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Oxygen-18, deuterium and carbon-13 content of organic matter from litter and humus layers in Podzolic soils
CATENA
Vol. 10, 159-166
Braunschweig1983
OXYGEN-18, DEUTERIUM AND CARBON-13 CONTENT OF ORGANIC MATI'ER FROM LITTER AND HUMUS LAYERS IN PODZOLIC SOILS M. Balabane, Paris SUMMARY In this study, a depletion in the O-18 content of soil organic matter from surface horizons relative to the plant parent material is reported. The relation between isotopic composition of land plants and environmental conqlj,tions is,not concealed by the processes leading to humic substance formation as shown by'°O and "H contents of organic matter from H layer. Besides, carbon-13 contents show no systematic relationship with climatic parameters. RESUME L'analyse de la teneur en oxyg~ne-18 des couches L, F et H de l'horizon A0 de podzol montre un appauvrissement en isotope lourd de la mati~re d6compos6e par rapport/l la v6g6tation dont elle d6rive. Les teneurs en O-18 et deuterium de la couche H tendent/~ montrer que la relation qui existe entre la composition isotopique de la cellulose et certains param~tres du milieu n'est pas masqu6e par le d6but de d6composition subi par la liti~re dans le sol. Les teneurs en carbone-13 sont, quant/l elles, ind6pendantes des param~tres climatiques. 1. INTRODUCTION Natural abundances in stable isotopes represent a tool in the understanding of hydrological, geochemical and biochemical processes. Carbon-13 analyses in plants have been used extensively since the first survey carried out by CRAIG (1953). Influence of photosynthetic pathway is well established (BENDER 1971, SMITH & EPSTEIN 1971). Some studies attempted to relate dl3c in plants to climatic parameters (LIBBY & PANDOLFI 1974, LERMAN & LONG 1978). As to 2H/1H and 180/160, variation of these ratios in precipitations mainly depends on climate (CRAIG 1961). Systematic differences between deuterium content of land plants and environmental water were reported (SCHIEGL 1970). Besides, the part played by photosynthetic processes was outlined (ZIEGLER et al. 1976, ESTEP & HOERING 1978, 1980). Studies on deuterium content of tree rings were made for paleoclimatic purposes (SCHIEGL 1974, WILSON & GRINSTED 1975, YAPP & EPSTEIN 1977). The oxygen isotopic studies are more complicated because of interactions between atmosphere IH20,CO2,02] and plant Iplant water and photosynthetic processes}. Oxygen-18 analyses alone or combined with deuterium analyses in both plant water (DONGMANN et al. 1974, EPSTEIN et al. 1977, FERHI & LETOLLE 1979) and plant material (FERHI & LETOLLE 1977a, 1977b) were carried out. Analyses of tree rings were also run (LIBBYet al. 1976, GRAY & THOMPSON 1979). However, only a limited number of studies concerns soil organic matter. Some data are
160
BALABANE
available for peat and coals (SCHIEGL 1970, 1972). Carbon-13 and deuterium contents of humic substances were studied as possible tracers for humification in soils (NISSENBAUM et al. 1972, 1974). Data on isotopic composition (180, 2H, ~3C)of soil organic matter are presented here. Samples are from L, Fand H layers of raw humus in podzolic soils. Variations are observed: 1) In H layers, in relation with different environmental conditions; 2) In L, Fand H layers, in relation with different stages of decay.
2. 2.1.
MATERIAL AND METHODS SAMPLES
These are plant residues from litter (L) and fermentation (F) layers and organic fine substance from humus substance (H) layer in raw humus, surface horizon of podzolic soils (BABEL 1975); collected in different climatic areas: cold (Sweden and Canada), temperate (Belgium and France) and equatorial (French Guyana). Before analysis, plant residues are rinsed in distilled water. In H layers, inorganic compounds (mainly quartz grains) are removed by shaking and decanting in distilled water. Samples are dried at 70"C for 72 hours then heated (50 to 60°C) under vacuum. Possible exchange of hydrogen with water was checked. A sample of 82H = -122~0 was shaken for 72 hours at 25"C in water of d2H = 500/00. After this, the sample analysis gave -120 d2H°/00 which agrees with the original value within the analytical error.
