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Black carbon—possible source of highly aromatic components of soil humic acids
Org. Geochem. Vol.23, No. 3, pp. 191-196, 1995
Pergamon
0146-6380(95)00003-8
Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0146-6380/95$9.50+ 0.00
Black carbon--possible source of highly aromatic components of soil humic acids LUDWIG H A U M A I E R and W O L F G A N G ZECH Lehrstuhl ffir Bodenkunde und Bodengeographie, Universit/it Bayreuth, D-95440 Bayreuth, Germany (Received 24 June 1994; returned for revision 26 July 1994; accepted 16 January 1995)
Abstract--Structural features and chemical composition of highly aromatic soil humic acids strongly suggest that these humic acids are derived from black carbon (charred plant residues, soot) and not from native plant materials. Humic acids from laboratory-oxidized black carbon show remarkable similarities to highly aromatic soil humic acids in their spectroscopic properties and chemical composition. Thus, black carbon is considered to be a possible source of the chemically most stable, aromatic soil carbon pool. Key words--black carbon, humic acids, aromaticity of humics, soil organic matter stability
INTRODUCTION
Soil humic substances commonly are considered to be formed during the decay of plant and microbial residues, either by selective preservation and transformation of constituents resistant to biodegradation, e.g. lignin (lignin theory) and aliphatic polymers, or by condensation of low-molecular-weight degradation products such as phenols, phenolic acids, sugars, and amino acids (polyphenol theory, sugar-amine condensation) (Stevenson, 1982; Hayes, 1991). The chemical structural models for the aromatic components of humic acids (HAs) have been proposed to consist of aromatic structures with a high degree of hydroxyl and carboxyl substitution (e.g. Flaig et al., 1975; Stevenson, 1982). The use of ~3C NMR spectroscopy in the study of humic substances has revealed that there is great variability in aromaticity of soil HAs (Hatcher et al., 1981). Although there have been some suggestions that phenols are in low concentration in some humic materials (Hatcher et al., 1989), it is clear (Preston and Schnitzer, 1987) that groups of relatively low acidity are present in humic materials and solution NMR is not definitive in excluding them. The source of phenolic components of humic materials is clear. Lignin, the most abundant aromatic biomacromolecule, as well as other aromatic plant constituents, such as tannins, are characterized by a high degree of oxygen substitution of aromatic rings, and there are no known reaction pathways which would lead to extensive removal of oxygen substituents from aromatic rings under the oxidizing conditions prevailing in most soils. Moreover, hydroxylation of aromatic rings is one of the primary steps in the
microbial degradation of aromatic compounds under aerobic conditions (Tschech, 1989). Condensation reactions, according to the polyphenol theory, should yield products containing appreciable amounts of phenolic carbons. Solid state NMR and other studies (Hatcher et aL, 1989; Tate et aL, 1990) show that the presence of phenols in some humic substances is low It might be asked, where do these materials come from? Although one possibility is that they are the final degradation products of non-phenolic components of lignin, there are other alternatives such as from carbonized materials produced during vegetation fires. Biomass burning leads to a significant production of charred plant materials (Seller and Crutzen, 1980), and heating soil organic matter results in a considerable increase in aromaticity of the remains, at the expense of carboxyl groups and aliphatic structures (Almendros et aL, 1992). Oxidation of these materials would produce highly aromatic carboxylated humic materials. The possible significance of carbonized materials, such as charcoal and cinder, for the formation of humic materials has been addressed by Kumada (1983). Shindo et aL (1986a, b, c) compared optical properties, IR spectra, and X-ray diffraction patterns of HAs from volcanic ash soils and from charred plant residues. They concluded, from the observed similarities, that charring processes could be one possible mechanism for the formation of HAs in volcanic ash soils in Japan. In this work we present spectroscopic and elemental compositional data which address the possible contribution of carbon black/charcoal/char material to humic materials, which are formed by subsequent oxidation of carbonized mat~ials. 191
Ludwig Haumaier and W o l f g a n g Zech
192
Table I. Chemical characteristics of soil humic acids Per cent on moisture and ash free basis Sample origin Soil depth
Atomic ratios
C
H
N
O*
H/C
O/C
59.9
3.8
2.4
33.9
0.76
0.42
55.0 58.1 59.9 59.7
3.6 3.2 2.6 3.2
5.0 4.9 4.7 4.1
36.4 33.8 32.8 33.0
0.78 0.66 0.52 0.64
0.50 0.44 0.41 0.41
59.9 60.3 59.8 60.0 60.4 60.5
3.6 3.5 3.7 3.2 3. I 3.0
3.4 3.3 3.4 3.4 3.3 3.7
33.1 32.9 33. I 33.4 33.3 32.9
0.72 0.69 0.74 0.64 0.61 0.59
0.41 0.41 0.42 0.42 0.41 0.41
57.1 60.4 60.7 60.6
4.2 2.9 2.3 2.9
3.2 2.7 2.4 2.7
35.5 34.0 34.5 33.9
0.88 0.57 0.45 0.57
0.47 0.42 0.43 0.42
Stagno-Dystric Gleysol, German)' 0-5 cm
Umbric Leptosol, Spain I)-20 cm 2 0 4 0 cm 41~70 cm 70-10(; cm
Chernozem, German) ~ 5 cm 10 15cm 20-25 cm 30 35 cm 40--45 cm 60-70 cm
Umbric Andosol, Japan 0-5 cm 5 34 cm 34-70 cm 70- 90 cm
*Calculated by difference.
