- Email: [email protected]
Dynamics of soil organic matter as reflected by natural 13C abundance in particle size fractions of forested and cultivated oxisols
0038-0717/92sS.00+ 0.00 Copyright Q 1992Pergamon Press plc
Soil Bid. B&kern. Vol. 24, No. 3, pp. 275-277, 1992 Printed in Great Britain. All rights reserved
SHORT COMMUNICATION
DYNAMICS OF SOIL ORGANIC MATTER AS REFLECTED BY NATURAL 13C ABUNDANCE IN PARTICLE SIZE FRACTIONS OF FORESTED AND CULTIVATED OXISOLS TORBEN A. BONDE,‘* BENT T. CHRISTENSEN*and CARLOS C. CERR$ ‘University of Linkliping, Department of Water and Environmental Studies, S-581 83 LinkSping, Sweden, ‘Askov Experimental Station, Vejenvej 55, DK-6600 Vejen, Denmark and 3Centro de Energia Nuclear na Agricultura (CENA), Cx. Postal 96, 13400 Piracicaba SP, Brazil (Accepted 29 September 1991) Soil organic matter (SOM) encompasses a diversity of organic compounds with decomposition rates which vary continuously due to the complex interactions of biological, chemical and physical processes in soils. Turnover times of individual SOM components are greatly influenced by their association with the mineral part of the soil. Functional analysis of SOM dynamics most commonly results in classifications of SOM into a few entities with widely differing turnover rates. The Century Model (Parton et al., 1987, 1989), for example, partitioned SOM into three fractions with turnover times of about l-5. 20-40, and 200-1500 yr. A number of studies ranging from tracer experiments and various forms of incubations to physical fractionation have suggested that such fractions can validly represent functional units of SOM occurring in nature (Parton et al., 1987). Fractionation of soil according to particle size (clay, silt, and sand-size fractions) yields organo-mineral fractions with distinctly different properties in terms of organic matter turnover (e.g. Tiessen and Stewart. 1983; Christensen, 1987a). A number of studies have shown that clayassociated organic matter is important in medium-term SOM turnover, whereas silt-bound organic matter participates in longer-term turnover (Christensen, 1987a). However, the potential use of organic matter in different soil particle-size classes as estimates of SOM fractions in modelling remains to be evaluated. A recently applied method based on natural 13C abundance and principles of isotopic dilution allows determination of long-term SOM dynamics (e.g. Cerri et al., 1985; Balesdent et al., 1987, 1988). The method has been applied to field experiments, where one type of vegetation is replaced by another having a different “C isotopic composition, e.g. where tropical forest (C, vegetation) is replaced by sugar-cane or grass pasture (C, vegetation). The natural i3C abundance of SOM corresponds closely to that of the vegetative cover, and replacement of vegetation will thus gradually change the isotopic composition. Differences in isotopic signature between various SON fractions together with information on the length of time since the change-over from one vegetation type to the other will allow assessment of relative turnover rates of the fraction. The present study was based on the combined use of particle-size fractionation and the natural “C tracer technique in order to evaluate the dynamics of SOM within particle-size fractions, The experimental site was situated near Piracicaba in southeastern Brazil, where three adjacent areas with *Author for correspondence. 588 24:3-G
275
(1) natural forest vegetation and two clearfelled areas continuously cropped with sugar-cane for (2) 12 yr and (3) SOyr were selected. The forest site was sampled at depths of O-6 and 6-12cm, while the sites under sugarcane were sampled at 0-1Ocm. The soil is an Oxisol classified as clayey, kaolinitic, isotherm&, Typic Haplorthox according to USDA Soil Taxonomy. Detailed information on sampling procedure and the sites is given in Cerri et a/. (1985) and Vitorello et al. (1989). Particle-size fractions were isolated according to Christensen ( 1985). Briefly, 30 g soil samples were dispersed ultrasonically in 150 ml of water (300 W for 15 min) using a probe-type disintegrator. Clay and silt-size fractions were