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Charred organic carbon in German chernozemic soils

M . W . I . S C H M I D T a , J . O . S K J E M S T A D b , E . G E H R T c & I . K OÈ G E L - K N A B N E R a

aLehrstuhl fuÈr Bodenkunde, Technische UniversitaÈt MuÈnchen, 85350 Freising-Weihenstephan, Germany, bCSIRO, Land and Water,

Glen Osmond, SA 5064, Australia, and cNiedersaÈchsisches Landesamt fuÈr Bodenforschung, 30655 Hannover, Germany

Summary

Burning vegetation produces partly charred plant material which subsequently could contribute to the

highly refractory proportion of soil organic matter. The presence of charred organic carbon (COC) was

investigated in 17 horizons originating from nine soils from Germany and the Netherlands using a suite of

complementary methods (high-energy ultraviolet photo-oxidation, scanning electron microscopy, solid-

state 13C nuclear magnetic resonance, lignin analysis by CuO oxidation). Charred organic carbon could

not be detected in the A horizons of an Alisol and a Gleysol, but it contributed up to 45% of the organic

carbon and up to about 8 g kg±1 of the soil in a range of grey to black soils (Cambisol, Luvisol, Phaeozem,

Chernozem and Greyzem). All these soils have chernozemic soil properties (dark colour, A±C pro®le,

high base saturation, bioturbation). A 10-km colour sequence of four chernozemic soils, which were very

similar in chemical and physical properties, showed a strong relation between colour and the content of

COC. This suggests that the COC affects mainly soil colour in the sequence studied. Finely divided COC

seems to be a major constituent of many chernozemic soils in Germany. These results suggest that

besides climate, vegetation and bioturbation, ®re has played an important role in the pedogenesis of

chernozemic soils.

Introduction

Burning vegetation produces large amounts of highly refrac-

tory organic matter consisting of charcoal and partly charred

plant material on the surface and incorporated in the soil. We

call these forms of thermally altered organic carbon `charred

organic carbon' (COC). The COC can have a major impact on

composition, turnover and formation of soil organic matter. It

may increase the amount of aromatic C and contribute a

relatively inert type of carbon to the soil organic matter pool

(Skjemstad et al., 1996). From COC highly aromatic humic

acids can be extracted (Haumaier & Zech, 1995; Skjemstad

et al., 1996).

Using light microscopy several workers have found black-

ish, coal-like particles or ¯akes often with a cellular structure

and diameters between 2 and 10�m in numerous typical

Chernozems from the Central Russian highland forest steppe

on loess±loam (Kubiena, 1938; Yarilova, 1972; Pawluk,

1985), Canadian Chernozems (Pawluk, 1985) and other soil

types with dark A horizons (AltemuÈller, 1992). There is also

indirect evidence that COC produces highly aromatic humic

acids in volcanic ash soils (Hatcher et al., 1989), in

Chernozems (Kononova, 1966; Schnitzer, 1992), in a Vertisol

in Mali (Gehring et al., 1997), in an Argentinian Hapludoll

(Zech et al., 1997), and in Japanese volcanic ash soils (Golchin

et al., 1997). In a systematic study of northern Eurasian soils,

humic acid contents increased in the order Grey forest

soils < Dark grey forest soils < Chernozems (Kononova,

1966). Particles of COC were identi®ed by light microscopy

and proton-spin relaxation editing in 13C nuclear magnetic

resonance (NMR) spectroscopy (Golchin et al., 1997). Using a

combination of high-energy ultraviolet (UV) photo-oxidation,

scanning electron microscopy and solid-state 13C NMR

spectroscopy, Skjemstad et al. (1996) identi®ed COC and

determined its content in Australian soils. Australian grassland

soils which were under aboriginal ®re management, presum-

ably for thousands of years, are characterized by black A

horizons with up to 30% of the soil carbon present as COC,

whereas adjacent forested soils not subjected to regular

burning are grey and contain little COC (Skjemstad et al.,

1997).

To determine whether the black colour of some German

soils could also be attributed to COC, we studied a variety of

Chernozem-like soils. All of these soils had dark grey to black

A horizons, high base saturation, and bioturbation, and were

classi®ed as Chernozem, Greyzem, Phaeozem, Cambisol or

Luvisol (FAO, 1994). For simplicity, we use the more general

term `chernozemic soils' for these soils in this paper. As

references, two non-chernozemic light-coloured soils (Alisol,

Gleysol) were sampled.

R

Correspondence: M. W. I. Schmidt, Max-Planck-Institut fuÈr Biogeochemie,

PO Box 10 01 64, 07745 Jena, Germany.

E-mail: [email protected]

Received 7 April 1998; revised version accepted 21 December 1998

European Journal of Soil Science, June 1999, 50, 351±365

# 1999 Blackwell Science Ltd 351

To investigate the apparent coincidence between black

colour and the presence of COC we examined the chemical

structure of the soil organic matter in a colour sequence in soils

of part of the loess belt between Hildesheim and Braun-

schweig, Germany, where chernozemic soils are typical.

Recently, Gehrt (1999) reported a patchwork-like distribution

of black and grey soils south of Peine. Apart from colour, the

soil properties seemed almost identical, i.e. morphology (depth

of pro®le, horizons, bioturbation, position in the landscape,

water table, crop rotation) and physical and chemical proper-

ties (texture, bulk mineralogy, C and N contents, pH). In

previous studies, the correlation between organic carbon

content and soil colour, expressed as Munsell values, was

found to be predictable for Ap horizons (r2 > 0.9) within soil

landscapes, provided soil texture did not vary widely (Schulze

et al., 1993). However, the patchwork (diameter 5±15 km) of

black and grey chernozemic soils (Chernozems or Greyzems)

is visible in satellite images and in the ®eld. The factors

responsible for this apparent colour pattern remain unde®ned,

but they could be related to the structure of the soil organic

matter.

