different ecosystems. Our observations were carried out in spring and summer, that is, during period of high biosphere activity. The results may be useful at least for estimating the importance of the moment of soil air sampling in long-term isotope observations.

Materials and methods

We did our investigations in the Lublin Upland (south- eastern Poland), near Lublin city. The climate there is temperate, with the mean annual precipitation of 600 mm and the mean annual temperature of +7.3 °C. July is the wannest month, with the mean temperature of + 18 °C. The elevation of this area is about 200 m.

For our study we have selected three sites, repre- senting varieties of environments typical for the Lublin Upland. Site 1 was a cultivated field with winter wheat (Triticum aestivum), where the Orthic Luvisols devel- oped from loeslike material, and the horizon depths w e r e : Ap (0-25 cm), A3 (25-40 cm), Bt (40-70 cm), D (>70 cm). The second sampling site was located in an uncultivated grass-covered field, where the Orth- ic Luvisols developed from the loess. The dominant grass species at this site were Lotus corniculatus, Poa annua and Festuca pratensis, and a minor fraction was composed of Veronica triphyllos, Agropyron sp., Equi- setum arvense, Arenaria serphyllifolia and Poligonum bistorta. The soil horizons were at the depths: A1 (0-15 cm), A3 (15-30 cm), Bt~ (30-75 cm), Bt2 (75-125 cm), C (>125 cm). The third investigated site was a decid- uous forest where the Quercus sessilus and Carpinus betulus were the main tree species with an admixture of the Betula pendula. No herbaceous species were present. The forest grew on the Eutric Gleysols devel- oped from loeslike material. The soil horizon depths were: Ao (0-1 cm), Al (1-15 cm), gl ( 1 5 4 0 cm), g2 (40-120 cm), Cg (>120 cm). All of the mentioned plants photosynthesize carbon dioxide using the Calvin cycle (C3 plants), the mean 613C value of that plant

group being -27%o (Bender, 1971; Lowdon and Dyck, 1974; Martin et al., 1988; Nakamura et al., 1990; Smith and Epstein, 1971).

In order to avoid rapid moisture changes which may modify biosphere actvlty, we waited for a day without clouds proceeded by a few rainless days. Soil gas samples were taken out into 50 mL glass syringes by means of plastic probes permanently installed in the soil or a stainless steel probe which was forced into a desired depth and stayed unmoved in the soil for the whole time of observation. To avoid perturbations in the soil atmosphere, the sampling proceeded very slowly and took about 20 min. Each time the soil gas was sampled, the air temperature and the soil temper- ature at the 10 cm depth were measured by means of a mercury thermometer.

The carbon dioxide was separated from an air sam- ple criogenically by freezing it under reduced pressure at the temperature of liquid nitrogen. An admixture of nitrous oxide (which has the same mass as CO2, i.e. 44 and 45) was removed by passing the gas through hot copper wires, where N20 decomposed (Dudziak and Halas, 1990).

For the measurement of isotopic ratios of the car- bon and CO2 concentration in the air samples we used the modified mass spectrometer MI 1305. The 13C/12C ratios are given as relative deviations from the PDB standard:

(~13C = (Rsampxe/Rstandard -- 1) × 1031%o]

where R stands for the carbon isotope ratio. Each sam- ple was analysed twice and the precision of 0.25%o (including the separation procedure) was achieved, while the internal mass-spectrometer reproducibility was 0.1%o.

The carbon isotope composition in soil CO2 is relat- ed to the CO2 concentration in soil air, so we deter- mined it by measuring the quantity of carbon dioxide mass-spectrometrically with a relative error of 2 per- cent. We used the linear dependence of the ion beam intensity upon the pressure of gas in the inlet system at low pressure range. The details of the experimental methods are described in our previous papers: (Halas and Dudziak, 1989; Dudziak and Halas, 1990).

Results

The field with winter wheat (site 1)

The diurnal measurements in the field with winter wheat were carried out on May 3-4, 1989 at the depths

293

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H o u r s

Figure I. Diurnal variation of 613CO2 (A) and its concentration (B) in soil under winter wheat on May 3-4, 1989.