obtained by gravity ~dimentation, the sand-size fractions being isolated subsequently by dry sieving. The 13C isotopic composition of SOM was determined on a Finnigan mass spectrometer, and 613C values were calculated using the PDB-standard (Vitorello Ed al., 1989). Total C was determined by dry combustion in a Wosthof “Carmogram 12” analyser. The fractionation yielded about 55% clay, 15% silt, and 30% sand, with only a very smali loss of soil solids (Table 2). The distribution of soil particles was in accordance with results of a standard textural analysis based on chemical-mechanical dispersion (Table 1). The largest discrepancy between the two methods was found for fine sand 1 (20-63 pm) and for the silt fraction of forest soil and soil cultivated for 50 yr. The overall com~s~tion of the soil in terms of weight fractions of clay, silt, and sand particles did not change to any major extent during cultivation (Table 2). The silt-size particles contained the highest concentrations of total C (31-93 mg g-i), but accounted for only a minor proportion of whole soil C (23-35%). The clav contained most of the soil C (48-67%), while the sand Contained 7-17%. Clay, silt, and sand-size fractions contained on average 53, 3 I and 17% of whole soil C, respectively, in the forest soil. while the corresponding percentages after 50 yr of cropping were 67, 23. and 10%. Thus. cultivation caused a deuletion of organic C in the sand and silt fractions and a relative enrichment in organic C of clay. Delta 13Cvalues of soil organic C may increase as a result of humification (Vitorello et ai., 1989). The approximate one unit increase in 6 “C from sand to clay-size particles in the forest soil (Table 2) may reflect as increasing degree of humification of the associated organic matter. The enrichment in “C caused by cropping with sugar-cane was most pronounced in the sand-size fractions and less pronounced in the clay-size fraction. Vitorello ef al. (1989) and Cerri et al. (1985) did not observe significantly different Si3C values of size fractions < 200 pm for the soils cropped with 1
216
Short
communications
Table 1. Standard textural analyses of samples taken from 0 to 6 and 6 to 12 cm depth in soil under natural forest vegetation (FS) and from 0 to 1Ocm in soil cultivated with sugar cane for 12 (X-12) or 50 yr (SC-SO) after forest clearing. Values are expressed as mg g ’ whole soil dry wt
FS, O-6 cm FS, 6-12cm SC-12, 0-IOcm SC-50, O-IO cm
Total C
OM
Clay <2um
46.9 22.4 15.2 14.1
80 38 26 24
564 455 577 632
Silt 2-20 /Am
Fine sand 1 20-63 urn
Fine sand 2 63-200 urn
Coarse sand 200-2000 u m
58 181 126 73
26 25 21 33
124 142 132 125
148 158 117 113
Table 2. Dry weight, total C content and delta ‘% values of particle size fractions isolated from soil samples taken in O-6 and 6-l 2 cm depth under natural forest vegetation (FS) and from O-IO cm depth in soil cultivated with sugar cane for 12 (SC-12) or 50 yr (SC-SO) after forest clearing. Mean values (n = 3) with standard deviations in parentheses Particle Clay <2um Fraction
weight,
mg gm’
FS, O-6 cm FS, 6-12 cm SC-12, O-IOcm SC-50, O-10 cm Total
C content,
mg g
FS, O-6 cm FS, 6-12 cm SC-12,0-1Ocm SC-50, O-10 cm Della
“C
value
whole
499 530 583 551
soil dry
36.8 27.2 21.3 20.9
Fine sand 1 20-63 urn
Fine sand 2 63-200 urn
Coarse sand 200-2000 urn
145 (8) 125 (9) 122 (6) 127 (8)
64(i) 66 (2) 71 (4) 88 (8)
134 (3) 131 (2) 118(3) 127 (2)
143 (I) 138 (3) 101 (6) 101 (5)
(4.0) (1.8) (0.2) (1.2)
18.9 (3.9) ll.6(9.6) 2.6 (0.8) 2.6 (0.2)
14.1 (1.9) 11.4(6.1) 3.7 (2.1) 1.9 (0.3)
(0.20) (0.48) (0.07) (0.09)
- 27. I (0.22) - 27.0 (0.74) -22.9 (0.34) -21.7(0.09)
Sum of fractmns
wf
(12) (8) (11) (15)
’ ,fraction dry
size fraction
Silt 2-20 urn
983 988 995 992
(3) (3) (2) (6)
wt
(3.9) (1.9) (2.6) (0.7)
92.8(11.0) 49.8(1.7) 33.1 (1.1) 31.2 (0.2)
26.9 10.5 8.4 9.5
o/m
FS, O-6 cm ’ FS. 6-12 cm SC:12,0-1Ocm SC-50, O-10 cm
- 25.9 (0.58) -25.7 (0.051 -24.8 iO.14j -23.0(0.17)
-26.6(0.12) -26.6 (0.20) -23.9(0.16) -21.2(0.15)
-26.8 -27.0 - 24.1 -21.6