EXPERIMENTAL
Details of the soils used which originate from Spain, Germany and Japan are listed in Table 1. Humic acids were extracted with a mixture of 0.1 M NaOH and 0.1 M Na4P207 and purified by repeated treatment with dilute HCI-HF solution (Schnitzer, 1982). Elemental compositional data for the humic material is also given in Table 1. Some carbonaceous materials were also prepared for study. Grass char was obtained by lighting withered grass on a 30 × 30 cm soil block. The black fibers of char left behind by the smooth fire were collected and ground. Soot was scraped from the inner walls of a Jotul stove in which predominantly spruce wood had been burnt. Finely ground barley straw was heated under a stream of N 2 in a muffle furnace at 300°C for 2 h. These samples were subjected to oxidation which was achieved by refluxing with 25% (w/w) HNO3 for 4 h (charred grass, soot), by treatment with 6.5% (w/w) H202 at room temperature for 96 h (soot), or by a similar treatment with H202 made 0.5 M in NaOH (charred barley straw). For the preparation of HAs, oxidized black carbon samples were isolated by centrifugation, washed with water, and extracted with
0.5 M NaOH. HAs were precipitated from the extracts by acidification to pH 1 with 10M HCI, purified by reprecipitation from alkaline solution, washed with 0.01 M HCI, and freeze-dried. Percentages of carbon converted to HAs were 77% (charred grass/HNO3), 23% (soot/HNO3), 19% (soot/H202), and 48% (charred barley straw/H202/NaOH), respectively. Chemical compositions of the materials are given in Table 2. Solution ~3C N M R spectra were obtained on a BRUKER AM 500 spectrometer. Conditions for obtaining spectra were as follows. Spectrometer frequency was 125 MHz; inverse-gated decoupling. Acquisition time was 0.33 s, delay time was 1.67 s and the line-broadening factor was 100 Hz. To obtain spectra, 150 mg of the sample was dissolved in 3 ml of 0.5 M NaOD solution. Elemental analyses were performed by Mikroanalytisches Laboratorium Ilse Beetz, Kronach, Germany.
RESULTS AND DISCUSSION
Solution 13C NMR spectra of HAs, from the various soils examined, are shown in Fig. 1. Similar spectra have been published, e.g. by Calderoni and
Table 2. Chemical characteristics of black carbon samples and humic acids obtained by chemical oxidation Per cent on moisture free basis Sample (oxidizing agent)
Atomic ratios
C
H
N
ash
O*
H/C
O/C
Charred grass Soot Charred barley straw
18.0 65.1 68.3
0.9 3.2 4.2
0.9 3.4 0.9
75.9 5.1 10.3
4.3 23.2 16.3
0.60 0.59 0.73
0.18 0.27 0.18
Charred grass HAs ( H N O s ) Soot HAs ( H N O s ) Soot HAs ( H 2 0 2 ) Charred barley straw HAs (H202/NaOH)
56.8 53.8 64.9 64.4
1.5 1.9 3.6 4.4
4.2 3.7 4.0 0.6
3.9 0.6 0.7 1.1
33.6 40.0 26.8 29.5
0.31 0.42 0.66 0.81
0.44 0.56 0.31 0.34
*Calculated by difference.
Black carbon--source of soil humic acids
LJ c) J
d)
e)
f) g)
__f I 250
I 200
1 150
I 1O0 ppm
I 50
I 0
I - 50
Fig. 1. Solution J3C NMR spectra of humic acids from (a) Stagno-Dystric Gleysol (0-5 cm), (b) Umbric Leptosol (0-20 cm), (c) Umbric Leptosol (40-70 cm), (d) Chernozem (0-5 cm), (e) Chernozem (60-70 cm), (f) Umbric Andosol (0-5 cm), (g) Umbric Andosol (5-34 cm). Schnitzer (1984), Schnitzer and Preston (1986), and Arshad et al. (1988). The typical feature is the high abundance of peaks due to carboxyl groups (175ppm) and non-oxygenated aromatic carbon (around 130 ppm). Minor resonances in other regions of the spectra diminish as depth in the soil profile increases [cf. Fig. l(b/c), (d/e), if/g)], i.e. when the
193
influence of young organic matter (fresh aboveground or root litter) decreases. This indicates that carbon species, other than carboxyl and nonoxygenated aromatic carbon, comprise the younger carbon fraction. Increasing age of soil organic matter with increasing soil depth, impressively, has been demonstrated, e.g. by Scharpenseel (1993). Carbonsubstituted aromatic rings bearing carboxyl groups are obviously the structural units most resistant to mineralization. Figure 2 shows J3C NMR spectra of HAs obtained from the various types of black carbon prepared from grass, soot, and barley straw and oxidized artificially in the laboratory. Here again, the typical feature is the low abundance, or absence, of resonances other than those of carboxyl and non-oxygenated aromatic carbon. It is clear that these spectra could be those of components of the highly aromatic humic materials with NMR spectra shown in Fig. 1. Absence of aliphatic resonances (0-100ppm) and higher abundance of carboxyl