-20

o -22 o

5,~ -24

-l. lOcm; 6 =-2.93C-17.62; R2=0.98] -17.25; R:Z=0.97}

, l , I , ~ .I -26 1.0 1.5 2.0 25 3.0 3.5

C O 2 concentrat ion [%]

Figure 2. Relationship between carbon isotope composition and CO2 concentration in soil under winter wheat (May 1989).

of 10 and 50 cm simultaneously. The observed varia- tions in the ~13C and CO2 concentration were surpris-

ingly high (Figure 1). At 10 cm depth the CO2 concen- tration varied from 1.1% to 2.6% while the ~13C value varied by 4%o. The range of the changes diminished with depth, but at 50 cm they were still noticeable. The maximum ~]3C (and the minimum of the CO2 concentration) appeared at dawn and in the morning, and the minimum ~ 3 C (with the maximum CO2 con- centration) -in the late afternoon. In the evening the differences between the ~3C/~2C ratios at two inves- tigated depths did not exceed 0.5%o, and at the same time the differences in the CO2 concentration were the Feast (about 0.5%). For any given depth, the CO2 con- centration and ~ 3 C correlated very well (Figure 2).

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Hours

Figure 3. Diurnal variation in an uncultivated grass-field and in a nearby located field with barley (5-6 July 1991): A - air and soil temperature; B - 613C of soil CO2; C - CO2 concentration in soil air.

Uncultivated grass-field and field with barley (site 2)

We carried out three times our study at this site. For the first t ime the observations were made on June 20-21, 1990 at the depths of 30 and 90 cm in an uncultivat- ed grass-field, and simultaneously, in the proximity of this site but in cultivated soil in a field of green barley at the depth of 35 cm. The investgations were repeated in the same site a year later i.e. on July 5-6, 1991 but at smaller depths (10 cm, 30 cm, and 20 cm under bar-

294

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0.6 o'~ t -

0.5 o . m

0.4

0.3 ~

8 0,2 ~

o O

Figure 4. Diurnal variation in an uncultivated grass-field (23-24 May 1995: A - air and soil temperature; B - 6~3CCO2 and its concentration in soil air at 30 cm depth.

-15 - 0.7

* "'~ o.6 8 7 - 1 7 , ) ~ \ " # '

0 " = x ~- , " 4 1 ~ 0,4 ~c -19 41" 8

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4 6 8 10 12141618202224 2 4 6 8 1012141618 2022 H o u r s

Figure 5. Diurnal variation of 613CO2 and its concentration in deciduous forest soil at 30 cm depth: A - 28-29 May 1989; B - 5 -6 August 1990 (the end of long, period of heat and dryness, the soil was dry); C - 15-16 August 1990 (relatively humid soil, two days after a fairly long period of rain),

ley). Both investigations took place in very hot periods with the mean soil temperature at the depth of 10 cm reaching 22 °C or 20 °C, and the maximum air tem- perature exceeding 30 °C or 27 °C respectively. In this way the obtained data may be taken as representative of the hottest summer periods.

The results of these diurnal observations do not show any significant variations (values obtained in July 1991 are presented in Figure 3). The 613C value varied by no more than 1%o and the CO2 concentration by no more than 0.2 of the respective average. Therefore, we can speak about some tendencies only, for example - a tendency to decrease the CO2 concentration and increase its 13C/12C ratio at night which is similar to the character of the changes recorded under winter wheat.

The observations at this site were repeated on May 23-24, 1995 when the thermal conditions of the soil were different. After a rather long period of cold and wet weather the mean soil temperature at 10 cm depth was about 12 °C, i.e. essentially lower than in pre- vious investigations. This time the observations were performed at the depth 30 cm only and the results are presented in Figure 4.

The CO2 concentration varies very little although its increase in the afternoon is noticeable. On the other hand the variations of 613C are larger than in previ- ous observations and exceed 1.5%o with a well-marked minimum in the evening (like in the field with winter wheat).