2.2.
ISOTOPIC ANALYSIS
For lSo/160 ratio analysis (HARDCASTLE & FRIEDMAN 1974, FERHI 1980) the organic material is pyrolysed in presence of diamond at 1250°C. The carbon monoxide formed is converted to carbon dioxide by a high voltage discharge. The CO2 is analysed on a mass spectrometer. 2H/1H and 13C/12C analyses consist in a combustion of organic material in an oxygen atmosphere. Water and carbon dioxide formed are separated by trapping in glass-vessels at liquid nitrogen and dry-ice temperatures. The CO2 is analysed on a mass spectrometer. The water is reduced with hot uranium (BIGELEISEN 1952), the hydrogen evolved is recovered with the intermediate formation of uranium hydride (FRIEDMAN & HARDCASTLE 1970), Hydrogen is then analysed on a mass spectrometer. Results are given in d notation: dX = [(R sample/g standard)q] • 103 °/00 where X = 13C; 180 or 2H. R = 13C/12C; 180/160 or 2H/1H. dl3c are given relative to the PDB standard (CRAIG 1957) and d2H and d180 relative to SMOW (CRAIG 1961). The standard deviation "o" is respectively + 0,1°/00, + 20/00 and + 0,1°/oo.
STABLE ISOTOPES CONTENT, HUMUS LAYERS, PODZOLS
3.
161
RESULTS AND DISCUSSION
3.1.
PLANT RESIDUES FROM L AND F LAYERS [81SO}.
Leaves and needles from L and F layers analysis shows (Table 1) a depletion of about 3 81800/00 between the non or slightly decomposed litter and plant residues from F layer. After decomposition, the proportion oflignin increases due to varying rates of.decay for different tissue groups. This may account for the observed depletion as lignin has a lower 180 content than cellulose (GRAY & THOMPSON 1977).
Tab. h 81SO OF PLANT RESIDUES FROM RAW HUMUS LAYERS. (1). Al80°/oo = d180 (L) - 8180 (F). (2). Replicate measurements. Sampling locations Willerzie (Belgium)
Sample
layer ~0-1'8%o
leaves
27,07
(Fagus sylvatica)
24,21 (2)
~0-18%o(1 2,71
24,51 1st p r o { i l e
leaves
F~re-en-Tardenois (F#ance)
(Ouercus)
2nd p r o f i l e
Mont P i l a t
3.2.
(France)
27,11 27 • O0 F
23,73
leaves
L
23,27
( B e t u l a verrucosa)
F
20,95
needles
L
23,91
(Abies a l b a }
F
21,10
3,32 2,82
2,81
SOIL ORGANIC FINE SUBSTANCE FROM H LAYER
The data (Table 2) indicate similar behaviour of 180 and 2H contents in relation to sampling locations: - depletion in the heavy isotopes in cold areas (Sweden and Canada) with dlSo of 14 to 200/00 and average 82H of-110°/0o. - similar compositions for temperate and equatorial areas with averages for 8180 and d2H of respectively 220/00 and -700/00. Carbon-13 contents do not seem to be related to climatic areas.
3.2.1.