peaks in spectra of the nitric acid-oxidized samples indicate that the nitric acidoxidation procedure was more effective in oxidizing the black carbon samples, notably their aliphatic constituents, than were the hydrogen peroxide treatments. The relatively weak carboxyl peaks in the spectra of H202-oxidized samples also indicate a lower degree of oxidation compared to soil HAs. Nonetheless, even this low degree of oxidation was sufficient to produce alkali-soluble HAs. Distinct signals of phenolic (150 ppm) and methoxyl carbon (56 ppm) in the spectrum of H 2O2-oxidized soot point to the presence of lignin-like moieties in this type of soot. Generally, the N M R spectra corroborate the findings of Kumada (1983) and Shindo et al. (1986a, b,c) who found similarities between HAs obtained from charred residues and soil HAs by means of other spectroscopic methods. Valuable information on the sources of humic substances can be obtained from their elemental composition as presented in a van Krevelen plot of atomic H/C vs atomic O/C (Hedges, 1990). Figure 3 shows such data for the soil HAs used in this study, for some plant constituents, and for black carbon from various sources. The principal reactions responsible for compositional changes, dehydration/ hydration and oxidation, are represented by straight lines. The composition of pure hexane soot lies closest to that of graphitic carbon. Dehydration is the main process in the carbonization of plant materials. One can easily recognize that the direction of compositional changes, necessary to transform the chars to HAs, is oxidation for the less carbonized materials, whereas oxidation plus hydration are needed for pure soot or the more strongly carbonized chars. Both types of reaction can readily be expected to occur in soils. Native plant materials such as carbohydrates or lignin have to pass through a dehydration step in either case to get the elemental composition of highly aromatic HAs in an oxidative process. Whereas this
194
Ludwig Haumaier and Wolfgang Zech ation of the black carbon samples to HAs are oxidation and hydration as suggested for the formation of soil HAs. Some of the confusion concerning differences in the composition and stability of soil organic matter unravels, when black carbon is recognized as the source of the highly aromatic soil materials, i.e. as the precursor of the chemically most stable fraction of soil organic matter. Presumably, most soils contain humification products of both native plant materials, on the one hand, and charred residues on the other hand. At a given level of charred residue reserves, aromaticity of HAs depends mainly on the position of equilibrium between input and mineralization of plant residues. Aromaticity may be high as a result of
process may occur microbiologically there is no reason to exclude it as a possible pathway during carbonization. Figure 4 shows corresponding data for the black carbon samples used in this study and HAs prepared therefrom. Soot from domestic burning is much less carbonized than pure hexane soot. Understandably, compositions of charred grass and charred barley straw are similar to those of charred cellulose samples. The lower H/C ratios for the nitric acidoxidized products can be attributed to the selective removal of hydrogen-rich aliphatic constituents in this rather drastic procedure. This agrees well with the N M R data. In the hydrogen peroxide treatments, however, the processes responsible for transform-
(b)
OCH 3
(c)
(d)
I
I
I
I
I
!
I
250
200
150
100
50
0
-50
ppm
Fig. 2. Solution 13C NMR spectra of humic acids from laboratory-oxidized black carbon, (a) charred grass/HNO 3, (b) soot/HNO 3, (c) soot/H202, (d) charred barley straw/H202/NaOH.
Black carbon--source of soil humic acids 2.0
I
I
I
1.6
et al., 1989) can be due to the long periods of burial
I cellulose
•
3250//1 Miscanth us • ~ o ¢'
1.2 HIC
lignin •
~ ' ~
lh
? 7
• 350o•. -=
0.8
4h',,~°o\
~%
450% 8h [ 500o• 12h= ~ 0.4
HAsl i /"
,~ o.---
vv'~ ~xidation
0.0 0.0
I 0.2
I 0.4
I 0.6
I 0.8
1.0
O/C Fig. 3. Atomic H/C vs atomic O/C diagram for soil HAs used in this study, plant materials, and various types of black carbon. ©, soil HAs; O, lignin (van Krevelen, 1950); A, cellulose and cellulose chars obtained by heating for 5min at several temperatures (Sekiguchi et al., 1983); i , Miscanthus sinensis and chars obtained at 250°C for 1 h, 4 h, 8 h, 12 h (Shindo, 1991); V, particle-size fractions of chars from grassland fire (Shindo, 1991); 41,, hexane soot (Akhter et al., 1985).