Deciduous forest (site 3)

The measurements in deciduous forest were performed on May 28-29, 1989 and again in August 1990 at the depth of 30 cm each time. The samples obtained on August 5-6, 1990 were taken from dry soil at the end of long-lasting period of heat and dryness, whereas those obtained on August 15-16, 1990 were taken from rel- atively humid soil, two days after a rather long-lasting rain. The mean soil temperatures on the investigating days of May 1989 and August 1990 at 10 cm depths were 12.0 °C, 16.0 °C and 16.8 °C, with the maxi- mum differences between dawn and afternoon of 1.7 °C, 1.7 °C and 1.1 °C. The air temperature in the forest reached its maximum in the afternoon and it was 21 °C, 28 °C and 26 °C, respectively.

The diurnal variations recorded in May 1989 and on August 15-16, 1990 were similar (Figures 5A and 5C). They have a sine form with the maxima of the 13Cfl2C ratio which correspond to the minima of the CO2 concentration in the late afternoon, while the min-

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1)6 = -10 40C -13.111 R 2= 0.96 • . 1 2) 5 = -26.37C -12.31: R 2= 0.93

-16 ~ , k ' ~ 3 i ~ = -12.44C -14.941 R2= 0.95

-18 ~ •

, ,e - 2 0

- 2 2 J I ,

0.1 0.3 05 07

CO 2 concentration [%]

Figure 6. Relationship between carbon isotope composition and CO2 concentration in the soil of a deciduous forest. 1 -28-29 May 1989; 2 - 5-6 August 1990 (soil was dry); 3 - 15-16 August 1990.

ima of the F3Cfl2C ratio and the maxima of the CO2 concentration appeared at dawn. The 613C varied by 4%0 while the COx content in the soil air at sunrise was over twice as large as in the afternoon. The variations recorded for dry soil were less pronounced and less regular than for humid soil, but their range was the same (Figure 5B). The CO2 concentration was in good correlation with 613C each time (Figure 6).

D i s c u s s i o n

The CO2 concentration in the soil is dependent on two proceses: variations of the rate of input (sources) and variation of the rate of output. The major sources of carbon dioxide in the soil are microbial respiration and root respiration. An additional source is atmospheric CO2 and its contribution is proportionally higher when C Q concentration is lower. The observed so far 24- hours respiration patterns are summarized briefly in the introductory section of this paper. Concerning the car- bon isotope composition, the CO2 produced during the microbial decomposition of organic matter is slightly depleted in isotope 13C (up to 1%o). (Nakamura et al., 1990) and also the 13cflZc ratio of respired CO2 is similar or lower in relation to the whole plant (Hsu and Smith, 1972; Smith, 1971). The average 613C value of the atmospheric CO2 is reported to be -8%,) (lnoue and Sugimura, 1985; Mook et al., 1983).

Output of CO2 is mostly dependent on the rate of diffusion of this gas to the surface, and the main fac- tors affecting diffusion coefficients are moisture, tex- ture and structure of soil. Diffusion of COz through soil air in the soil porosity is an isotope fractionat- ing process, leading to 13C moving slower (D~rr and

296

Mtinnich, 1980). The changes of soil respiration rate influence the 613C value in soil CO2, even when the dif- fusion coefficient and 13C/12C ratio of the net respired CO2 remains constant throughout the day. This effect has been calculated for the simple models of soil being in steady-state conditions, i.e. the respiration rate is the same for the whole soil profile, or it is decreasing exponentially with depth (Cerling, 1984; Cerling et al., 1991; Hesterberg and Siegenthaler, 1991). According to these models the ~13C in soil CO2 decreases with depth and with rise of respiration rate. So, an increase in soil CO2 concentration (by root or microbial respi- ration) leads to a depletion in 13C of soil CO2. These models showed also that the atmospheric component is likely to be important only when soil respiration rate is very low or at very shallow depths (less than 10 cm). But the variations of CO2 concentration or 13C02/12C02 ratio in atmospheric air near the ground change the boundary conditions for carbon dioxide dif- fusion from the soil to the atmosphere and may influ- ence the carbon isotope composition in soil CO2. Such situation may take place for instance in dense forest, where the exchange of air with the overlaying global air is diminished.