Oxygen-18concentration
The 180 content of soil organic matter is represented in figure 1-A with respect to latitude of sampling locations. In temperate and cold areas (45 to 700N), our data fit a curve almost parallel to that of plant cellulose (figure I-B) in the same range of latitude (FERHI & LETOLLE 1979). The intercept is 3 to 50/00 lower in good agreement with the 30/00 depletion
162
BALABANE
Tab. 2: ORGANICFINE SUBSTANCE FROM H LAYERS: SAMPLING LOCATIONS AND dlSO, d2H and 8jaC. The analyticalprecision of these measurements is, respectively, + 0,1; 5-2 and 5- 0,1 °/00. Sampling
locations
Abisko (Sweden)
~0-18°Ioo (SMOW)
~D°loo ~C-13°Ioo (SMOW) (PDB)
14,07
~-26,05 I
• 26 05 Trosken
(Sweden}
16,94
-98,56
-27,33
-124,31
-25,68
16,56
Thor Lake (Canada)
17,92
-120,09
123 90 Lake o f t h e Noods (Canada) Nillerzie
(Belgium)
F ~ r e - e n - T a r d e n o i s (France)
20,60
1-27,22
22,07
1-69,39
22,56
70 03
-27,23
23,85
-75,46
-25,09
-73,64
-26,90
-60,61
-27,30
22,99
S~ Malo de P h l l y
(France)
22,45 22,29
Mont P l l a t Hourtin
(France)
(France)
Kourou (French Guyana)
22,99 21,94
-73,36
-27,68
~23,45
-60,59
-28,25
23~16
observed between plant residues from L and F layers. Raw humus from warm and dry areas is scarce and data are missing in this interval. The equatorial sample exhibits the 30/00 de~81etionregarding plant cellulose• FERHI et al. (1979) showed the influence of ground water O content and relative humidity on d 18O of plant cellulose. In cold areas where ground water is depleted of heavy isotopes and relative humidity is high, a depletion of 1SO content of cellulose is noted. Temperate and equatorial areas, in spite of very different temperatures, show similar 180 content due to high relative humidity in equatorial regions compensating high temperatures in the process of transpiration of plants. Our data suggest that dlSO in humus still reflects environmental conditions of plant growth. An attempt to relate d180 of organic fine substance to temperatures in sampling locations from cold and temperate areas gave a linear correlation with mean temperatures from April to September (r = 0,94; figure 2). Considering the standard deviation, the accuracy ( + 3°C) is not as good as may be expected from this good correlation. With mean annual temperatures, the correlation coefficient is smaller (0,81), but the slope of 0,46 is in agreement with values of 0,45 and 0,41 for 180 content of tree rings reported by GRAY & THOMPSON (1979) and BURK & STUIVER (1979).
STABLEISOTOPESCONTENT,HUMUSLAYERS,PODZOLS
163
Latitude ("N) I
30
'
I
I
'
I
I
I
I
28 26 24 •
•
o
u
•
~ 20
0
16 14
i '
7b
'
60
'
510
'
410'
310'
210 ' ! 0 I
,
Fig. 1: VariationofdlSo (A) of organic fine substancein raw humus formations, (B) of plant cellulose (FERHI et al. 1979), as related to latitude of sampling locations. mean
Aprml to S e p t e m b e r
24
'
. . . .
'
temperatures(°C) '
'
"
'
• ie
22
2o
:IE b Fig. 2: Relative 180 content of organic fine substance from H layer as related to mean April to September temperatures in cold and temperate areas. (Meteorological data: monthly mean temperatures averaged over 20 years: 1950-1970).
3.2.2.
Deuterium
16
,~,2t ol0,21 r = 0,94 n=8 n=B
/ 14
/
; 4
12
,,,_ 16
20
concentration
d2H shows similar variations as dlSo: relative depletion in cold areas and enrichment in warmer temperate areas. The best correlation coefficient (0,93) is obtained with mean annual temperature. The slope and the intercept are similar to those obtained for peat by S C H I E G L (1972). In figure 4, a linear regression is presented between d2H and d180. More results are needed to ascertain this relationship.
164
BALABANE meon onnuol
temperoture
i
i
(°C)
D
1
i
-70
-BO
~ -90 ~2 u% 0
.~ qo0 62H= 3.z,-Bt - 105,6
Fig. 3: _ Relative ZHcontent oforganic fine substance from H layer as related to mean annual temperatures in cold and temperate areas. (Meteorological data: monthly mean temperatures averaged over 20 years: 19501970).
-110
-120
,/-4
;
2
;
1B 5 0°/** (SMOW) '
1
'
i
'
i
,
i
-60
-70
-80 0 L/3
"~ -90
~-100
52H=B,~IBo- 270 -120
Fig. 4: 16
3.2.3.
Carbon-13
18
ZO
22
24
6ZH versus dl80 of organic fine substance from raw humus formations.
concentration
The values (table 2) fall between -28 and -25°/00 within the range of carbon from C3 plants following the Calvin cycle. No systematic relationship with climatic parameters appears. The most negative value (-28,2) represents a sample from French Guyana from a place with a dense vegetation. This low carbon isotope value could be related to the canopy effect (LERMAN & LONG 1978).