relative enrichment of charring products, due to low input or rapid mineralization of native plant materials, or low because of high input and slow mineralization of plant debris. For example, highly aromatic H A s in mineral horizons of podzols, low in microbial activity, may be due to the retention of plant debris in organic surface horizons, i.e. due to low input of fresh materials to the mineral soil. Their occurrence in chernozems, on the contrary, may be the result of rapid mineralization of litter input due to high microbial activity and cultivation. The exceptionally high aromaticity of HAs obtained from buried soils (Calderoni and Schnitzer, 1984; Hatcher 2.C
~O~o~"
B
o B/H202 o S/H202
G• S* 0.4
G/HNO3 o S/HNO3 O
oxidation =
0.0 0.0
= 0.2
i 0.4
= 0.6
t 0.8
without input of fresh organic matter. Gradual mineralization of all components, less resistant to decomposition, leaves behind the more refractory oxidation products of black carbon. High contents and stability of organic matter in some anthropogenic soils (e.g. Zech et al., 1990; Blackmore et al., 1990) can be attributed to the utilization of combustion residues and even soot (Wainwright and Kiliham, 1982) as fertilizers. Furthermore, it is easily explained why long-term field trials yield only a temporary rise in soil organic matter content, even with high inputs of organic manure. In contrast to black carbon, organic manures do not possess the resistance to mineralization, and they obviously do not acquire it during decomposition. In summary we believe there is no a priori evidence to suggest that black carbon may not be a precursor to aromatic substances in some soils. The extent of this contribution is unknown, but predictably it should occur in areas with a high incidence of fires and subsequent high rates of oxidation. One possible way of testing the importance of this theory might be by measuring the aromatic ring cluster size of aromatic natural humic acids and carbonized and oxidized materials. Carbonized and then oxidized materials should have large cluster sizes. The cluster size of highly aromatic humic acids is unknown but could be measured by chemical shielding anisotropy measurements (Wilson, 1987). This would be an excellent test of the contribution of carbonized and then oxidized materials to soil humic acids. Associate E d i t o r - - A . G. Douglas Acknowledgements--Dr S. Arai (National Institute of Agro-
Environmental Sciences, Tsukuba, Japan), Dr M. Frfihauf (Martin-Luther-Universit~it Halle-Wittenberg, Germany), and Dr F. Gil-Sotres (Universidad de Santiago, Santiago de Compostela, Spain) are gratefully acknowledged for providing soil or humic acid samples. Mr R. Blasek is thanked for the preparation of humic acid samples, Mr T. Engelbrecht for the preparation of the figures. We are very much obliged and indebted to Dr M. Wilson (CSIRO, North Ryde, Australia) for his efforts in reviewing the manuscript. REFERENCES
12
H/C 0.8
195
1.0
O/C
Fig. 4. Atomic H/C vs atomic O/C diagram for black carbon samples used in this study and HAs derived therefrom. O, black carbon samples (B, charred barley straw; G, charred grass; S, soot); C), HAs.
Akhter M. S., Chughtai A. R. and Smith D. M. (1985) The structure of hexane soot I: spectroscopic studies. Appl. Spectrosc. 39, 143-153. Almendros G., Gonzfilez-Vila F. J., Martin F., FrOnd R. and Liidemann H.-D. (1992) Solid state NMR studies of fire-induced changes in the structure of humic substances. Sci. Total Environ. 117/118, 63-74. Arshad M. A., Schnitzer M. and Preston C. M. (1988) Characterization of humic acids from termite mounds and surrounding soils, Kenya. Geoderma 42, 213-225. Blackmore A. C., Mentis M. T. and Scholes R. J. (1990) The origin and extent of nutrient-enriched patches within a nutrient-poor savanna in South Africa. J. Biogeogr. 17, 463-470. Calderoni G. and Schnitzer M. (1984) Effects of age on the chemical structure of paleosol humic acids and fulvic acids. Geochim. Cosmochim. Acta 48, 2045-2051.
196
Ludwig Haumaier and Wolfgang Zech
Flaig W., Beutelspacher H. and Rietz E. (1975) Chemical composition and physical properties of humic substances. In Soil Components, Vol. 1: Organic Components (Edited by Gieseking J. E.), pp. 1 211. Springer, Berlin. Hatcher P. G., Schnitzer M., Dennis L. W. and Maciel G. E. (1981) Aromaticity of humic substances in soils. Soil Sci. Soc. Am. J. 45, 1089-1094. Hatcher P. G., Schnitzer M., Vassallo A. M. and Wilson M. A. (1989) The chemical structure of highly aromatic humic acids in three volcanic ash soils as determined by dipolar dephasing NMR studies. Geochim. Cosmochim. Acta 53, 125-130. Hayes M. H. B. (1991) Concepts of the origins, composition, and structures of humic substances. In Advances in Soil Organic Matter Research: The Impact on Agriculture and the Environment (Edited by Wilson W. S.), pp. 3-22. The Royal Society of Chemistry, Cambridge. Hedges J. I. (1990) Compositional indicators of organic acid sources and reactions in natural environments. In Organic Acids in Aquatic Ecosystems (Edited by Perdue E. M. and Gjessing E. T.), pp. 43~53. Wiley, Chichester. van Krevelen D. W. (1950) Graphical statistical method for the study of structure and reaction processes of coal. Fuel 29, 269-284. Kumada K. (1983) Carbonaceous materials as a possible source of soil humus. Soil Sci. Plant Nutr. 29, 383-386. Preston C. M. and Schnitzer M. (1987) L3C NMR of humic substances: pH and solvent effects. J. Soil Sci. 38, 667~78. Scharpenseel H. W. (1993) Major carbon reservoirs of the pedosphere; source sink relations; potential of D~4C and 613C as supporting methodologies. Water Air Soil Pollut. 