sugar cane. This inconsistency as compared to our resultsmay be related to differences in size-fraction limits and separation procedures. Vitorello et al. (1989) exposed soil samples to two subsequent overnight extractions with sodiumpyrophosphate (adjusted to pH 11.5 with NaOH) before size fractionation. The rationale of extracting whole soil samples with strong chemical reagents before attempting to isolate intact organomineral separates seems questionable, since the extractions applied removed 33-37% of whole soil C. It has been demonstrated that organic matter in various size fractions have a differential extractability when exposed to alkali solutions (Anderson et al., 1981; Catroux and Schnitzer, 1987), and a redistribution of organic material amongst size fractions induced by chemical treatment cannot be excluded. Based on data in Table 2, the amount of total C derived from forest (Cr) can be calculated for the cultivated sites using the equation: C, = C, x (4 - d,)/(dr
- &).
where C, denotes total C of sample; ds and d,, average 6 “C values for the respective size fractions in the samples from
- 27.1 -26.9 -21.3 -22.7
(0.20) (0.94) (2.36) (0.98)
sugar-cane fields and the forest, respectively, while 6 ‘C of sugar-cane residues, d,, equals - 13%~ The resulting values (Table 3) show a rapid decline in forest soil C in all size fractions during the first 12 yr of cropping and a much slower decrease during the following years. However, the initial decomposition of SOM in sand and silt is more pronounced than that of the clay fraction. During 12 yr of cropping, 83 and 93% of the initial forest C decomposed in the coarse sand and fine sand 2 (63-200pm) fractions, respectively, while in the clay only 40% of the forest-derived C decomposed during the initial 12 yr of cropping (Table 3). Forest C in the silt and the fine sand 1 (20-63 pm) declined by 63 and 64% during the same period. The following 38-yr cropping period resulted in a much smaller decline in forest derived C, i.e. about 10% in the clay and silt, and I- 7% in the sand fractions. The turnover time of the forest derived C, which decomposed in the various size fractions during 50 yr of cropping may be estimated assuming that the C stems from one labile fraction exclusively and that the kinetics of degradation are determined by the C remaining after 12 yr of cropping. The calculated turnover times of about 59, 6 and 4yr in the clay, silt and sand-size fractions, respectively. compares
Table 3. Total C derived from forest and remaining in soil cultivated with sugar cane for 12 (K-12) or 50 yr (SC-SO). Samples were taken O-12 cm depth in soil under natural forest vegetation (FS) and from 0 to 10 cm in soils cultivated with sugar cane after forest clearnina. Mean values calculated from data in Table 2 Fine sand I 20-63 urn
Fine sand 2 63-200 urn
Coarse sand 200-2000 urn
Total C deriuedfrom foresr, mg gm’,fraction dry WI FS, O-l 2 cm 32.0 71.3 SC-12,0-1Ocm 19.5 26.5 SC-50, O-10 cm 16.3 19.7
18.7 6.7 5.9
15.3 1.1 0.9
12.8 2.2 1.3
Proportion of forestderived C remaining, % FS, O-12 cm 100 Z-12, 0-1Ocm 61 SC-50. 0- 10 cm 51
100 36 32
100 7 6
100 17 10
Clay <2um
Silt 2-20 urn
100 31 28
Short communications fairly well to the turnover of the active (l-5 yr) and slow fraction (20-40yr) as indicated in the Century Model (Patton et al., 1987). The remaining forest C in the particle size fractions after 50yr of cropping may constitute the passive fraction. Although size fractionation yields organomineral separates which differ in turnover times, individual size fractions clearly encompass pools of organic C which are differentially susceptible to biological decomposition. Organic matter associated with a particular size fraction cannot be considered a uniform entity. This study shows that individual size fractions contained at least two organic matter pools with widely differing turnover times. In addition, the various size fractions seemed to contain stable organic C with approximately the same turnover, while having differing turnovers of the labile component. The silt fraction OM was more labile than clay associated OM, which is generally not the case in temnerate soils (Christensen. 