STABLEISOTOPESCONTENT,HUMUSLAYERS,PODZOLS 4.
165
CONCLUSION
The results show a depletion of lSo content of organic matter from H layer regarding parent plant material. This depletion seems to take place in the first steps of decay processes of litter. Furthermore, relation between climatic areas and t51SOof plant cellulose is main° rained although decomposition has occured. This suggests that 180 content in humus still reflects environmental conditions of plant growth. Variations in d2H of organic matter from H layers with sampling locations are similar to those observed for t~lSo. Carbon-13 content does not seem to be related to climatic parameters. Further analyses of organic matter from organo-mineral horizons of different soil types are necessary to settle the role ofpedogenetic and climatic parameters in the isotopic composition of soil organic matter.
ACKNOWLEDGEMENTS
I wish to thank Professor G. BOCQUIER, Il LETOLLE, P. ROGNON and Dr. A. FERHI for helpful scientific discussions and critical reading of the manuscript.
REFERENCES
BABEL, U. (1975): Micmmorphology of soil organic matter. In: Soil Components, vol. 1. Organic components, chap. 7. J.E. Gieseking, Soringer Verlag Press, N.Y. BENDER, M. (1971): Variations in the laC/lzC ratios of plants in relation to the pathway of photosynthetic CO2 fixation. Phytochem. 10, 1239-1244. BIGELEISEN, J., PERLMAN, M.L., PROSSER, H.C. (1952): Conversion ofhydrogenic materials to hydrogen for isotope analysis. Analytical chemistry 24, 8, 1356-1357. BURK, ILL. & STUIVER, M. (1979): Factors affecting18O/ 16,O ratios in cellulose. In: Proceedings of the Int. Meeting on Stable Isotopes in tree ring research, N.Y., G.C. Jacoby, 68-70. CRAIG, H. (1953): The geochemistry of the stable carbon isotopes. Geoch. and Cosmoch. Acta 3, 53-92. CRA/G, H. (1957): Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analysis of carbon dioxide. Geoch. and Cosmoch. Acta 12, 133-149. CRAIG, H. (1961): Isotopic variations in meteoric waters. Science 133, 1702-1703. CRAIG, H. (1961): Standard for reporting concentrations of deuterium and oxygen-18 in natural waters. Science 133, 1.8.33-1834. DONGMANN, G., NURNBERG, H.W., FORSTEL, H. & WAGENER, K. (1974): On the enrichment of H20 in the leaves of transpiring plants. Rad. and environ, biophys. 11, 41-52. EPSTEIN, S., THOMPSON, P. & YAPP, J.C. (1977): Oxygen and hydrogen isotopic ratios in plant cellulose. Science 198, 4323, 1209-1215. ESTEP, M.F. & HOERING, T.C. (1978): The organic geochemistry of the stable hydrogen isotopes. 4th int. conf. geochr, cosmochr, and isotope geology, Aspen, Colorado, 108-109. ESTEP, M.F. & HOERING, T.C. (1980): Biogeochemistry of the stable hydrogen isotopes. Geoch. and Cosmoch. Acta 44, 1197-1206. FERHI, A.M. & LETOLLE, Il (1977): Variation de la composition isotopique de l'oxyg6ne organique de quelques plantes en fonction de leur milieu de vie. Compte-rendu de l'Acad6mie des Sciences 284, 1887-1889. FERHI, A.M. & LETOLLE, Il (1977): Transpiration and evaporation as the principal factors in oxygen isotope variations of organic matter in land plants. Physiologic v6g6tale 15, 2, 363-370. FERHI, A.M. & LETOLLE, Il (1979): Relation entre le milieu climatique et les teneurs en ISO de la cellulose des plantes terrestres. Physiologic v6g6tale 17, 1,107-117. FERHI, A.M. (1980): Variation des dlsO de la mati6re organique d'origine v6g6tale. Application h l'6tude des pal6oenvironnements. Th6se Doct. Etat, Univ. P. et M. Curie, Paris.