The content of COC in soils can be determined by the

technique of high-energy UV photo-oxidation in combination

with 13C NMR spectroscopy. Previous studies by Skjemstad

et al. (1993) showed that several materials found in soil,

including wood, lignin and humic acids, could all be destroyed

by the high-energy photo-oxidation process, provided they

were exposed to ultraviolet radiation in the presence of excess

oxygen. In subsequent studies Skjemstad et al. (1996) found

that this method could differentiate between natural soil

organic matter and COC.

In this work we pose two separate questions: (i) what is the

potential contribution of COC to chernozemic soils from

Germany, and (ii) is there a relation between soil colour and

the chemical structure of soil organic matter? To answer these

questions we investigated the contribution of COC to the

chemical composition of these soils, using a suite of

complementary methods, including elemental analysis, CuO

oxidation of lignin compounds, high-energy UV photo-

oxidation, solid-state 13C NMR spectroscopy and scanning

electron microscopy. The soils we chose originate from sites

under agriculture and forest.

Materials and methods

Soils

As references two non-chernozemic soils were sampled. One is

a Haplic Alisol (soil 1) located in the eastern hills in Siggen,

Schleswig-Holstein, under a beech and oak (Melico±Fagetum)

vegetation. The mean age of the stand is 90±100 years, and

forest on this site is documented since the 13th century

(Schimming et al., 1993). The other is a Dystric Gleysol (soil

2) at Wageningen, the Netherlands, in a ®eld that has been

under agriculture for 100 years. Both mineral fertilizers and

cow manure have been applied, with a typical crop rotation of

maize, wheat and potato, to this ®eld.

The chernozemic soils we studied are as follows. A Humic

Cambisol (soil 3) from loess overlying WuÈrmian ¯uvic gravel

was sampled 30 km south of MuÈnchen. It has been under

coniferous vegetation for at least 100 years. The A horizons

display classic chernozemic soil properties, i.e. deep black A

horizons directly overlying the carbonaceous C horizons, high

base saturation and bioturbation. This soil receives 1400 mm

of rain per year on average, which is unusually large for

chernozemic soils. A Humic-stagnic Luvisol from the Baltic

coast close to Groûenbrode and developed from Weichselian

till (Schimming et al., 1993) is our soil 4. It lies in an isolated

patch of black soils on the peninsula neighbouring the island of

Fehmarn. The occurrence of these chernozemic soils is

focused around a few islands in the southwestern Baltic Sea

(Samsoe, Poel, Fehmarn). Because of their apparent unique-

ness, the black soils of Fehmarn have attracted the attention of

soil scientists over many years (Wolff, 1930; Schlichting,

1953). The pedogenesis of this soil has been related to that of

Chernozems, although a steppe vegetation during the Holo-

cene could not be proven by pollen analysis (Schmitz, 1955).

Other explanations were introduced to explain the dark colour,

i.e. clay mineralogy (Schimming et al., 1993) and slow

degradation of organic matter because of a high water table

(Rohdenburg & Meyer, 1968). No conclusive explanation for

the exceptional dark colour of this soil, however, has been

generally accepted. Our soil 5 is a Haplic Phaeozem north of

Halle/Saale in Seeben and has been farmland since the

beginning of this century (Schmidt et al., 1996).

Our sites 6±9 represent a colour sequence changing from

black to grey (Chernozems and Greyzems) developed on loess

in the region of Hildesheim-Braunschweig, northern Germany

(Gehrt, 1999). Except for the variation in colour, soil

properties are very similar. Gehrt (1999) has suggested that

differences in colour might be related to variation in clay

mineralogy arising from parent loess and underlying Cretac-

eous sediments. According to Bailly (1972), variations in soil

colour due to differences in palaeo-vegetation can be excluded.

The soils were described, sampled and classi®ed according

to established procedures, and horizons were designated

according to the German Soil Survey Description (AG-Boden,

1994; FAO, 1994). Table 1 gives a detailed description of the

investigated soils.

Sample pretreatment

Roots and visible plant remains were removed from the

samples where possible. The samples were frozen and freeze-

dried. Soil aggregates were crushed and the fraction > 2 mm

was removed by dry sieving. The pH (in 0.01 M CaCl2) was

measured with a glass electrode in the supernatant suspension

of a 2.5:1 (water:soil by weight).

L

352 M. W. I. Schmidt et al.

# 1999 Blackwell Science Ltd, European Journal of Soil Science, 50, 351±365

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Charred organic carbon in German chernozemic soils 353


Elemental analysis

For elemental analysis, a subsample was milled in a ball mill

for 10 min. Carbon and nitrogen contents were determined by

dry combustion in duplicate with an Elementar Vario EL. The

minimum detectable amounts were 0.1 6 0.3 g kg±1 for C and

N. For the various fractions investigated during the photo-

oxidation procedure, carbon was determined by a modi®ed

chromic acid digestion procedure (Heanes, 1984).

Photo-oxidation

Prior to the photo-oxidation procedure, each soil was saturated

with Na+ to facilitate the separation of silt and clay particles.

Na+ saturation was completed by shaking 10 g of soil in 50 ml

of 5 M NaCl in centrifuge tubes and centrifuging at 850 g for

10 min and ®ltering the supernatant (GF/A glass ®lter paper).

Samples were washed with 1 M NaCl twice and then dialysed

against distilled water until free of salt. Each sample was

transferred to a 250-ml beaker with 100 ml of water and treated

with ultrasonic energy for 10 s using a Branson B-30 soni®er

set to 50% power and 50% pulse and ®tted with a probe

(13 mm diameter). The soni®ed sample was allowed to stand

for 3 min to allow large particles to settle, and the supernatant

was poured through a nest of sieves (200 and 53�m) into a

measuring cylinder. Ultrasonic treatment and decantation was

repeated twice. After the third soni®cation, the remaining

sample was transferred to the cylinder through the sieves, and

the < 53-�m fraction was made to 500 ml.