70, 43 IM42. Schnitzer M. (1982) Organic matter characterization. In Methods o f Soil Analysis, Part 2. Chemical and Microbiological Properties (Edited by Page A. L., Miller R. H. and Keeney D. R.), 2nd Ed., Chapter 30, pp. 581-594. American Society of Agronomy, Soil Science Society of America, Madison, WI. Schnitzer M. and Preston C. M. (1986) Analysis of humic acids by solution and solid-state carbon-13 nuclear magnetic resonance. Soil Sci. Soc. Am. J. 50, 326 331. Seiler W. and Crutzen P. J. (1980) Estimates of gross and
net fluxes of carbon between the biosphere and the atmosphere from biomass burning. Climatic Change 2, 207 247. Sekiguchi Y., Frye J. S. and Shafizadeh F. 0983) Structure and formation of cellulosic chars. J. Appl. Polym. Sci. 28, 3513 3525. Shindo H. (1991) Elementary composition, humus composition, and decomposition in soil of charred grassland plants. Soil Sci. Plant Nutr. 37, 651~557. Shindo H., Matsui Y. and Higashi T. (1986a) Humus composition of charred plant residues. Soil Sci. Plant Nutr. 32, 475~478. Shindo H., Higashi T. and Matsui Y. (1986b) Comparison of humic acids from charred residues of Susuki (Eulalia, Miscanthus sinensis A.) and from the A horizons of volcanic ash soils. Soil Sci. Plant Nutr. 32, 579-586. Shindo H., Matsui Y. and Higashi T. (1986c) A possible source of humic acids in volcanic ash soils in Japan-charred residue of Miscanthus sinensis. Soil Sci. 141, 84 87. Stevenson F. J. (1982) Humus Chemistry. Wiley, New York. Tate K. R., Yamamoto K., Churchman G. J., Meinhold R. and Newman R. H. (1990) Relationships between the type and carbon chemistry of humic acids from some New Zealand and Japanese soils. Soil Sci. Plant Nutr. 36, 611~21. Tschech A. (1989) Der anaerobe Abbau yon aromatischen Verbindungen. Forum Mikrobiol. 12, 251-264. Wainwright M. and Killham K. (1982) Microbial transformations of some particulate pollution deposits in soils A source of plant-available nitrogen and sulphur. Plant Soil 65, 297-301. Wilson M. A. (1987) N M R Techniques and Applications in Geochemistry and Soil Chemistry. pp. 259-262. Pergamon Press, Oxford. Zech W., Haumaier L. and Hempfling R. (1990) Ecological aspects of soil organic matter in tropical land use. In Humic Substances in Soil and Crop Sciences: Selected Readings (Edited by MacCarthy P., Clapp C. E., Malcolm R. L. and Bloom P. R.), pp. 187-202. American Society of Agronomy, Soil Science Society of America, Madison, WI.
Pergamon
0146-6380(95)00003-8
Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0146-6380/95$9.50+ 0.00
Black carbon--possible source of highly aromatic components of soil humic acids LUDWIG H A U M A I E R and W O L F G A N G ZECH Lehrstuhl ffir Bodenkunde und Bodengeographie, Universit/it Bayreuth, D-95440 Bayreuth, Germany (Received 24 June 1994; returned for revision 26 July 1994; accepted 16 January 1995)
Abstract--Structural features and chemical composition of highly aromatic soil humic acids strongly suggest that these humic acids are derived from black carbon (charred plant residues, soot) and not from native plant materials. Humic acids from laboratory-oxidized black carbon show remarkable similarities to highly aromatic soil humic acids in their spectroscopic properties and chemical composition. Thus, black carbon is considered to be a possible source of the chemically most stable, aromatic soil carbon pool. Key words--black carbon, humic acids, aromaticity of humics, soil organic matter stability
INTRODUCTION
Soil humic substances commonly are considered to be formed during the decay of plant and microbial residues, either by selective preservation and transformation of constituents resistant to biodegradation, e.g. lignin (lignin theory) and aliphatic polymers, or by condensation of low-molecular-weight degradation products such as phenols, phenolic acids, sugars, and amino acids (polyphenol theory, sugar-amine condensation) (Stevenson, 1982; Hayes, 1991). The chemical structural models for the aromatic components of humic acids (HAs) have been proposed to consist of aromatic structures with a high degree of hydroxyl and carboxyl substitution (e.g. Flaig et al., 1975; Stevenson, 1982). The use of ~3C NMR spectroscopy in the study of humic substances has revealed that there is great variability in aromaticity of soil HAs (Hatcher et al., 1981). Although there have been some suggestions that phenols are in low concentration in some humic materials (Hatcher et al., 1989), it is clear (Preston and Schnitzer, 1987) that groups of relatively low acidity are present in humic materials and solution NMR is not definitive in excluding them. The source of phenolic components of humic materials is clear. Lignin, the most abundant aromatic biomacromolecule, as well as other aromatic plant constituents, such as tannins, are characterized by a high degree of oxygen substitution of aromatic rings, and there are no known reaction pathways which would lead to extensive removal of oxygen substituents from aromatic rings under the oxidizing conditions prevailing in most soils. Moreover, hydroxylation of aromatic rings is one of the primary steps in the