166
BALABANE
FRIEDMAN, I. & HARDCASTLE, K. (1970): A new technique for pumping H gas. Geo. and Cosmo. Acta 34, 125-126. GRAY, J. & THOMPSON, P. (1977): Climatic informations from lSO/160 analysis of cellulose, lignin and whole wood from tree rings. Nature 270, 708-709. GRAY, J. &THOMPSON, P. (1979): Natural variations in the 180 content of cellulose. In: Proceedings ofthe Int. Meeting on Stable Isotopes in tree ring research, N.Y., G.C. Jacoby, 84-92. HARDCASTLE, K. & FRIEDEMAN, I. (1974): A method for oxygen isotope analysis of organic material. Geophys. research letters 1, 165-167. LERMAN, J.C., LONG, A. (1978): 13C as climate indicator; factors controlling stable carbon isotopes ratios in tree rings. 4th int. conf. geochr, cosmochr, and isotope geology, Aspen, Colorado, 248-250. LIBBY, L.M. & PANDOLFI, L.J. (1974): Temperature dependence of isotope ratios in tree rings. Proc. Nat. Acad. Sci. USA 71, 6, 2482-2486. LIBBY, L.M. et al. (1976): Isotopic tree thermometers. Nature 261, 284-288. NISSENBAUM, A. & KAPLAN, I.R. (1972): Chemical and isotopic evidence for the in situ origin of marine humic substances. Limnology and Oceanography 17, 4, 570-582. NISSENBAUM, A. & SCHALLINGER, K.M. (1974): The distribution of the stable carbon isotopes (13C/12C) in fractions of soil organic matter. Geoderma 11, 2. 137-145. NISSENBAUM, A. (1974): Deuterium content of humic acids from marine and non-marine environmerits. Marine chemistry 2, 59-63. SCHIEGL, W.E. & VOGEL, J.C. (1970): Deuterium content of organic matter. Earth and planetary science letters 7, 307-313. SCHIEGL, W.E. (1972): Deuterium content of peat as a palaeoclimatic recorder. Science 175, 512-513. SCHIEGL, W.E. (1974): Climatic significance of deuterium abundance in growth rings of Picea. Nature 251,582-584. SMITH, B.N. & EPSTEIN, S. (1971): Two categories of 13C/12Cratios for higher plants. Plant. Physiology 47, 3,380-384. WILSON, A.T. & GRINSTED, M.J. (1975): Paleotemperatures from tree rings and the D/H ratio of cellulose as a biochemical thermometer. Nature 257, 387-388. YAPP, C.J. & EPSTEIN, S. (1977): Climatic implications of D/H ratios of meteoric water over North America (9500-22000 BP) as inferred from ancient wood cellulose C-H hydrogen. Earth and planetary science letters 34, 333-350. ZIEGLER, H., OSMOND, C.B., STICHLER, W. & TRIMBORN, P. (1976): Hydrogen isotope discrimination in higher plants: Correlations with photosynthetic pathway and environment. Planta (Berlin) 128, 85-92.
Address of author: May Balabane, D~partement de Grologie Dynamique, Universit6 Pierre et Marie Curie 4, Place Jussieu, 75230 Paris c~dex 05, France.