Aliquots of the < 53-�m suspension, comprising » 2.5 mg

organic C, were placed in quartz test tubes and made up to

20 ml with water. To keep the sample in suspension, air was

introduced to the bottom of each tube through stainless-steel

capillary tubing (50 ml min±1). A stainless-steel cold-®nger

condenser was inserted to maintain room temperature. All

suspensions where photo-oxidized at 2.5 kW for 2 h (Skjem-

stad et al., 1996). After photo-oxidation, suspensions were

transferred to centrifuge tubes, 0.5 ml of saturated Al2(SO4)3

solution added and the samples centrifuged at 850 g for 10 min

and washed twice with water. Several samples (30±50) of each

soil were photo-oxidized and combined prior to HF treatment

(Skjemstad et al., 1994) and then freeze-dried for subsequent13C NMR studies.

Solid-state 13C NMR spectroscopy

Samples of the bulk soils treated with HF and the photo-

oxidized < 53-�m residues were packed in a 7-cylindrical

zirconia rotor (diameter 7 mm) with Kel-F caps and spun at

5 kHz at a frequency of 50.309 MHz in a Doty Scienti®c MAS

probe. Spectra were obtained at a 13C resonance frequency of

50.3 MHz on a Varian Unity 200 spectrometer using magic

angle spinning (MAS) and a standard cross-polarization (CP)

pulse sequence (Wilson, 1987). A contact time of 1 ms was

used, and recycle delay time was 0.3 s to ensure complete

relaxation between scans (recycle delay > 7 T1H). The spectra

were plotted between ±100 and 300 p.p.m. using a Lorentzian

line broadening of 50 kHz and a Gaussian function broadening

of 0.004 s. Chemical shift assignments are given in

Table 2. The NMR spectra were divided into ®ve chemical

shift regions representing alkyl C (0±46 p.p.m.), O±alkyl C

(46±110 p.p.m.), aryl C (110±145 p.p.m.), O±aryl C (145±165

p.p.m.), carbonyl C (165±190 p.p.m.) and aldehyde/ketone C

(190±220 p.p.m.).

Determination of the content of charred organic carbon

Spectra were corrected for spinning sidebands from aromatic C

(230 and 30 p.p.m.) and carbonyl C (273 and 73 p.p.m.) by

integrating the signal intensities between 220 and 250 p.p.m.,

respectively 250 and 295 p.p.m. Assuming that the correspond-

ing sidebands at 30 and 73 p.p.m. are of equal size, integrals

were doubled and added to the aromatic and carbonyl region

and subtracted from the alkyl C and O±alkyl C region.

Spinning sidebands from O±aryl C spectra were not differ-

entiated because of their small contributions to the total signal

intensities. The content of COC is determined by the amount

of aryl carbon corrected for lignin content if following photo-

oxidation a recognizable O±aryl C peak is observed. For the

L

Table 2 Chemical shift assignment of peaks in the solid-state 13C CPMAS NMR spectra which is referenced to tetramethylsilane = 0 p.p.m.

(Wilson, 1987; LuÈdemann & Nimz, 1973)

Chemical shift

range /p.p.m. Assignment

220±190 Aldehyde, ketone

190±160 Carbonyl carbons

160±140 Aromatic COR or CNR carbons

140±120 Aromatic C-H carbons, guaiacyl C-2, C-6 in lignin, ole®nic carbons

120±100 Anomeric carbon of carbohydrates, C-2, C-6 of syringyl units in lignin

100±60 Carbohydrate-derived structures (C-2 to C-5) in hexoses, C-� of some amino acids, higher alcohols

60±45 Methoxyl groups and C-6 of carbohydrates and sugars, C-� of most amino acids

45 to ±10 2°, 3° and 4° carbons in alkyl structures, methyl carbons



samples studied here such a correction was considered

unnecessary.

In materials that contain carbon nuclei remote from protons

(> 4±5 bond lengths), effective cross-polarization will not

occur, and these nuclei will not be detected by CPMAS

spectroscopy (Alemany et al., 1983; Snape et al., 1989). As a

result, highly aromatic structures, such as those found in COC,

will be underestimated by CPMAS spectroscopy. Bloch decay

does not require the proximity of protons, and relies on

relaxation processes through the C nuclei. To analyse all

samples by Bloch decay for COC content would be totally

impractical, however, considering the long recycle times

required (typically > 60 s) and the small amount of sample

available from the photo-oxidation method. We therefore

applied an empirically determined correction to the CPMAS

data in an attempt to correct for the underestimation of

aromatic carbon. Skjemstad & Taylor (unpublished data)

compared a number of photo-oxidized samples by both

CPMAS and Bloch decay. These data were used to develop

an equation to transform CPMAS-determined aryl C data to

that determined by Bloch decay (r2 = 0.98). Using this

correction, the COC content (g COC kg±1 soil) was calculated

as equal to the corrected aryl C contribution to the total signal

intensity (arylcorr). Although this approach suffers from some

uncertainty, it does provide a conservative measure of the

content of COC. From the corrected aryl C content (arylcorr) of

the < 53-�m fraction and its relative contribution to the bulk

soil, it is possible to calculate the content of COC in that

fraction normalized to the soil mass. To exclude differences in

organic C content in the different horizons, the COC content

was also normalized to the soil C content.

Scanning electron microscopy

Scanning electron microscopy (SEM) was carried out on a

Cambridge Stereoscan S250 on samples coated with 20±30 nm

of carbon. Elemental characterization was done using a Link

AN1000 EDX analyser.