Moreover, different soil moisture, plant vegetation period and plant activity, localization of the sources (i.e. depth) or possible seasonal shift of 13c/lZc in CO2 generated by roots and by microbial decay influence the diurnal changes of 613C. This is important when we compare the changes observed at different sites or in various months at the same site. Generally speaking the problem of diurnal variations is multifactorial and the factors are in interaction. So often there is not possible to clearly separate the effects of each factor.

While analysing the diurnal variation in the soil under winter wheat (Figure 1) it is necessary to discuss the significance of the two major sources of carbon dioxide in the soil - microbial respiration and root res- piration. At the beginnig of May the wheat was 60 cm high and it well shaded the soil surface. As a result, the soil temperature was relatively low (about 11.5 °C at the 10 cm depth) and it varied between day and night within no more than 1.2 °C. This did not influence the microbial respiration very much and could not cause variations in soil CO2 concentration big enough to be observed. In this case the microbial respiration was a minor source in comparison to root respiration. Hence, the most important reason of the recorded variations seems to be the diurnal variation in roots respiration. At the beginning of May there was a rapid growth of wheat and its roots became the main source of CO2 in

the soil. This inference was confirmed by the fact that in the seasonal observations at this time we recorded an occurrence of the maximum CO2 concentration in the soil air, while in the next month the CO2 content decreased in spite of an increase in soil temperature (Dudziak, 1995).

The root respiration rate is at its maximum during the day and this causes an increase of the carbon diox- ide concentration in the soil air (Harris and van Bavel, 1957; Huck et al., 1962). This caused the observed 13C-depletion in soil CO2 which agrees with the mod- els mentioned above, though the conditions in the soil somewhat depart from steady-state. This process might largely contribute to the results recorded.

An additional reason of the observed variations at this site seems to be the differences in the carbon iso- tope composition of winter wheat and soil organic mat- ter. When the variations of microbial respiration rate during a day are low, the participation of the main sources to the total soil respiration varies, which influ- ences on the 13C/12C ratio in soil carbon dioxide. The ~13C value of wheat may vary from - 33%0 to -27%o although as low as -33%e was observed in a laboratory experiments (MacDowall and Lowdon, 1989; Mad- havan et al., 1991; Martin et al., 1988; Nakamura et al., 1990; Troughton and Card, 1975) while the 613C of soil or organic matter in this cultivated field was from -26.5.%o to -25%o (Dudziak, 1995). The isotopic composition of soil CO2 is enriched in 13C relative to respired CO2 by at least 4.4%0 as predicted by the dif- ference in diffusion coefficients for 12C02 and 13C02 (Cerling et al., 1991). The average value of ~13CO2 at the depth of 50 cm was -25%o (Figure 1), so the root respired carbon dioxide should be respectively 13C depleted and its dI3C is lower than -27%o accept- ed as the mean value of the C3 plant group. Thus, dl3C of CO2 produced in the soil is more negative dur- ing the day than at night. Although the average ~3C of CO2 derived from microbial resirpation has to be equal to 613C of the plant material which constitutes the soil organic matter, but in the cultivated field the plant species are changed every year and the 6~3C of newly growing plants may differ from that of the old soil organic matter.

Similar variations of the CO2 concentration in soil air or soil respiration rates were observed by Witkamp (1969), Curie (1975), Nobel and Palta (1989); howev- er, they were explained, essentially, as the influence of soil temperature variations.

The small effects which were observed in unculti- vated grass-field during hottest summer periods (Fig-

ure 3) may be explained by the high soil temperature, at which the CO2 production from the decay of soil organic matter by micro-organisms is high in relation to the root respiration, which in such conditions must have been only a minor fraction of the total CO2 pro- duced (plant cover activity is probably low). Even rel- atively high variation in soil temperature (4-5 °C at the depth of 10 cm) did not influence the CO2 con- centration very much. Although this concentration is about five-six times lower than under wheat, but it confirms the importance of large contribution of root respiration in the total CO2 produced for the appear- ance of significant diurnal variations (like in the soil under wheat). The observations presented in Figure 4 confirm this idea, because at low soil temperature in May enhanced contribution of root respiration in the soil, which makes the diurnal changes of ~13C in CO2 more pronounced.

