Composition and distribution of organic matter in physical fractions of a rehabilitated mine soil...

$: Composition and distribution of organic matter in physical fractions of a rehabilitated mine soil rich in lignite-derived carbon$
Ž .Geoderma 98 2000 177–192www.elsevier.nlrlocatergeoderma

Composition and distribution of organic matter inphysical fractions of a rehabilitated mine soil rich in

lignite-derived carbon

C. Rumpel a,), I. Kogel-Knabner b, H. Knicker b, R.F. Huttl a¨ ¨a Department of Soil Protection and RecultiÕation, Brandenburg Technical UniÕersity of Cottbus,

P.O. Box 10 13 44, D-03013 Cottbus, Germanyb Lehrstuhl fur Bodenkunde, Technische UniÕersitat Munchen,¨ ¨ ¨

D-85350 Freising-Weihenstephan, Germany

Received 11 November 1999; received in revised form 25 April 2000; accepted 1 May 2000

Abstract

After open-cast mining operations in the eastern states of Germany, extremely acid overburdenŽ .material with a high lignite content up to 50 g OCrkg soil is relocated and deposited at a spoil

bank. For rehabilitation, these substrates are amended with alkaline ash from lignite-fired powerstations and reforested with coniferous or broad-leaved trees. The aim of this study was to identifythe allocation of lignite and recently formed humified compounds in soil compartments as

Žseparated by physical fractionation procedures. Samples were taken from the soil surface 0–5 cm,. Ž . ŽAi horizon and from the subsoil 1 m depth, Cv horizon under red oak Quercus rubra, age 36

.years . Organic matter in bulk soils as well as in particle-size and density fractions was analysedfor elemental composition, magnetic susceptibility, chemical structure by solid-state 13C CPMASNMR spectroscopy and lignite content by 14C activity measurements. The contribution of lignin tothe particle size fractions was estimated using CuO oxidation. In the bulk soil as well as in thephysical fractions, carbon species related to lignite and carbon species related to plant litter occurin mixture. The 2000–630 mm fraction was found to be dominated by plant litter compoundswhereas the 630–6.3 mm fractions consist mainly of aromatic and aliphatic carbon, most probablyindicative of lignite and carbonaceous particles which were added to the Ai horizon duringamelioration and airborne contamination with lignite combustion products. In the -2.0 mmfraction, CuO oxidation indicated that aromatic C compounds are derived from lignite rather thanfrom recent plant lignin compounds. Physical fractionation with subsequent characterisation of the

) Corresponding author. Tel.: q49-355-78-1162; fax: q49-355-78-1170.Ž .E-mail addresses: [email protected] C. Rumpel ,

Ž [email protected] I. Kogel-Knabner .¨

0016-7061r00r$ - see front matter q2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0016-7061 00 00060-4

( )C. Rumpel et al.rGeoderma 98 2000 177–192178

Ž .organic matter present in the fractions revealed that recently formed soil organic matter SOMconsists mainly of partly altered plant residues rather than humic substances. q 2000 ElsevierScience B.V. All rights reserved.

Keywords: lignite; organic matter; particle size fractionation; density fractionation; magneticsusceptibility; 13C NMR spectroscopy; radiocarbon dating

1. Introduction

In industrial areas, especially mining districts, soils contain several types oforganic matter. In mine soils of the eastern part of Germany, lignite particles,carbonaceous particles from combustion and airborne carbonaceous particles canoccur in mixture with plant-derived material and have an impact on the chemical

Ž . Žcomposition of the soil organic matter SOM Schmidt et al., 1996; Rumpel et.al., 1998a,b . The organic matter types present in bulk soil samples of lignite-

Ž .containing mine soils under forest were characterised by Rumpel et al. 1998b .Ž .The differentiation of organic matter derived from plant material recent carbon

Ž .and organic matter derived from the lignite mining industry lignite carbon iscrucial for understanding the dynamics of the recent SOM and soil development.Such differentiation may be achieved by particle size or density fractionation.However, up to now research on lignite-containing mine soils concentrated onbulk soil samples.

In natural soils, physical fractionation procedures applying mechanical disrup-tion and separation by particle size andror density have been used for elucidat-

Ž .ing SOM dynamics e.g. Tiessen and Stewart, 1983 . With these methods, SOMŽ .compartments have been separated Oades, 1988; Andreux et al., 1995 , which

Žwere found to differ in chemical structure and bioavailability Baldock et al.,.1992; Kogel-Knabner and Ziegler, 1993; Guggenberger et al., 1994, 1995 .¨

Usually, the degree of decomposition increases with decreasing particle size.ŽPlant fragments are found in the light and coarse fractions Hassink and

.Dalenberg, 1996 and humic substances accumulate preferentially in the heavyŽ .and fine fractions Tisdall and Oades, 1982 . Recently, a calibration procedure

became available which allows for physical fractionation of soils which containŽ .coal or combusted particles Schmidt et al., 1999 . Fractionation of the organic

matter of lignite-containing mine soils could lead to the separation of organicmatter types present.