CuO oxidation

Alkaline CuO/NaOH oxidation was carried out by the method

of KoÈgel-Knabner (1995). Triplicate subsamples containing

» 50 mg organic C were added to 2 M NaOH in Te¯on vials

together with CuO. The vials were sealed under N2 and heated

for 2.5 h at 170°C. After ®ltration, humic acids were removed

by acidi®cation and centrifuging. The supernatant was

extracted with conventional clean-up columns (Baker,

Germany), and silyl derivatives were separated and quanti®ed

by GC-FID (gas chromatography-¯ame ionization detection).

The total yields were determined for vanillyl (vanillin + va-

nillic acid), syringyl (syringealdehyde + syringic acid) and

cinnamyl units (p-coumaric acid + ferulic acid), respectively.

Amounts are given in g per kg of organic carbon (g VSC kg±1

C). Relative standard deviation for the detection of phenolic

products is 9% (n = 10). The p-hydroxyl compounds are not

included in the sum of phenolic oxidation products because

they can also be derived from non-lignin structures. The ratios

of acid-to-aldehyde of vanillyl (ac:al)V and syringyl (ac:al)S

and of syringyl-to-vanillyl (S:V) are calculated as mass ratios.

Colour

The colour of dry soil samples was determined with Munsell

colour charts. For standardization and better resolution, the

degree of darkness was measured with an ASD ®eld spectro-

meter under sunlight, and is reported as Y-CIE value, which

can be converted to Munsell values and corresponds closely to

human observations (Schulze et al., 1993).

Results and discussion

We now report the results obtained from 17 horizons,

originating from nine soils, and discuss them. First, the A

horizons of two non-chernozemic light-coloured soils (soils 1

and 2) are compared. Second, data for A horizons originating

from three chernozemic soils (soils 3±5) are presented, and

®nally a colour sequence of four chernozemic soils (soils 6±9)

with a gradual change from black to grey colour is

investigated.

Non-chernozemic soils

The Ah horizon of the Haplic Alisol (soil 1) contains 15.1 g

organic C kg±1 soil (Table 3). A major proportion of the bulk

soil C (57%) is in the < 53-�m fraction, subsequently

investigated by 13C CPMAS NMR spectroscopy (Figure 1).

The spectrum of the < 53-�m fraction is dominated by peaks in

the alkyl C region (±10 to 45 p.p.m.) with major peaks

representing methylene carbon (33 and 30 p.p.m.). Signals in

the O±alkyl C region (45±110 p.p.m.) indicate the presence of

polysaccharide and alcoholic structures. Signals near 56 p.p.m.

can be attributed to methoxyl groups and to C-�. The shoulder

at 65 p.p.m. is due to C-6 carbon in polysaccharides. Signals

near 74 p.p.m. correspond with ring carbon C-5, C-3 and C-2,

whereas resonances at 104 p.p.m. are due to anomeric C-1

carbon. Some resonances in the aryl C region (145±110 p.p.m.)

are attributable to protonated and alkyl-substituted aryl carbon

(116 and 131 p.p.m.). These signals for aryl C in combination

with O±aryl C structures (165±145 p.p.m.) are typical for the

presence of lignin units, whereas COC would be characterized

by signals in the aryl C region centred around 130 p.p.m. with

few additional peaks in the O±aryl C region. The carbonyl C

region (190±165 p.p.m.) is dominated by a set of resonances at

173 p.p.m. most likely due to amide carbon. The aldehyde/

ketone C region (220±190 p.p.m.) reveals only small signal

intensities, which here can be considered to be of little

importance (Preston & Ripmeester, 1983).

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All labile organic matter is removed by photo-oxidation for

2 h, and only physically or chemically protected matter

remains (Skjemstad et al., 1996). In fact, most of the organic

carbon is removed by this treatment, leaving about 10% of the

bulk soil C (Table 3). Comparing the resulting spectrum with

the spectrum obtained before photo-oxidation, only minor

alterations are apparent in shape and integrated areas.

Although peak heights in the alkyl C and O±alkyl C regions

change, the integrated areas vary only within the error range.

Intensities decrease in the aryl C and ketone C region (±2%)

while intensities for O±aryl and carboxyl C slightly increase

accordingly (+1%). These alterations probably can be

explained by a greater line broadening due to the smaller C

content of the sample after photo-oxidation. As a result, the

organic carbon resistant to photo-oxidation presumably is

protected physically inside organo-mineral complexes, reveal-

ing a bulk chemical structure similar to the carbon fraction

prior to the treatment. Skjemstad et al. (1996) observed similar

results for other soils. Aromatic carbon is present mainly in

lignin-derived phenols, as clearly demonstrated by typical

signals at 130 p.p.m. in combination with peaks in the O±aryl

C region. As a result, the presence of COC cannot be

determined by this technique in the Ah horizon of the Haplic

Alisol.

The Ap horizon obtained from the Dystric Gleysol (soil 2)

comprises 17.5 g C kg±1 dry soil with 50% of the bulk soil C in

the fraction < 53�m. The spectrum of this fraction reveals

signals mainly in the chemical shift regions, as discussed

above. The main differences occur in the aryl C region with a

more distinct peak around 130 p.p.m. with shoulders at 120

and 140 p.p.m. indicating the presence of lignin. Contributions

from aryl C are slightly larger (8% of the total signal intensity)

compared with the Alisol (7%), suggesting greater contribu-

tions of lignin to the organic matter.