Vol. 10, 159-166
Braunschweig1983
OXYGEN-18, DEUTERIUM AND CARBON-13 CONTENT OF ORGANIC MATI'ER FROM LITTER AND HUMUS LAYERS IN PODZOLIC SOILS M. Balabane, Paris SUMMARY In this study, a depletion in the O-18 content of soil organic matter from surface horizons relative to the plant parent material is reported. The relation between isotopic composition of land plants and environmental conqlj,tions is,not concealed by the processes leading to humic substance formation as shown by'°O and "H contents of organic matter from H layer. Besides, carbon-13 contents show no systematic relationship with climatic parameters. RESUME L'analyse de la teneur en oxyg~ne-18 des couches L, F et H de l'horizon A0 de podzol montre un appauvrissement en isotope lourd de la mati~re d6compos6e par rapport/l la v6g6tation dont elle d6rive. Les teneurs en O-18 et deuterium de la couche H tendent/~ montrer que la relation qui existe entre la composition isotopique de la cellulose et certains param~tres du milieu n'est pas masqu6e par le d6but de d6composition subi par la liti~re dans le sol. Les teneurs en carbone-13 sont, quant/l elles, ind6pendantes des param~tres climatiques. 1. INTRODUCTION Natural abundances in stable isotopes represent a tool in the understanding of hydrological, geochemical and biochemical processes. Carbon-13 analyses in plants have been used extensively since the first survey carried out by CRAIG (1953). Influence of photosynthetic pathway is well established (BENDER 1971, SMITH & EPSTEIN 1971). Some studies attempted to relate dl3c in plants to climatic parameters (LIBBY & PANDOLFI 1974, LERMAN & LONG 1978). As to 2H/1H and 180/160, variation of these ratios in precipitations mainly depends on climate (CRAIG 1961). Systematic differences between deuterium content of land plants and environmental water were reported (SCHIEGL 1970). Besides, the part played by photosynthetic processes was outlined (ZIEGLER et al. 1976, ESTEP & HOERING 1978, 1980). Studies on deuterium content of tree rings were made for paleoclimatic purposes (SCHIEGL 1974, WILSON & GRINSTED 1975, YAPP & EPSTEIN 1977). The oxygen isotopic studies are more complicated because of interactions between atmosphere IH20,CO2,02] and plant Iplant water and photosynthetic processes}. Oxygen-18 analyses alone or combined with deuterium analyses in both plant water (DONGMANN et al. 1974, EPSTEIN et al. 1977, FERHI & LETOLLE 1979) and plant material (FERHI & LETOLLE 1977a, 1977b) were carried out. Analyses of tree rings were also run (LIBBYet al. 1976, GRAY & THOMPSON 1979). However, only a limited number of studies concerns soil organic matter. Some data are
160
BALABANE
available for peat and coals (SCHIEGL 1970, 1972). Carbon-13 and deuterium contents of humic substances were studied as possible tracers for humification in soils (NISSENBAUM et al. 1972, 1974). Data on isotopic composition (180, 2H, ~3C)of soil organic matter are presented here. Samples are from L, Fand H layers of raw humus in podzolic soils. Variations are observed: 1) In H layers, in relation with different environmental conditions; 2) In L, Fand H layers, in relation with different stages of decay.
2. 2.1.
MATERIAL AND METHODS SAMPLES
These are plant residues from litter (L) and fermentation (F) layers and organic fine substance from humus substance (H) layer in raw humus, surface horizon of podzolic soils (BABEL 1975); collected in different climatic areas: cold (Sweden and Canada), temperate (Belgium and France) and equatorial (French Guyana). Before analysis, plant residues are rinsed in distilled water. In H layers, inorganic compounds (mainly quartz grains) are removed by shaking and decanting in distilled water. Samples are dried at 70"C for 72 hours then heated (50 to 60°C) under vacuum. Possible exchange of hydrogen with water was checked. A sample of 82H = -122~0 was shaken for 72 hours at 25"C in water of d2H = 500/00. After this, the sample analysis gave -120 d2H°/00 which agrees with the original value within the analytical error.
2.2.
ISOTOPIC ANALYSIS
For lSo/160 ratio analysis (HARDCASTLE & FRIEDMAN 1974, FERHI 1980) the organic material is pyrolysed in presence of diamond at 1250°C. The carbon monoxide formed is converted to carbon dioxide by a high voltage discharge. The CO2 is analysed on a mass spectrometer. 2H/1H and 13C/12C analyses consist in a combustion of organic material in an oxygen atmosphere. Water and carbon dioxide formed are separated by trapping in glass-vessels at liquid nitrogen and dry-ice temperatures. The CO2 is analysed on a mass spectrometer. The water is reduced with hot uranium (BIGELEISEN 1952), the hydrogen evolved is recovered with the intermediate formation of uranium hydride (FRIEDMAN & HARDCASTLE 1970), Hydrogen is then analysed on a mass spectrometer. Results are given in d notation: dX = [(R sample/g standard)q] • 103 °/00 where X = 13C; 180 or 2H. R = 13C/12C; 180/160 or 2H/1H. dl3c are given relative to the PDB standard (CRAIG 1957) and d2H and d180 relative to SMOW (CRAIG 1961). The standard deviation "o" is respectively + 0,1°/00, + 20/00 and + 0,1°/oo.