microbial degradation of aromatic compounds under aerobic conditions (Tschech, 1989). Condensation reactions, according to the polyphenol theory, should yield products containing appreciable amounts of phenolic carbons. Solid state NMR and other studies (Hatcher et aL, 1989; Tate et aL, 1990) show that the presence of phenols in some humic substances is low It might be asked, where do these materials come from? Although one possibility is that they are the final degradation products of non-phenolic components of lignin, there are other alternatives such as from carbonized materials produced during vegetation fires. Biomass burning leads to a significant production of charred plant materials (Seller and Crutzen, 1980), and heating soil organic matter results in a considerable increase in aromaticity of the remains, at the expense of carboxyl groups and aliphatic structures (Almendros et aL, 1992). Oxidation of these materials would produce highly aromatic carboxylated humic materials. The possible significance of carbonized materials, such as charcoal and cinder, for the formation of humic materials has been addressed by Kumada (1983). Shindo et aL (1986a, b, c) compared optical properties, IR spectra, and X-ray diffraction patterns of HAs from volcanic ash soils and from charred plant residues. They concluded, from the observed similarities, that charring processes could be one possible mechanism for the formation of HAs in volcanic ash soils in Japan. In this work we present spectroscopic and elemental compositional data which address the possible contribution of carbon black/charcoal/char material to humic materials, which are formed by subsequent oxidation of carbonized mat~ials. 191
Ludwig Haumaier and W o l f g a n g Zech
192
Table I. Chemical characteristics of soil humic acids Per cent on moisture and ash free basis Sample origin Soil depth
Atomic ratios
C
H
N
O*
H/C
O/C
59.9
3.8
2.4
33.9
0.76
0.42
55.0 58.1 59.9 59.7
3.6 3.2 2.6 3.2
5.0 4.9 4.7 4.1
36.4 33.8 32.8 33.0
0.78 0.66 0.52 0.64
0.50 0.44 0.41 0.41
59.9 60.3 59.8 60.0 60.4 60.5
3.6 3.5 3.7 3.2 3. I 3.0
3.4 3.3 3.4 3.4 3.3 3.7
33.1 32.9 33. I 33.4 33.3 32.9
0.72 0.69 0.74 0.64 0.61 0.59
0.41 0.41 0.42 0.42 0.41 0.41
57.1 60.4 60.7 60.6
4.2 2.9 2.3 2.9
3.2 2.7 2.4 2.7
35.5 34.0 34.5 33.9
0.88 0.57 0.45 0.57
0.47 0.42 0.43 0.42
Stagno-Dystric Gleysol, German)' 0-5 cm
Umbric Leptosol, Spain I)-20 cm 2 0 4 0 cm 41~70 cm 70-10(; cm
Chernozem, German) ~ 5 cm 10 15cm 20-25 cm 30 35 cm 40--45 cm 60-70 cm
Umbric Andosol, Japan 0-5 cm 5 34 cm 34-70 cm 70- 90 cm
*Calculated by difference.
EXPERIMENTAL
Details of the soils used which originate from Spain, Germany and Japan are listed in Table 1. Humic acids were extracted with a mixture of 0.1 M NaOH and 0.1 M Na4P207 and purified by repeated treatment with dilute HCI-HF solution (Schnitzer, 1982). Elemental compositional data for the humic material is also given in Table 1. Some carbonaceous materials were also prepared for study. Grass char was obtained by lighting withered grass on a 30 × 30 cm soil block. The black fibers of char left behind by the smooth fire were collected and ground. Soot was scraped from the inner walls of a Jotul stove in which predominantly spruce wood had been burnt. Finely ground barley straw was heated under a stream of N 2 in a muffle furnace at 300°C for 2 h. These samples were subjected to oxidation which was achieved by refluxing with 25% (w/w) HNO3 for 4 h (charred grass, soot), by treatment with 6.5% (w/w) H202 at room temperature for 96 h (soot), or by a similar treatment with H202 made 0.5 M in NaOH (charred barley straw). For the preparation of HAs, oxidized black carbon samples were isolated by centrifugation, washed with water, and extracted with
0.5 M NaOH. HAs were precipitated from the extracts by acidification to pH 1 with 10M HCI, purified by reprecipitation from alkaline solution, washed with 0.01 M HCI, and freeze-dried. Percentages of carbon converted to HAs were 77% (charred grass/HNO3), 23% (soot/HNO3), 19% (soot/H202), and 48% (charred barley straw/H202/NaOH), respectively. Chemical compositions of the materials are given in Table 2. Solution ~3C N M R spectra were obtained on a BRUKER AM 500 spectrometer. Conditions for obtaining spectra were as follows. Spectrometer frequency was 125 MHz; inverse-gated decoupling. Acquisition time was 0.33 s, delay time was 1.67 s and the line-broadening factor was 100 Hz. To obtain spectra, 150 mg of the sample was dissolved in 3 ml of 0.5 M NaOD solution. Elemental analyses were performed by Mikroanalytisches Laboratorium Ilse Beetz, Kronach, Germany.