The lignin signature, as determined by CuO oxidation,

provides information on the structure of aromatic carbon and

re¯ects the vegetation from which the soil organic matter was

derived (Ertel & Hedges, 1984). Lignin from gymnosperm

wood produces mainly vanillyl-type (V) oxidation products,

whereas lignin from angiosperm wood produces syringyl units

(S) in addition to vanillyl units. Large yields of cinnamyl units

(C) are characteristic of non-woody tissues of both types of

plants. The proportion of lignin-derived phenols (g VSC kg±1

C) and the ratio acid:aldehyde (ac:ad) for syringyl (S) and

vanillyl (V) phenols varies among plant species (Sarkanen &

Ludwig, 1971; Ertel & Hedges, 1984). Large ratios (ac:ad)V

and (ac:ad)S indicate a greater degree of decomposition of

lignin phenols. As detected by CuO oxidation, proportions of

lignin-derived phenols are greater in the Ap horizon of the

Gleysol (11.7 g VCS kg±1 C) than in the Alisol (8.5 g VCS kg±1

C; Table 3), which con®rms conclusions from NMR spectro-

scopy. As in the Ah horizon of the Alisol, 10% of the bulk soil

C resists photo-oxidation. Again, signal intensities in the 13C

CPMAS spectrum of this fraction show only small alterations,

except for an increase in the aryl C region (15%) compared

with the untreated sample (8%). In the Gleysol these signals

can be assigned to lignin phenols, as indicated by typical

shoulders at 153 p.p.m.

In summary, we could not detect COC in the A horizons of

the non-chernozemic soils (Alisol, Gleysol) by the procedure

of photo-oxidation followed by 13C CPMAS NMR spectro-

scopy.

Chernozemic soils (soils 3±5)

The Humic Cambisol (soil 3) shows similar C:N ratios and pH

values in both horizons studied, whereas texture and the

content of organic matter differ. The AxhBv horizon contains

slightly more clay (30.3 mass percentage) compared with the

overlying Axh horizon (25.7 mass percentage). The C and N

contents are greater in the Axh horizon than in the underlying

AxhBv horizon, which probably re¯ects the larger input of

roots and leaves mixed into this horizon by bioturbation. Also

in this soil, the < 53-�m fraction contains a major proportion of

the bulk soil C in both the Axh horizon (54%) and the AxhBv

horizon (70%).

The 13C CPMAS spectra (Figure 2) display similar peak

heights and integrated areas for alkyl C, carboxyl C and ketone

C. Some differences between the spectra can be found in the

O±alkyl C region and in the aryl and O±aryl C regions.

Contributions from O±alkyl C are greater in the Axh horizon

(30%) than in the underlying AxhBv horizon (23%), in

accordance with the larger proportion of polysaccharides

derived from plant litter in the Axh horizon. The contributions

from aryl C (Table 3) are greater in the Axh horizon (28%) and

in the AxhBv horizon (31%) than in soils 1 and 2 (< 8%).

These large contributions can be attributed only partly to lignin

compounds. As demonstrated before, lignin typically produces

signals in the aryl C region in combination with resonances in

the O±aryl C region (165±145 p.p.m.), but in the spectra

obtained from the Humic Cambisol, signals in the O±aryl C

L

Figure 1 Solid-state 13C NMR spectra of the investigated A

horizons from (1) the Haplic Alisol and (2) the Dystric Gleysol

before (left) and after high-energy UV photo-oxidation (right).



region are weak, suggesting that lignin contributes only part of

the observed aromatic carbon. This is con®rmed by small

yields of CuO oxidation products (< 2.9 g VSC kg±1 C)

compared with those of soils 1 and 2 (Table 3). These results

indicate the presence of unsubstituted aromatic carbon in both

horizons of the Phaeozem. If this signal were due to COC, the

material would resist photo-oxidation, and the resulting

spectrum would show large signal intensities for aryl C. In

fact, a larger proportion of soil organic carbon resists photo-

oxidation (Axh 16% and AxhBv 30%) than in soils 1 and 2

(< 11%). The spectra of both horizons, after photo-oxidation,

are dominated by aryl C peaks at 130 p.p.m. with spinning

sidebands present at 230 and 30 p.p.m. At 30 p.p.m. these

signals overlap with resonances of alkyl C, and minor signals

indicate the presence of some O±alkyl C (75 p.p.m.) and

carboxyl C (170 p.p.m.). Skjemstad et al. (1993) showed that

carbonyl bands remaining after photo-oxidation are most likely

due to carboxylic acids. The absence of signals in the O±alkyl

C region indicates the absence of carbohydrate carbon which is

typical for COC.

To investigate the nature of this material further we

observed its morphology by scanning electron microscopy

(SEM). Micrographs are shown in Figure 3(a,b). Many of the

larger particles have a morphology characteristic of the xylem

structure of woody material. These and many of the ®ne

particles were probed with an EDX system and found to

contain no elements with an atomic weight greater than

sodium, except for sulphur. Few particles could be identi®ed as

mineral. This suggests that many of the particles are organic

and probably are COC.

If a major proportion of aromatic carbon is present in

unsubstituted structures, this poses questions about the

quantitative reliability of the CPMAS spectra obtained from

soil 3. In cross-polarization (CP) experiments, the polarization

energy for carbon nuclei is transferred via hydrogen nuclei

with an ef®ciency depending on the proximity of these nuclei.

Consequently, the carbon nuclei in unsubstituted structures are

underestimated by CP experiments relative to Bloch decay

experiments, in which carbon nuclei are polarized directly.