STABLE ISOTOPES CONTENT, HUMUS LAYERS, PODZOLS
3.
161
RESULTS AND DISCUSSION
3.1.
PLANT RESIDUES FROM L AND F LAYERS [81SO}.
Leaves and needles from L and F layers analysis shows (Table 1) a depletion of about 3 81800/00 between the non or slightly decomposed litter and plant residues from F layer. After decomposition, the proportion oflignin increases due to varying rates of.decay for different tissue groups. This may account for the observed depletion as lignin has a lower 180 content than cellulose (GRAY & THOMPSON 1977).
Tab. h 81SO OF PLANT RESIDUES FROM RAW HUMUS LAYERS. (1). Al80°/oo = d180 (L) - 8180 (F). (2). Replicate measurements. Sampling locations Willerzie (Belgium)
Sample
layer ~0-1'8%o
leaves
27,07
(Fagus sylvatica)
24,21 (2)
~0-18%o(1 2,71
24,51 1st p r o { i l e
leaves
F~re-en-Tardenois (F#ance)
(Ouercus)
2nd p r o f i l e
Mont P i l a t
3.2.
(France)
27,11 27 • O0 F
23,73
leaves
L
23,27
( B e t u l a verrucosa)
F
20,95
needles
L
23,91
(Abies a l b a }
F
21,10
3,32 2,82
2,81
SOIL ORGANIC FINE SUBSTANCE FROM H LAYER
The data (Table 2) indicate similar behaviour of 180 and 2H contents in relation to sampling locations: - depletion in the heavy isotopes in cold areas (Sweden and Canada) with dlSo of 14 to 200/00 and average 82H of-110°/0o. - similar compositions for temperate and equatorial areas with averages for 8180 and d2H of respectively 220/00 and -700/00. Carbon-13 contents do not seem to be related to climatic areas.
3.2.1.
Oxygen-18concentration
The 180 content of soil organic matter is represented in figure 1-A with respect to latitude of sampling locations. In temperate and cold areas (45 to 700N), our data fit a curve almost parallel to that of plant cellulose (figure I-B) in the same range of latitude (FERHI & LETOLLE 1979). The intercept is 3 to 50/00 lower in good agreement with the 30/00 depletion
162
BALABANE
Tab. 2: ORGANICFINE SUBSTANCE FROM H LAYERS: SAMPLING LOCATIONS AND dlSO, d2H and 8jaC. The analyticalprecision of these measurements is, respectively, + 0,1; 5-2 and 5- 0,1 °/00. Sampling
locations
Abisko (Sweden)
~0-18°Ioo (SMOW)
~D°loo ~C-13°Ioo (SMOW) (PDB)
14,07
~-26,05 I
• 26 05 Trosken
(Sweden}
16,94
-98,56
-27,33
-124,31
-25,68
16,56
Thor Lake (Canada)
17,92
-120,09
123 90 Lake o f t h e Noods (Canada) Nillerzie
(Belgium)
F ~ r e - e n - T a r d e n o i s (France)
20,60
1-27,22
22,07
1-69,39
22,56
70 03
-27,23
23,85
-75,46
-25,09
-73,64
-26,90
-60,61
-27,30
22,99
S~ Malo de P h l l y
(France)
22,45 22,29
Mont P l l a t Hourtin
(France)
(France)
Kourou (French Guyana)
22,99 21,94
-73,36
-27,68
~23,45
-60,59
-28,25
23~16
observed between plant residues from L and F layers. Raw humus from warm and dry areas is scarce and data are missing in this interval. The equatorial sample exhibits the 30/00 de~81etionregarding plant cellulose• FERHI et al. (1979) showed the influence of ground water O content and relative humidity on d 18O of plant cellulose. In cold areas where ground water is depleted of heavy isotopes and relative humidity is high, a depletion of 1SO content of cellulose is noted. Temperate and equatorial areas, in spite of very different temperatures, show similar 180 content due to high relative humidity in equatorial regions compensating high temperatures in the process of transpiration of plants. Our data suggest that dlSO in humus still reflects environmental conditions of plant growth. An attempt to relate d180 of organic fine substance to temperatures in sampling locations from cold and temperate areas gave a linear correlation with mean temperatures from April to September (r = 0,94; figure 2). Considering the standard deviation, the accuracy ( + 3°C) is not as good as may be expected from this good correlation. With mean annual temperatures, the correlation coefficient is smaller (0,81), but the slope of 0,46 is in agreement with values of 0,45 and 0,41 for 180 content of tree rings reported by GRAY & THOMPSON (1979) and BURK & STUIVER (1979).