1987a). Thus, it is important to take into consideration the soil mineralogy if estimates of SOM fractions in modelling among other things are based on size fractions. Classification of SOM in terms of functionally relevant pools is crucial for current models simulating agroecosystems and grasslands (Parton et al., 1989). Successful development of techniques for direct measurement of pool sizes would represent a major step towards appropriate verification of models and the revision of inherent concepts. The ‘C isotope provides a valuable contribution to such classification and model validation in the relatively few long-term field experiments available. Application of physical fractionation may further contribute to the validation of model concepts, in particular if combined with measurements of soil microbial biomass, mineralization studies and isotope techniques (Christensen, 1987b; Paustian and Bonde, 1987). Standard procedures for physical fractionation of soils are, however, much needed, and current methods should be carefully evaluated before being adopted for a wide range of soils (Christensen, 1992). Acknowledgements-The present investigation was financially supported by a grant to Professor Thomas Rosswall from the Swedish Agency for Research Cooperation with Developing Countries (SAREC) and was part of the IAEA sponsored Brazilian research programme “Amazonia Un”. We are grateful to Reynaldo L. Victoria for the isotopic analysis. REFERENCES Anderson D. W., Saggar S., Bettany J. R. and Stewart J. W. B. (1981) Particle size fractions and their use in studies of soil organic matter: I. The nature and distribution of forms of carbon, nitrogen, and sulphur. Soil Science Society of America Journal 45, 767-772.
277
Balesdent J., Mariotti A. and Guillet B. (1987) Natural 13C abundance as a tracer for studies of soil organic matter dynamics. Soil Biology & Biochemistry 19, 25-30. Balesdent J., Wagner G. H. and Mariotti A. (1988) Soil organic matter turnover in long-term field experiments as revealed by carbon-13 natural abundance. Soil Science Society of America Journal 52, 118-124. Catroux G. and Schnitzer M. (1987) Chemical, spectroscopic, and biological characteristics of the organic matter in particle size fractions separated from an Aquoll. Soil Science Society of America Journal 51, 1200-1207. Cerri C. C., Balesdent J., Feller C., Victoria R. and Plenecassagne A. (1985) Application du tracage isotropique nature1 en “C a l’etude de la dynamique de la matidre organique dans les soIs. Comtes Rendus de I’dcademie des Sciences de Paris. T. 300, Serie II 9, 423-428. Christensen B. T. (1985) Carbon and nitrogen in particle size fractions isolated from Danish arable soils by ultrasonic dispersion and gravity sedimentation. Acta Agriculturae Scandinavica 35, 175-187. Christensen B. T. (1987a) Decomposability of organic matter in particle size fractions from field soils with straw incorporation. Soil Biology & Biochemistry 19, 429-435. Christensen B. T. (1987b) Use of particle size fractions in soil organic matter studies. INTECOL BuNetin 15, 113-123. Christensen B. T. (1992) Physical fractionation of soil and organic matter in primary particle size and density separates. Advances in Soil Science. In press. Parton W. J.. Schimel D. S.. Cole C. V. and Oiima D. S. (1987) Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Science Society of America Journal 51, 1173-I 179. Parton W. J., Sanford R. L., Sanchez P. A., Stewart J. W. B., Bonde T. A., Crossley D., van Veen H. and Yost R. (1989) Modelling soil organic matter in tropical soils. In Dynamics of Soil Organic Matter in Tropical Ecosystems (D. C. Coleman, J. M. Oades and G. Uehara, Eds), pp. 153-171. University of Hawaii Press, Honolulu, Hawaii. Paustian K. and Bonde T. A. (I 987) Interpreting incubation data on nitrogen mineralization. INTECOL Bulletin 15, 101-l 12. Tiessen H. and Stewart J. W. B. (1983) Particle-size fractions and their use in studies of soil organic matter: II. Cultivation effects on organic matter composition in size fractions. Soil Science Society of America Journal 41, 509-514. Vitorello V. A., Cerri C. C., Andreux F., Feller C. and Victoria R. L. (1989) Organic matter and natural carbon13 distribution in forested and cultivated oxisols. Soil Science Society of America Journal 53, 773-778.