The samples contain little carbon. Long recycle times would

therefore be required in Bloch decay experiments, and it would

take several days to obtain a single spectrum which would still

give a poor signal-to-noise ratio. With so many samples Bloch

decay experiments were quite impractical. We therefore

introduced an empirically determined correction for contribu-

tions of unsubstituted aromatic carbon (for details see

Materials and methods). We assumed that the resulting

corrected signal intensities for unsubstituted aromatic carbon

(arylcorr) all originate from COC. For the < 53-�m fractions the

results are expressed relative to the bulk soil, i.e. normalized to

organic C and to soil mass of the bulk soil, respectively (Table

3). In the < 53-�m fractions from both horizons of the

Cambisol, organic carbon is almost exclusively present in

unsubstituted aromatic structures with negligible contributions

from other C species, as revealed by corrected aromaticity

(Table 3). Expressed relative to the bulk soil, COC contributes

30% to the bulk soil C in the AxhBv horizon, which is

equivalent to 7.4 g COC kg±1 bulk soil. In the Axh horizon,

COC contributes less to the bulk soil C (15%) and to the bulk

soil mass (5.6 g COC kg±1 bulk soil).

As a result, the Humic Cambisol contains considerable

amounts of a material which (i) resists photo-oxidation, (ii)

consists of unsubstituted aromatic carbon, and (iii) displays a

morphology typical of cell wall material from plants, which

provides evidence for the presence of charred plant remains or

COC.

Three horizons from a Humic-stagnic Luvisol (soil 4)

with dark A horizons were studied. Progressing from Axp

to AxhBt horizons, the C and N contents decrease, whereas

clay contents increase (8.3 to 17.7 mass percentage clay;

R

Figure 2 Solid-state 13C NMR spectra of the investigated A

horizons from (3) the Humic Cambisol, (4) the Humic-stagnic

Luvisol and (5) the Haplic Phaeozem before (left) and after high-

energy UV photo-oxidation (right). Asterisks (*) indicate spinning

sidebands from carbonyl C (273 p.p.m.) and aromatic C

(230 p.p.m.). For simplicity, the corresponding sidebands at 73 and

30 p.p.m., respectively, were not labelled.



L

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Table 1). The C:N ratios are uniformly small (12±13), and

pH is constantly neutral (6.9±7.0) throughout the pro®le.

For all three horizons the < 53-�m fraction represents

> 57% of the soil C.

The 13C CPMAS spectra (Figure 2) obtained from the Axp

and Axh horizons demonstrate that contributions from aryl C

(15±21% of the total signal intensity) are intermediate between

those detected for the Humic Cambisol (soil 3) (28±31%) and

those observed for the Alisol and the Gleysol (< 8%). Typical

signals for lignin structures in the O±aryl C region indicate that

some of the signal intensities in the aromatic region can be

attributed to lignin structures. This conclusion is supported by

larger proportions of lignin-derived phenols in the Axp (12.2 g

VSC kg±1 C) and in the Axh horizon (8.0 g VSC kg±1 C). After

photo-oxidation, C spectra show a dominant peak at 130 p.p.m.

and a large contribution in the aryl C regions (> 34% of the

total signal intensities). Weak signals in the O±aryl C region

(4%) indicate that lignin structures contribute in only a minor

way to the organic carbon present in this fraction. Microscopic

investigation (Figure 3c) demonstrates the presence of ®nely

divided angular particles with a cellular morphology, char-

acteristic of COC. Contrasting with the samples from the

Humic Cambisol, the particles are much ®ner and are more

dif®cult to attribute exclusively to COC. This is also re¯ected

by the spectra, which in addition to signals in the aryl C region

show peaks at 30, 75, 106 and 175 p.p.m. indicating the

presence of alkyl C, O±alkyl C and carbonyl C. Progressing

through the pro®le, calculated contributions of COC to the

bulk soil C are smallest in the Axp horizon (4% of the bulk soil

C), largest for the Axh horizon (15%) and intermediate (8%) in

the AxhBt horizon. The same pattern is true for the absolute

amounts of COC in these horizons.

As a typical representative of the chernozemic soils in the

Halle-Magdeburg area, a Haplic Phaeozem under agricultural

practice was studied (soil 5). Only minor variations between

Axp and Axh horizons were observed for C and N content, pH

and texture. For both horizons, > 71% of the bulk soil C was

recovered in the < 53-�m fraction for photo-oxidation. The

spectra obtained from these fractions are almost identical to

those obtained from the non-fractionated soils as presented

previously (Schmidt et al., 1996), showing that the fraction

investigated can be regarded as representative for the bulk soil.

Because of the presence of peaks at 153 p.p.m., some of the

signal intensity in the aromatic C region can be attributed to

lignin structures, as con®rmed by large proportions of lignin-

derived phenols (> 15.2 g VSC kg±1 C).

Almost identical amounts of organic C resist photo-

oxidation in the Axp and Axh horizons (17 and 18% of the

bulk soil C, respectively). Also the 13C CPMAS spectra

(Figure 2) are similar, with the largest intensities in the aryl C

region peaking at 130 p.p.m. (Axp 39% and Axh 46% of the

total signal intensity). The absence of distinct signals near

153 p.p.m. suggests that aryl signals can be assigned to COC,

which is supported by scanning electron microscopy of the

fraction obtained from the Axh horizon (Figure 3d). As can be

seen from Table 3, contributions of COC increase with depth,

both normalized to the bulk soil C (Axp 9% and Axh 15% of

the bulk soil C) as well as normalized to the bulk soil mass.

Again, the presence of COC could not be detected in the

light-coloured non-chernozemic soils (Haplic Alisol, Dystric

Gleysol), whereas in chernozemic soils (Humic Cambisol,

Humic-stagnic Luvisol, Haplic Phaeozem) COC contributed

up to 30% to the bulk soil carbon with largest contents in the

subsurface horizons (Axh, AxhBv or Ah) and smallest contents

in surface horizons (Axh, Axp). These proportions of COC are

similar to those reported by Skjemstad et al. (1996) for some

Australian soils.