The differences in average ~I3C (at similar CO2 concentration between May and July in grass-covered field (Figures 3 and 4) may be due to seasonal shift of 13C/12C ratio in the CO2 sources, or may be caused by the variation of diffusion coefficient in the soil. The soil in May was humid, thereby it had low air porosity, which results in lower gas diffusion rate when com- pared to July. In the mentioned gas diffusion models the lowering of the diffusion coefficient decreases t3C content in soil CO2 (Dudziak and Halas, 1996), which agrees with our observations.

Another reason of the small effect observed in June and July might be very short night period when those observations were made.

The variations observed in the deciduous forest in humid soil. (Figure 5) were just the opposite to those recorded under winter wheat where the highest 3~3C values and lowest CO2 concentration took place before sunrise (Figure 1). Thus, the variations of the microbal respiration rate caused by temperature changes, or the variations of diurnal root respiration rate could not be simply related to the observed variations of soil gas properties.

Diurnal variations of the same type have also been observed in the investigations of the CO2 flux from forest soils, where the night-time CO2 evolution rates were even over twice higher than the daytime rates. Witkamp (1969) has suggested that convection was flushing the CO2 upward from the litter and soil hori- zons, while the soil was warmer than the overlying air at night. However the increase of the CO2 flux had begun several hours before the air temperature dropped below the soil temperature. Edwards and

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Sollins (1973) discovered that the highest contribu- tion to the night peak of the CO2 evolution originated from the litter. They also found an inverse correlation between the CO2 flux and the vapour pressure deficit in atmospheric air which indicated that moisture and temperature may be important parameters determining the rate of the CO2 movement from the forest floor to the atmosphere.

The variations of the CO2 concentration and its ¢~13C in the atmospheric air may influence the soil car- bon dioxide. In the coniferous forest they are large, because at dawn the CO2 content in the atmosphere is nearly twice as high as in the afternoon, and 613C is by 5%e lower (Szaran, 1990). In a dense deciduous forest near the ground or in the litter the variations may be even higher. Based on the diffusion model of CO2 transport from the soil (Cerling et al., 1991 ; Hesterberg and Siegenthaler, 1991 ) we calculated the influence of (~13C changes in atmospheric CO2 on the carbon iso- tope composition of soil CO2. There was assumed, that an increase in atmospheric CO2 concentration is a result of simple mixing of atmospheric air contain- ing 0.035% CO2 with ~J3C = -8%~ and respired CO2 with 313C = -27%c. When the soil respiration rate is low, an increase of atmospheric CO2 concentration up to 0.1% may deplete 13C content in soil CO2 at the depth of 30 cm even a few permil. Moreover, the CO2 concentration in this site strongly increased with depth down to l m (Dudziak, 1995), so the changes near the surface easier influenced the CO2 concentration and the 13Cfl2C ratio at the sampled depth of 30 cm. Although this process only partly explains the more than two-fold increase of CO2 concentration in soil air, this effect may contribute to the recorded changes when CO2 concentration is low.

In order to explain large diurnal variation in CO2 concentration at depth of 30 cm in deciduous forest by the diffusion model we have to assumed that upper- most layer of soil and litters may be considered as an soil-to-atmosphere interlace with higher CO2 content and lower 6J3C than atmospheric air. This interface has his own 24-hours period of variation with predom- inantly higher amplitude than that in atmospheric air. Therefore, the diffusion model with periodically vary- ing boundary condition (COe concentration and c~I3C at the soil "surface") may explain not only strong ampli- tude of variation but also the retardation of peak in CO2 concentration (accompanied by lowermost 3~3C) to late night hours (Figure 5). The diffusion model pre- dicts that the amplitude of these variations will be lower and lower with increase of depth (in similar manner as

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diurnal temperature variation inasmuch as both vari- ations are described by similar equations: the second Fic's law and the heat conductivity equation, respec- tively) while the retardation will be larger and larger with depth.

Moreower, it is noteworthy that then there is large variation in evapotranspiration rates in day and night periods, but it is not easy to estimate its influence on the 13c/lZc ratio in the soil CO2 in the investigated forest.