Soil Bid. B&kern. Vol. 24, No. 3, pp. 275-277, 1992 Printed in Great Britain. All rights reserved
SHORT COMMUNICATION
DYNAMICS OF SOIL ORGANIC MATTER AS REFLECTED BY NATURAL 13C ABUNDANCE IN PARTICLE SIZE FRACTIONS OF FORESTED AND CULTIVATED OXISOLS TORBEN A. BONDE,‘* BENT T. CHRISTENSEN*and CARLOS C. CERR$ ‘University of Linkliping, Department of Water and Environmental Studies, S-581 83 LinkSping, Sweden, ‘Askov Experimental Station, Vejenvej 55, DK-6600 Vejen, Denmark and 3Centro de Energia Nuclear na Agricultura (CENA), Cx. Postal 96, 13400 Piracicaba SP, Brazil (Accepted 29 September 1991) Soil organic matter (SOM) encompasses a diversity of organic compounds with decomposition rates which vary continuously due to the complex interactions of biological, chemical and physical processes in soils. Turnover times of individual SOM components are greatly influenced by their association with the mineral part of the soil. Functional analysis of SOM dynamics most commonly results in classifications of SOM into a few entities with widely differing turnover rates. The Century Model (Parton et al., 1987, 1989), for example, partitioned SOM into three fractions with turnover times of about l-5. 20-40, and 200-1500 yr. A number of studies ranging from tracer experiments and various forms of incubations to physical fractionation have suggested that such fractions can validly represent functional units of SOM occurring in nature (Parton et al., 1987). Fractionation of soil according to particle size (clay, silt, and sand-size fractions) yields organo-mineral fractions with distinctly different properties in terms of organic matter turnover (e.g. Tiessen and Stewart. 1983; Christensen, 1987a). A number of studies have shown that clayassociated organic matter is important in medium-term SOM turnover, whereas silt-bound organic matter participates in longer-term turnover (Christensen, 1987a). However, the potential use of organic matter in different soil particle-size classes as estimates of SOM fractions in modelling remains to be evaluated. A recently applied method based on natural 13C abundance and principles of isotopic dilution allows determination of long-term SOM dynamics (e.g. Cerri et al., 1985; Balesdent et al., 1987, 1988). The method has been applied to field experiments, where one type of vegetation is replaced by another having a different “C isotopic composition, e.g. where tropical forest (C, vegetation) is replaced by sugar-cane or grass pasture (C, vegetation). The natural i3C abundance of SOM corresponds closely to that of the vegetative cover, and replacement of vegetation will thus gradually change the isotopic composition. Differences in isotopic signature between various SON fractions together with information on the length of time since the change-over from one vegetation type to the other will allow assessment of relative turnover rates of the fraction. The present study was based on the combined use of particle-size fractionation and the natural “C tracer technique in order to evaluate the dynamics of SOM within particle-size fractions, The experimental site was situated near Piracicaba in southeastern Brazil, where three adjacent areas with *Author for correspondence. 588 24:3-G
275
(1) natural forest vegetation and two clearfelled areas continuously cropped with sugar-cane for (2) 12 yr and (3) SOyr were selected. The forest site was sampled at depths of O-6 and 6-12cm, while the sites under sugarcane were sampled at 0-1Ocm. The soil is an Oxisol classified as clayey, kaolinitic, isotherm&, Typic Haplorthox according to USDA Soil Taxonomy. Detailed information on sampling procedure and the sites is given in Cerri et a/. (1985) and Vitorello et al. (1989). Particle-size fractions were isolated according to Christensen ( 1985). Briefly, 30 g soil samples were dispersed ultrasonically in 150 ml of water (300 W for 15 min) using a probe-type disintegrator. Clay and silt-size fractions were obtained by gravity ~dimentation, the sand-size fractions being isolated subsequently by dry sieving. The 13C isotopic composition of SOM was determined on a Finnigan mass spectrometer, and 613C values were calculated using the PDB-standard (Vitorello Ed al., 1989). Total C was determined by dry combustion in a Wosthof “Carmogram 12” analyser. The fractionation yielded about 55% clay, 15% silt, and 30% sand, with only a very smali loss of soil solids (Table 2). The distribution of soil particles was in accordance with