Colour sequence (soils 6±9)

The apparent coincidence of dark soil colour with the presence

of COC raises questions of a potential relation between these

two variables. Soil colour and organic matter content are more

closely related for soils occurring together in landscapes and

having similar texture and parent material than for soils over a

wide geographic region or widely varying in texture (Schulze

et al., 1993). We therefore studied the relation between them in

a 10-km colour sequence of four chernozemic soils. The soils

studied had developed from almost identical parent material

(loess) and have similar chemical and physical properties, and

they are all under agriculture with similar crop rotations. The

striking feature is a gradual change from black to grey. For a

more accurate quanti®cation of the small differences in colour,

the Munsell value of each soil was determined by measuring

the re¯ectance in the laboratory with a diffuse re¯ectance

spectrometer and expressed as Y-CIE value (Table 1).

The C contents (Table 1) for the ploughed horizons (Axp)

vary between 5.0 and 23.6 g C kg±1 soil, whereas the subsoils

vary less (Axh 11.9±17.8 g C kg±1), and show variations

typically found in this region (Gehrt, 1999). The C:N ratios

are uniformly small (9±14) in all horizons. Also, the texture is

similar in all soils and horizons, with small sand contents

(< 3.3 mass percentage) and typically large silt contents (77.0±

80.2 mass percentage). Clay contents (16.8±23.9 mass

percentage) are often slightly larger in the Axh horizons than

in the overlying Axp horizons. There is a little variation in the

pH (7.0±7.6).

For the sequence investigated (Table 3), the sum of lignin-

derived phenols is typically largest in the Axp horizons (6.6±

17.8 g VSC kg±1 C) and somewhat smaller in the correspond-

ing Axh horizons (1.7±10.4 g VSC kg±1 C). This indicates the

larger input of plant material from agricultural crops in the

ploughed horizon than in the Axh horizons. As demonstrated

by the acid:aldehyde ratios, the degree of lignin decomposition

varies only slightly in the horizons investigated: (ac:ad)V 0.18±

0.45; (ac:ad)S 0.46±0.74. Also, the ratios of syringyl:vanillyl

phenols (S:V) are similar (1.10±2.17), con®rming the larger

input from gymnosperms than from the angiosperm vegetation

R



of the forest (soil 1 < 0.38) and mixed deciduous vegetation

(0.90) (Sarkanen & Ludwig, 1971; Ertel & Hedges, 1984). As

a result, the lignin signature suggests that the sources of

organic matter are currently similar in the soils of the colour

sequence.

Taking into account the similar pedogenetic factors and soil

properties, including the similar C content, one could expect a

similar soil colour (Schulze et al., 1993). However, in

progressing from soil 6 to soil 9, the colour changes gradually

from black to grey. The visual assessment is con®rmed by the

spectrometrically determined Y-CIE values, which are an

equivalent to the Munsell values (Table 1). Small Y-CIE

values correspond with dark colours, while larger ones

correspond with lighter colours (Schulze et al., 1993).

In the colour sequence, the < 53-�m fraction comprises a

major proportion of the bulk soil carbon (56±83% of the bulk

soil C). The 13C CPMAS NMR spectra obtained from these

fractions (Figure 4, left side) are fairly uniform, except for the

aromatic C regions. Signals from aryl C (Table 3) are strongest

in the Axh horizon of soil 6 (37% of the total signal intensity)

and weakest in the Axp horizon of soil 9 (12%). After photo-

oxidation only small proportions of carbon are left, consis-

tently decreasing from soil 6 to soil 9 for both Axp (21 to 10%)

and Axh horizons (45 to 10%). As revealed by the corrected

aryl C value (arylcorr, Table 3) in the Axh horizon of soil 6,

almost all (95%) organic carbon is present in unsubstituted

aromatic structures. Also here, scanning electron microscopy

con®rms an abundance of what appears to be COC in this

fraction, which is also true for the Axh horizon from soil 7.

The largest proportion of COC in the colour sequence can be

detected in this Axh horizon from soil 6. Here, COC

contributes 45% of the soil organic carbon, which is equivalent

to 7.6 g COC kg±1 soil.

To visualize a potential interdependence between COC

content (Table 3), lignin compounds (Table 3) and soil

colour (Table 1), data are displayed as a colour sequence

from black to grey in Figure 5. To exclude variations

resulting from different C contents, the contents of COC

and lignin compounds are normalized to organic carbon

content. Progressing from black to grey (from soils 6 to 9)

two patterns for colour are evident. First, the colour of the

soil generally becomes lighter, i.e. the re¯ection expressed

as Y-CIE value increases, except for the almost identical

soils 8 and 9. Second, the Axp horizons are darker than the

corresponding Axh horizons. Parallel to these trends,

contributions of COC to the bulk soil C decrease

L

Figure 4 Solid-state 13C NMR spectra of the investigated Axp and

Axh horizons from the colour sequence before (left) and after high-

energy UV photo-oxidation (right). Asterisks (*) indicate spinning

sidebands from carbonyl C (273 p.p.m.) and aromatic C

(230 p.p.m.). For simplicity, the corresponding sidebands at 73 and

30 p.p.m., respectively, were not labelled.

Figure 5 Content of charred organic carbon (COC) and lignin

compounds detected in the Axp and Axh horizons of the

chernozemic soils 6±9 displayed as a colour sequence. Data for the

COC and lignin are displayed on the left axis; note different units

for these parameters. Re¯ection is given as Y-CIE value which is

an equivalent to the Munsell value.



consistently in the Axp horizons (19 to 2%) and in the Axh

horizons (45 to 3%), while proportions of lignin compounds

increase, again with soils 8 and 9 being almost identical.