STABLEISOTOPESCONTENT,HUMUSLAYERS,PODZOLS
163
Latitude ("N) I
30
'
I
I
'
I
I
I
I
28 26 24 •
•
o
u
•
~ 20
0
16 14
i '
7b
'
60
'
510
'
410'
310'
210 ' ! 0 I
,
Fig. 1: VariationofdlSo (A) of organic fine substancein raw humus formations, (B) of plant cellulose (FERHI et al. 1979), as related to latitude of sampling locations. mean
Aprml to S e p t e m b e r
24
'
. . . .
'
temperatures(°C) '
'
"
'
• ie
22
2o
:IE b Fig. 2: Relative 180 content of organic fine substance from H layer as related to mean April to September temperatures in cold and temperate areas. (Meteorological data: monthly mean temperatures averaged over 20 years: 1950-1970).
3.2.2.
Deuterium
16
,~,2t ol0,21 r = 0,94 n=8 n=B
/ 14
/
; 4
12
,,,_ 16
20
concentration
d2H shows similar variations as dlSo: relative depletion in cold areas and enrichment in warmer temperate areas. The best correlation coefficient (0,93) is obtained with mean annual temperature. The slope and the intercept are similar to those obtained for peat by S C H I E G L (1972). In figure 4, a linear regression is presented between d2H and d180. More results are needed to ascertain this relationship.
164
BALABANE meon onnuol
temperoture
i
i
(°C)
D
1
i
-70
-BO
~ -90 ~2 u% 0
.~ qo0 62H= 3.z,-Bt - 105,6
Fig. 3: _ Relative ZHcontent oforganic fine substance from H layer as related to mean annual temperatures in cold and temperate areas. (Meteorological data: monthly mean temperatures averaged over 20 years: 19501970).
-110
-120
,/-4
;
2
;
1B 5 0°/** (SMOW) '
1
'
i
'
i
,
i
-60
-70
-80 0 L/3
"~ -90
~-100
52H=B,~IBo- 270 -120
Fig. 4: 16
3.2.3.
Carbon-13
18
ZO
22
24
6ZH versus dl80 of organic fine substance from raw humus formations.
concentration
The values (table 2) fall between -28 and -25°/00 within the range of carbon from C3 plants following the Calvin cycle. No systematic relationship with climatic parameters appears. The most negative value (-28,2) represents a sample from French Guyana from a place with a dense vegetation. This low carbon isotope value could be related to the canopy effect (LERMAN & LONG 1978).
STABLEISOTOPESCONTENT,HUMUSLAYERS,PODZOLS 4.
165
CONCLUSION
The results show a depletion of lSo content of organic matter from H layer regarding parent plant material. This depletion seems to take place in the first steps of decay processes of litter. Furthermore, relation between climatic areas and t51SOof plant cellulose is main° rained although decomposition has occured. This suggests that 180 content in humus still reflects environmental conditions of plant growth. Variations in d2H of organic matter from H layers with sampling locations are similar to those observed for t~lSo. Carbon-13 content does not seem to be related to climatic parameters. Further analyses of organic matter from organo-mineral horizons of different soil types are necessary to settle the role ofpedogenetic and climatic parameters in the isotopic composition of soil organic matter.
ACKNOWLEDGEMENTS
I wish to thank Professor G. BOCQUIER, Il LETOLLE, P. ROGNON and Dr. A. FERHI for helpful scientific discussions and critical reading of the manuscript.
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BALABANE
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Address of author: May Balabane, D~partement de Grologie Dynamique, Universit6 Pierre et Marie Curie 4, Place Jussieu, 75230 Paris c~dex 05, France.