results of a standard textural analysis based on chemical-mechanical dispersion (Table 1). The largest discrepancy between the two methods was found for fine sand 1 (20-63 pm) and for the silt fraction of forest soil and soil cultivated for 50 yr. The overall com~s~tion of the soil in terms of weight fractions of clay, silt, and sand particles did not change to any major extent during cultivation (Table 2). The silt-size particles contained the highest concentrations of total C (31-93 mg g-i), but accounted for only a minor proportion of whole soil C (23-35%). The clav contained most of the soil C (48-67%), while the sand Contained 7-17%. Clay, silt, and sand-size fractions contained on average 53, 3 I and 17% of whole soil C, respectively, in the forest soil. while the corresponding percentages after 50 yr of cropping were 67, 23. and 10%. Thus. cultivation caused a deuletion of organic C in the sand and silt fractions and a relative enrichment in organic C of clay. Delta 13Cvalues of soil organic C may increase as a result of humification (Vitorello et ai., 1989). The approximate one unit increase in 6 “C from sand to clay-size particles in the forest soil (Table 2) may reflect as increasing degree of humification of the associated organic matter. The enrichment in “C caused by cropping with sugar-cane was most pronounced in the sand-size fractions and less pronounced in the clay-size fraction. Vitorello ef al. (1989) and Cerri et al. (1985) did not observe significantly different Si3C values of size fractions < 200 pm for the soils cropped with 1
216
Short
communications
Table 1. Standard textural analyses of samples taken from 0 to 6 and 6 to 12 cm depth in soil under natural forest vegetation (FS) and from 0 to 1Ocm in soil cultivated with sugar cane for 12 (X-12) or 50 yr (SC-SO) after forest clearing. Values are expressed as mg g ’ whole soil dry wt
FS, O-6 cm FS, 6-12cm SC-12, 0-IOcm SC-50, O-IO cm
Total C
OM
Clay <2um
46.9 22.4 15.2 14.1
80 38 26 24
564 455 577 632
Silt 2-20 /Am
Fine sand 1 20-63 urn
Fine sand 2 63-200 urn
Coarse sand 200-2000 u m
58 181 126 73
26 25 21 33
124 142 132 125
148 158 117 113
Table 2. Dry weight, total C content and delta ‘% values of particle size fractions isolated from soil samples taken in O-6 and 6-l 2 cm depth under natural forest vegetation (FS) and from O-IO cm depth in soil cultivated with sugar cane for 12 (SC-12) or 50 yr (SC-SO) after forest clearing. Mean values (n = 3) with standard deviations in parentheses Particle Clay <2um Fraction
weight,
mg gm’
FS, O-6 cm FS, 6-12 cm SC-12, O-IOcm SC-50, O-10 cm Total
C content,
mg g
FS, O-6 cm FS, 6-12 cm SC-12,0-1Ocm SC-50, O-10 cm Della
“C
value
whole
499 530 583 551
soil dry
36.8 27.2 21.3 20.9
Fine sand 1 20-63 urn
Fine sand 2 63-200 urn
Coarse sand 200-2000 urn
145 (8) 125 (9) 122 (6) 127 (8)
64(i) 66 (2) 71 (4) 88 (8)
134 (3) 131 (2) 118(3) 127 (2)
143 (I) 138 (3) 101 (6) 101 (5)
(4.0) (1.8) (0.2) (1.2)
18.9 (3.9) ll.6(9.6) 2.6 (0.8) 2.6 (0.2)
14.1 (1.9) 11.4(6.1) 3.7 (2.1) 1.9 (0.3)
(0.20) (0.48) (0.07) (0.09)
- 27. I (0.22) - 27.0 (0.74) -22.9 (0.34) -21.7(0.09)
Sum of fractmns
wf
(12) (8) (11) (15)
’ ,fraction dry
size fraction
Silt 2-20 urn
983 988 995 992
(3) (3) (2) (6)
wt
(3.9) (1.9) (2.6) (0.7)
92.8(11.0) 49.8(1.7) 33.1 (1.1) 31.2 (0.2)
26.9 10.5 8.4 9.5
o/m
FS, O-6 cm ’ FS. 6-12 cm SC:12,0-1Ocm SC-50, O-10 cm
- 25.9 (0.58) -25.7 (0.051 -24.8 iO.14j -23.0(0.17)
-26.6(0.12) -26.6 (0.20) -23.9(0.16) -21.2(0.15)
-26.8 -27.0 - 24.1 -21.6
sugar cane. This inconsistency as compared to our resultsmay be related to differences in size-fraction limits and separation procedures. Vitorello et al. (1989) exposed soil samples to two subsequent overnight extractions with sodiumpyrophosphate (adjusted to pH 11.5 with NaOH) before size fractionation. The rationale of extracting whole soil samples with strong chemical reagents before attempting to isolate intact organomineral separates seems questionable, since the extractions applied removed 33-37% of whole soil C. It has been demonstrated that organic matter in various size fractions have a differential extractability when exposed to alkali solutions (Anderson et al., 1981; Catroux and Schnitzer, 1987), and a redistribution of organic material amongst size fractions induced by chemical treatment cannot be excluded. Based on data in Table 2, the amount of total C derived from forest (Cr) can be calculated for the cultivated sites using the equation: C, = C, x (4 - d,)/(dr
- &).