RESULTS AND DISCUSSION
Solution 13C NMR spectra of HAs, from the various soils examined, are shown in Fig. 1. Similar spectra have been published, e.g. by Calderoni and
Table 2. Chemical characteristics of black carbon samples and humic acids obtained by chemical oxidation Per cent on moisture free basis Sample (oxidizing agent)
Atomic ratios
C
H
N
ash
O*
H/C
O/C
Charred grass Soot Charred barley straw
18.0 65.1 68.3
0.9 3.2 4.2
0.9 3.4 0.9
75.9 5.1 10.3
4.3 23.2 16.3
0.60 0.59 0.73
0.18 0.27 0.18
Charred grass HAs ( H N O s ) Soot HAs ( H N O s ) Soot HAs ( H 2 0 2 ) Charred barley straw HAs (H202/NaOH)
56.8 53.8 64.9 64.4
1.5 1.9 3.6 4.4
4.2 3.7 4.0 0.6
3.9 0.6 0.7 1.1
33.6 40.0 26.8 29.5
0.31 0.42 0.66 0.81
0.44 0.56 0.31 0.34
*Calculated by difference.
Black carbon--source of soil humic acids
LJ c) J
d)
e)
f) g)
__f I 250
I 200
1 150
I 1O0 ppm
I 50
I 0
I - 50
Fig. 1. Solution J3C NMR spectra of humic acids from (a) Stagno-Dystric Gleysol (0-5 cm), (b) Umbric Leptosol (0-20 cm), (c) Umbric Leptosol (40-70 cm), (d) Chernozem (0-5 cm), (e) Chernozem (60-70 cm), (f) Umbric Andosol (0-5 cm), (g) Umbric Andosol (5-34 cm). Schnitzer (1984), Schnitzer and Preston (1986), and Arshad et al. (1988). The typical feature is the high abundance of peaks due to carboxyl groups (175ppm) and non-oxygenated aromatic carbon (around 130 ppm). Minor resonances in other regions of the spectra diminish as depth in the soil profile increases [cf. Fig. l(b/c), (d/e), if/g)], i.e. when the
193
influence of young organic matter (fresh aboveground or root litter) decreases. This indicates that carbon species, other than carboxyl and nonoxygenated aromatic carbon, comprise the younger carbon fraction. Increasing age of soil organic matter with increasing soil depth, impressively, has been demonstrated, e.g. by Scharpenseel (1993). Carbonsubstituted aromatic rings bearing carboxyl groups are obviously the structural units most resistant to mineralization. Figure 2 shows J3C NMR spectra of HAs obtained from the various types of black carbon prepared from grass, soot, and barley straw and oxidized artificially in the laboratory. Here again, the typical feature is the low abundance, or absence, of resonances other than those of carboxyl and non-oxygenated aromatic carbon. It is clear that these spectra could be those of components of the highly aromatic humic materials with NMR spectra shown in Fig. 1. Absence of aliphatic resonances (0-100ppm) and higher abundance of carboxyl peaks in spectra of the nitric acid-oxidized samples indicate that the nitric acidoxidation procedure was more effective in oxidizing the black carbon samples, notably their aliphatic constituents, than were the hydrogen peroxide treatments. The relatively weak carboxyl peaks in the spectra of H202-oxidized samples also indicate a lower degree of oxidation compared to soil HAs. Nonetheless, even this low degree of oxidation was sufficient to produce alkali-soluble HAs. Distinct signals of phenolic (150 ppm) and methoxyl carbon (56 ppm) in the spectrum of H 2O2-oxidized soot point to the presence of lignin-like moieties in this type of soot. Generally, the N M R spectra corroborate the findings of Kumada (1983) and Shindo et al. (1986a, b,c) who found similarities between HAs obtained from charred residues and soil HAs by means of other spectroscopic methods. Valuable information on the sources of humic substances can be obtained from their elemental composition as presented in a van Krevelen plot of atomic H/C vs atomic O/C (Hedges, 1990). Figure 3 shows such data for the soil HAs used in this study, for some plant constituents, and for black carbon from various sources. The principal reactions responsible for compositional changes, dehydration/ hydration and oxidation, are represented by straight lines. The composition of pure hexane soot lies closest to that of graphitic carbon. Dehydration is the main process in the carbonization of plant materials. One can easily recognize that the direction of compositional changes, necessary to transform the chars to HAs, is oxidation for the less carbonized materials, whereas oxidation plus hydration are needed for pure soot or the more strongly carbonized chars. Both types of reaction can readily be expected to occur in soils. Native plant materials such as carbohydrates or lignin have to pass through a dehydration step in either case to get the elemental composition of highly aromatic HAs in an oxidative process. Whereas this
194
Ludwig Haumaier and Wolfgang Zech ation of the black carbon samples to HAs are oxidation and hydration as suggested for the formation of soil HAs. Some of the confusion concerning differences in the composition and stability of soil organic matter unravels, when black carbon is recognized as the source of the highly aromatic soil materials, i.e. as the precursor of the chemically most stable fraction of soil organic matter. Presumably, most soils contain humification products of both native plant materials, on the one hand, and charred residues on the other hand. At a given level of charred residue reserves, aromaticity of HAs depends mainly on the position of equilibrium between input and mineralization of plant residues. Aromaticity may be high as a result of
process may occur microbiologically there is no reason to exclude it as a possible pathway during carbonization. Figure 4 shows corresponding data for the black carbon samples used in this study and HAs prepared therefrom. Soot from domestic burning is much less carbonized than pure hexane soot. Understandably, compositions of charred grass and charred barley straw are similar to those of charred cellulose samples. The lower H/C ratios for the nitric acidoxidized products can be attributed to the selective removal of hydrogen-rich aliphatic constituents in this rather drastic procedure. This agrees well with the N M R data. In the hydrogen peroxide treatments, however, the processes responsible for transform-
(b)
OCH 3
(c)
(d)
I
I
I
I
I
!