The differences of average value of COz concen- tration between various observations are caused pre- dominantly by differences in soil respiration rates and in diffusion rate to the atmosphere. The microbial res- piration mainly depends on the content of soil organic matter, temperature and moisture, while the root respi- ration is affected by the type of plant cover, its period of vegetation, temperature and moisture. That is why the highest CO2 concentration was observed in the soil under winter wheat, because at the beginning of May wheat was at its maximum rate of growth.

The rate of diffusion output of CO2 mainly depends on diffusion coefficient, that is on such soil parame- ters as moisture, structure and texture. It is difficult to discuss the differences between various sites without adequate measurements, but influence of soil mois- ture changes at the same site is visible. The decrease of moisture in forest soil (Figure 5B) and connected increase of air porosity causes decrease of average CO2 concentration. This is the result of easier gas diffusion to the atmosphere and decrease of soil respiration rate. At dry period diurnal cycle in the soil was modified because during the day the CO2 concentration and 613 C stayed nearly constant. This confirms the importance of temperature and moisture for the soil processes.

One may see from the results presented in this paper, that the CO2 concentration at shallow depths is lower than in deeper layers (Figures 1 and 3) while the ~13C is respectively higher. This is the result of carbon dioxide diffusion to the atmosphere which is in agreement with the diffusion models. At shallower depth the soil air properties (CO2 concentration and its ~3C/12C ratio) bring closer to the atmospheric air properties.

The slope of the regression of 613C dependence upon the CO2 concentration showed large variabili- ty and appears to be depend on depth, season, soil moisture and site (Figures 2 and 6). However these differences are reasonable and are caused by the vari- ations in the average CO2 content. According to the diffusion models the increase of CO2 concentration in

soil due to increase of soil respiration rate, increase of CO2 concentration in atmospheric air or decrease of diffusion coefficient, causes the lowering of slope of regression, which was observed. This may be qualita- tively explained as follows: at higher CO2 content the 13c/lZc ratio has lower sensitivity to the CO2 varia- tions. A quantitative explanation of this effect will be published by Dudziak elsewhere.

Another reason for the short-term changes in soil air may be a rapid increase or decrease in atmospher- ic pressure. The water tables in the investigated sites are about 25 m below the surface, thus an increase of the pressure by 2% during the day may force the atmospheric air (with 613CO2 of -8%o) into the soil to the depth of 50 cm, which would be a very important factor in our study. That is why we observed the atmo- spheric pressure on the investigating days and the day before. However, the pressure changes did not exceed 0.3% and did not correlate with the CO2 changes, so this factor may be ignored.

Conclusions

Our study has confirmed the existence of strong diurnal cycles of ~13CO2 and its concentration in soil air. Until now the literature data have referred to CO2 concentra- tion only. The character and amplitude of the diurnal cycle of CO2 concentration and its 13C/12C ratio in soil air depends mainly on the type of environment, the period of plant vegetation and on soil temperature and moisture, i.e. on factors which determine the ratio of roots respiration to microbial respiration, and which determine the rates of CO2 output from the soil. Par- ticularly high variations occur at shallow depths of soil when the activity of plant cover is high, but the soil temperature is not too high. In such conditions the 6~3C variations have the amplitude of 4%e and the CO2 con- centration varies more than 100%. The maximum of the 13C/12C ratio and the minimum of the CO2 concen- tration in soil air my occur in the afternoon (deciduous forest), at dawn (winter wheat) or may not appear dis- tinctly at all, but in all cases the ~13C values correlate closely with the CO2 content.

The results of our investigations confirm the suppo- sition that "steady hours" of a day are very important when soil air is sampled for long-term observations, and that the sampling should not be made at shallow depths for this kind of study.

More information about the diurnal variations in soil air would be provided by simultaneous observa-

tions of temperature, CO2 concentration, 6~3C and moisture at different depths in the same site and at the same time as well as by measurements taken in different seasons of the year.

Acknowledgements

We are grateful to two anonymous referee's for sugges- tions which significantly improved "Discussion" sec- tion of this paper.

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