where C, denotes total C of sample; ds and d,, average 6 “C values for the respective size fractions in the samples from
- 27.1 -26.9 -21.3 -22.7
(0.20) (0.94) (2.36) (0.98)
sugar-cane fields and the forest, respectively, while 6 ‘C of sugar-cane residues, d,, equals - 13%~ The resulting values (Table 3) show a rapid decline in forest soil C in all size fractions during the first 12 yr of cropping and a much slower decrease during the following years. However, the initial decomposition of SOM in sand and silt is more pronounced than that of the clay fraction. During 12 yr of cropping, 83 and 93% of the initial forest C decomposed in the coarse sand and fine sand 2 (63-200pm) fractions, respectively, while in the clay only 40% of the forest-derived C decomposed during the initial 12 yr of cropping (Table 3). Forest C in the silt and the fine sand 1 (20-63 pm) declined by 63 and 64% during the same period. The following 38-yr cropping period resulted in a much smaller decline in forest derived C, i.e. about 10% in the clay and silt, and I- 7% in the sand fractions. The turnover time of the forest derived C, which decomposed in the various size fractions during 50 yr of cropping may be estimated assuming that the C stems from one labile fraction exclusively and that the kinetics of degradation are determined by the C remaining after 12 yr of cropping. The calculated turnover times of about 59, 6 and 4yr in the clay, silt and sand-size fractions, respectively. compares
Table 3. Total C derived from forest and remaining in soil cultivated with sugar cane for 12 (K-12) or 50 yr (SC-SO). Samples were taken O-12 cm depth in soil under natural forest vegetation (FS) and from 0 to 10 cm in soils cultivated with sugar cane after forest clearnina. Mean values calculated from data in Table 2 Fine sand I 20-63 urn
Fine sand 2 63-200 urn
Coarse sand 200-2000 urn
Total C deriuedfrom foresr, mg gm’,fraction dry WI FS, O-l 2 cm 32.0 71.3 SC-12,0-1Ocm 19.5 26.5 SC-50, O-10 cm 16.3 19.7
18.7 6.7 5.9
15.3 1.1 0.9
12.8 2.2 1.3
Proportion of forestderived C remaining, % FS, O-12 cm 100 Z-12, 0-1Ocm 61 SC-50. 0- 10 cm 51
100 36 32
100 7 6
100 17 10
Clay <2um
Silt 2-20 urn
100 31 28
Short communications fairly well to the turnover of the active (l-5 yr) and slow fraction (20-40yr) as indicated in the Century Model (Patton et al., 1987). The remaining forest C in the particle size fractions after 50yr of cropping may constitute the passive fraction. Although size fractionation yields organomineral separates which differ in turnover times, individual size fractions clearly encompass pools of organic C which are differentially susceptible to biological decomposition. Organic matter associated with a particular size fraction cannot be considered a uniform entity. This study shows that individual size fractions contained at least two organic matter pools with widely differing turnover times. In addition, the various size fractions seemed to contain stable organic C with approximately the same turnover, while having differing turnovers of the labile component. The silt fraction OM was more labile than clay associated OM, which is generally not the case in temnerate soils (Christensen. 