As a trend, progressing from dark to light soils in the colour

sequence (i) organic matter becomes less resistant to photo-

oxidation, (ii) contributions of COC to the bulk soil C

decrease, whereas (iii) contributions of lignin to the bulk soil C

increase. Following the Axp and the Axh horizons in the

colour sequence, we are tempted to conclude that soil colour

directly depends on the contribution of COC to soil organic

matter, but within the same soil pro®le this conclusion does not

seem to hold. For all the soils investigated, the surface

horizons (Ap, Axp, Axh) are darker than the corresponding

subsoils, except for soil 6 (Table 1), whereas contributions

from COC are always greatest in the subsoils (Table 3). The

lighter colour of the subsoils may be related to the smaller C

content. However, for soils 5 and 8 this explanation does not

hold, because the C contents are similar in the Axp and Axh

horizons. This observation shows that besides COC other

organic and inorganic pigments contribute to the soil's colour.

For the agricultural soils (4±9) and the forest soil (soil 3), the

contributions from COC are always greater in the subsoils than

in the surface horizons, both normalized to organic C content

and normalized to bulk soil mass. The same is true for the clay

content, except for soil 5 (Table 2). The apparent differences

between surface horizons (Ap, Axp, Axh) and subsoils may be

explained by primary differences during sedimentation of the

loess layers or by a subsequent eluviation within the pro®le

induced by cultivation.

Conclusions

The photo-oxidation method allowed us to identify charred

organic carbon (COC) in several chernozemic soils, but none

in the A horizons of the Haplic Alisol and the Dystric Gleysol.

It seems that COC is restricted to chernozemic soils, and this is

the ®rst time that COC has been detected in German

chernozemic soils. The COC contributes up to 45% of the

bulk organic carbon, which is equivalent to about 8 g kg±1 of

the bulk soil. A colour sequence of chernozemic soils, similar

in chemical and physical properties, showed a strong relation

between colour and the content of COC, suggesting that, at

least in the sequence studied, the COC dominates the colour of

the soil.

The presence of COC in chernozemic soils poses some

questions on its possible in¯uence on soil properties. The COC

might affect the soil physical properties (water holding

capacity, structural stability), chemical properties (pH, C

content, cation exchange capacity, sorption) and biological

properties (stability against biological degradation). For

example, standard determinations of the C content include

the highly recalcitrant COC, and so overestimate the content of

native soil organic matter.

The presence of COC also raises questions about the

pedogenesis of Chernozems. It is generally accepted that the

pedogenesis of Chernozems depends on continental dry

climate with little or no leaching. The characteristic deep

black A horizons are considered to develop as a result of

accumulation of organic matter and bioturbation. The

Chernozem belt developed from Siberia to Eastern Europe,

with isolated occurrences in Germany. There, Chernozems are

regarded as relic soils which developed about 8000 years ago

under a continental climate and were preserved under

favourable conditions, i.e. low precipitation or high ground

water table. Without these preserving conditions, Chernozems

are expected to be degraded by gradual decalci®cation and

accumulation of black clay±humus complexes in deeper

horizons, resulting in Phaeozems or Luvisols (MuÈckenhausen,

1977). However, the German chernozemic soils that we

studied display many properties typical of Chernozems,

although the present annual precipitation varies widely

between 480 and 1400 mm.

In the chernozemic soils we studied, the COC can be a

major contributor to the organic carbon, but the origin of this

®re-induced form of organic carbon remains unclear. It could

originate from natural vegetation ®res as well as from human

activity. Because it so recalcitrant, COC can be preserved in

the soil for a long time. Consequently, it could have originated

from ®res in the post-glacial vegetation, i.e. tundra, taiga and

later mixed deciduous forests. Today, the circumpolar taiga

may be an important zone for the production of charred

organic carbon from ®re (Wein, 1993). The COC could also

originate from post-mesolithic human use of ®re for the

clearing of the forests and subsequent agriculture. We sampled

the colour sequence in an area with an apparently random

pattern of patches with black and grey chernozemic soils on a

regional scale of some kilometres. This small-scale pattern

could be related to local human activity rather than to large

natural vegetation ®res. However, we have too little informa-

tion on the ®re and vegetation history to draw many

conclusions. The time of COC formation could be determined

by 14C analysis and provide further information on the history

of the soils.

If ®re was involved in the formation of the charred

proportion of soil organic matter in the chernozemic soils,

it may also be involved in the pedogenesis of other

chernozemic soils. This could explain the reported presence

of black ¯akes in numerous typical Chernozems from

central Russia (Kubiena, 1938; Yarilova, 1972; Pawluk,

1985), the high aromaticity of humic acids extracted from

other chernozemic soils (Kononova, 1966; Schnitzer, 1992;

Zech et al., 1997) as well as the relation between humic

acid content and soil colour for chernozemic soils from

northern Eurasia (Kononova, 1966).

Our results suggest that besides climate, vegetation and

bioturbation, ®re also plays an important role in the

pedogenesis of chernozemic soils.

R



Acknowledgements

The work was ®nancially supported by the Deutsche

Forschungsgemeinschaft (Ko 1035/6-1 and 2). A travel

grant was provided by the Massenberg foundation of the

University of Bochum. We thank Claus Schimming for

helping us select soils 1 and 6, and Lothar Beyer

(University of Kiel) for providing reference material from

soil 4. Walter Grottenthaler (Geological Survey of Bavaria)

provided samples and analytical information from soil 3.

We thank also Louis Maratos and Janine Taylor (CSIRO,

Land and Water) for soil fractionations and NMR spectro-

scopy. Experimental assistance for lignin analysis was

provided by Mathias Kempkens and Patrick GaÈrtner

(University of Bochum). Re¯ectance was measured by

Thomas Jarmer (University of Trier), and Hans-JuÈrgen

AltemuÈller (Braunschweig) provided helpful comments and

literature on charred particles in Chernozems.

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