I
250
200
150
100
50
0
-50
ppm
Fig. 2. Solution 13C NMR spectra of humic acids from laboratory-oxidized black carbon, (a) charred grass/HNO 3, (b) soot/HNO 3, (c) soot/H202, (d) charred barley straw/H202/NaOH.
Black carbon--source of soil humic acids 2.0
I
I
I
1.6
et al., 1989) can be due to the long periods of burial
I cellulose
•
3250//1 Miscanth us • ~ o ¢'
1.2 HIC
lignin •
~ ' ~
lh
? 7
• 350o•. -=
0.8
4h',,~°o\
~%
450% 8h [ 500o• 12h= ~ 0.4
HAsl i /"
,~ o.---
vv'~ ~xidation
0.0 0.0
I 0.2
I 0.4
I 0.6
I 0.8
1.0
O/C Fig. 3. Atomic H/C vs atomic O/C diagram for soil HAs used in this study, plant materials, and various types of black carbon. ©, soil HAs; O, lignin (van Krevelen, 1950); A, cellulose and cellulose chars obtained by heating for 5min at several temperatures (Sekiguchi et al., 1983); i , Miscanthus sinensis and chars obtained at 250°C for 1 h, 4 h, 8 h, 12 h (Shindo, 1991); V, particle-size fractions of chars from grassland fire (Shindo, 1991); 41,, hexane soot (Akhter et al., 1985).
relative enrichment of charring products, due to low input or rapid mineralization of native plant materials, or low because of high input and slow mineralization of plant debris. For example, highly aromatic H A s in mineral horizons of podzols, low in microbial activity, may be due to the retention of plant debris in organic surface horizons, i.e. due to low input of fresh materials to the mineral soil. Their occurrence in chernozems, on the contrary, may be the result of rapid mineralization of litter input due to high microbial activity and cultivation. The exceptionally high aromaticity of HAs obtained from buried soils (Calderoni and Schnitzer, 1984; Hatcher 2.C
~O~o~"
B
o B/H202 o S/H202
G• S* 0.4
G/HNO3 o S/HNO3 O
oxidation =
0.0 0.0
= 0.2
i 0.4
= 0.6
t 0.8
without input of fresh organic matter. Gradual mineralization of all components, less resistant to decomposition, leaves behind the more refractory oxidation products of black carbon. High contents and stability of organic matter in some anthropogenic soils (e.g. Zech et al., 1990; Blackmore et al., 1990) can be attributed to the utilization of combustion residues and even soot (Wainwright and Kiliham, 1982) as fertilizers. Furthermore, it is easily explained why long-term field trials yield only a temporary rise in soil organic matter content, even with high inputs of organic manure. In contrast to black carbon, organic manures do not possess the resistance to mineralization, and they obviously do not acquire it during decomposition. In summary we believe there is no a priori evidence to suggest that black carbon may not be a precursor to aromatic substances in some soils. The extent of this contribution is unknown, but predictably it should occur in areas with a high incidence of fires and subsequent high rates of oxidation. One possible way of testing the importance of this theory might be by measuring the aromatic ring cluster size of aromatic natural humic acids and carbonized and oxidized materials. Carbonized and then oxidized materials should have large cluster sizes. The cluster size of highly aromatic humic acids is unknown but could be measured by chemical shielding anisotropy measurements (Wilson, 1987). This would be an excellent test of the contribution of carbonized and then oxidized materials to soil humic acids. Associate E d i t o r - - A . G. Douglas Acknowledgements--Dr S. Arai (National Institute of Agro-
Environmental Sciences, Tsukuba, Japan), Dr M. Frfihauf (Martin-Luther-Universit~it Halle-Wittenberg, Germany), and Dr F. Gil-Sotres (Universidad de Santiago, Santiago de Compostela, Spain) are gratefully acknowledged for providing soil or humic acid samples. Mr R. Blasek is thanked for the preparation of humic acid samples, Mr T. Engelbrecht for the preparation of the figures. We are very much obliged and indebted to Dr M. Wilson (CSIRO, North Ryde, Australia) for his efforts in reviewing the manuscript. REFERENCES
12
H/C 0.8
195
1.0
O/C
Fig. 4. Atomic H/C vs atomic O/C diagram for black carbon samples used in this study and HAs derived therefrom. O, black carbon samples (B, charred barley straw; G, charred grass; S, soot); C), HAs.
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