1987a). Thus, it is important to take into consideration the soil mineralogy if estimates of SOM fractions in modelling among other things are based on size fractions. Classification of SOM in terms of functionally relevant pools is crucial for current models simulating agroecosystems and grasslands (Parton et al., 1989). Successful development of techniques for direct measurement of pool sizes would represent a major step towards appropriate verification of models and the revision of inherent concepts. The ‘C isotope provides a valuable contribution to such classification and model validation in the relatively few long-term field experiments available. Application of physical fractionation may further contribute to the validation of model concepts, in particular if combined with measurements of soil microbial biomass, mineralization studies and isotope techniques (Christensen, 1987b; Paustian and Bonde, 1987). Standard procedures for physical fractionation of soils are, however, much needed, and current methods should be carefully evaluated before being adopted for a wide range of soils (Christensen, 1992). Acknowledgements-The present investigation was financially supported by a grant to Professor Thomas Rosswall from the Swedish Agency for Research Cooperation with Developing Countries (SAREC) and was part of the IAEA sponsored Brazilian research programme “Amazonia Un”. We are grateful to Reynaldo L. Victoria for the isotopic analysis. REFERENCES Anderson D. W., Saggar S., Bettany J. R. and Stewart J. W. B. (1981) Particle size fractions and their use in studies of soil organic matter: I. The nature and distribution of forms of carbon, nitrogen, and sulphur. Soil Science Society of America Journal 45, 767-772.
277
Balesdent J., Mariotti A. and Guillet B. (1987) Natural 13C abundance as a tracer for studies of soil organic matter dynamics. Soil Biology & Biochemistry 19, 25-30. Balesdent J., Wagner G. H. and Mariotti A. (1988) Soil organic matter turnover in long-term field experiments as revealed by carbon-13 natural abundance. Soil Science Society of America Journal 52, 118-124. Catroux G. and Schnitzer M. (1987) Chemical, spectroscopic, and biological characteristics of the organic matter in particle size fractions separated from an Aquoll. Soil Science Society of America Journal 51, 1200-1207. Cerri C. C., Balesdent J., Feller C., Victoria R. and Plenecassagne A. (1985) Application du tracage isotropique nature1 en “C a l’etude de la dynamique de la matidre organique dans les soIs. Comtes Rendus de I’dcademie des Sciences de Paris. T. 300, Serie II 9, 423-428. Christensen B. T. (1985) Carbon and nitrogen in particle size fractions isolated from Danish arable soils by ultrasonic dispersion and gravity sedimentation. Acta Agriculturae Scandinavica 35, 175-187. Christensen B. T. (1987a) Decomposability of organic matter in particle size fractions from field soils with straw incorporation. Soil Biology & Biochemistry 19, 429-435. Christensen B. T. (1987b) Use of particle size fractions in soil organic matter studies. INTECOL BuNetin 15, 113-123. Christensen B. T. (1992) Physical fractionation of soil and organic matter in primary particle size and density separates. Advances in Soil Science. In press. Parton W. J.. Schimel D. S.. Cole C. V. and Oiima D. S. (1987) Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Science Society of America Journal 51, 1173-I 179. Parton W. J., Sanford R. L., Sanchez P. A., Stewart J. W. B., Bonde T. A., Crossley D., van Veen H. and Yost R. (1989) Modelling soil organic matter in tropical soils. In Dynamics of Soil Organic Matter in Tropical Ecosystems (D. C. Coleman, J. M. Oades and G. Uehara, Eds), pp. 153-171. University of Hawaii Press, Honolulu, Hawaii. Paustian K. and Bonde T. A. (I 987) Interpreting incubation data on nitrogen mineralization. INTECOL Bulletin 15, 101-l 12. Tiessen H. and Stewart J. W. B. (1983) Particle-size fractions and their use in studies of soil organic matter: II. Cultivation effects on organic matter composition in size fractions. Soil Science Society of America Journal 41, 509-514. Vitorello V. A., Cerri C. C., Andreux F., Feller C. and Victoria R. L. (1989) Organic matter and natural carbon13 distribution in forested and cultivated oxisols. Soil Science Society of America Journal 53, 773-778.