In this study, for the first time particle size and density fractionation of alignite-containing mine soil have been used to assess the composition anddistribution of SOM. Organic matter of the physical fractions was analysed forelemental composition and chemical structure by 13C NMR spectroscopy. 14Cactivity measurements were used to determine the lignite contribution. To studythe location of inorganic combustion products, magnetic susceptibility measure-

( )C. Rumpel et al.rGeoderma 98 2000 177–192 179

ments have been carried out. To obtain an indication of the contribution of thelignin component from recent organic matter CuO oxidation of the organicmatter in the particle size fractions was carried out. The objectives of this studywere:

v to identify the allocation of lignite and recently formed humic compounds insoil compartments as separated by physical fractionation procedures, and

v to characterise the recent organic matter present in these fractions.

2. Material and methods

2.1. Study site and sampling

The study site is situated in the Lusatian mining district in the eastern part ofGermany. During open-cast mining operations, the lignite-containing overbur-den material was excavated, relocated and deposited at a spoil bank. Afterdeposition, the site was amended with alkaline ashes from lignite-fired power

Ž .stations to prevent acidification Katzur and Haubold-Rosar, 1996 . These ashesŽ .can contain substantial amounts of OC Rumpel et al., 1998b . After afforesta-

Ž .tion with red oak Quercus rubra , plant litter accumulated at the soil surfaceand in the mineral material to form a mineral soil horizon, rich in recent carbon.During the initial stages of soil development, input of airborne carbonaceousparticles originating from lignite power plants or briquette fabrics may have

Ž .occurred. Soil samples were taken from the forest floor L, Oh horizons . Themineral soil horizon containing recent organic matter was sampled in 0–5 cmdepth and referred to as Ai horizon, the sandy parent material was sampled at 1

Ž .m depth Cv horizon . The designations of the soil horizons were chosenŽ .according to AG Bodenkunde 1994 . During sampling, SOM derived from the

decomposition of plant litter could not be distinguished morphologically fromthe lignite inherent to the parent material. Chemical parameters of the investi-gated soil are shown in Table 1.

Table 1Chemical parameters of the investigated soil

Ž . Ž . Ž .Sample Sampling pH H O OC grkg N grkg CrN Lignite carbon2Ž . Ž .depth cm % of total C

L 1 4.6 462 6.8 68 n.d.Oh 2 6.7 224 11.1 20 21.5"0.5Ai 0–5 6.9 110 5.5 20 47.0"0.4Cv 100 3.2 37 0.4 93 96.2"0.3


2.2. Sample pre-treatment, pH and magnetic susceptibility, carbon and nitrogencontent

Roots and visible plant remains were mechanically removed from the sampleswhere possible. The samples were air-dried and the fraction )2 mm wasremoved by dry sieving. For chemical analyses an aliquot was ground. The pHvalues were measured with a glass electrode in the supernatant of a 2.5r1Ž .wrw waterrsoil suspension. Magnetic susceptibility was measured with aForgenta ferromagnetic analyser FMA 5000. For calibration a Kappa-BridgeKY-2 was used. C and N measurements were recorded with a Leco CNH 1000analyser.

2.3. Particle size fractionation

Particle-size fractionation was carried out after ultrasonic dispersion after theŽ .)2 mm fraction had been removed by dry sieving Christensen, 1992 . The

amount of ultrasonic energy necessary for complete dispersion of the aggregatesŽ .was calibrated following the procedure suggested by Schmidt et al. 1999 . The

calorimetrically determined energy input was 150 Jrml. Three sand fractionsŽ .2000–630, 630–200 and 200–63mm were obtained by sieving. The clay

Ž . Ž .fraction -2 mm and the three silt fractions 63–20, 20–6.3 and 6.3–2 mmwere recovered after sedimentation. The soil suspension was filtered using

Ž .polysulfone membrane filters -0.45 mm . The solid material remaining on thefilter paper was recovered, freeze-dried and stored in a brown-glass bottle. Theaqueous phase recovered was analysed for its DOC content with a TOC-analyser.

2.4. Density fractionation

Ž 3The density fractionation to obtain three fractions -1.6 grcm ; 1.6–2.0 and3. Ž Ž .)2.0 grcm was carried out using sodium polytungstate Na H W O =6 2 12 40

. Ž .H O as suggested by Golchin et al. 1994 . Five grams of a soil were covered2

with approximately 50 ml of sodium polytungstate solution. Thereafter, anultrasonic treatment was applied to destroy aggregates. The suspension wascentrifuged at 3000 rpm for 20 min to separate fractions of different densityŽ 3 3.-1.6 grcm ; 1.6–2.0 and )2.0 grcm . The fractions were recovered by

Ž .filtration under suction on filterpaper Whatman, GFA and dried at 408C.

2.5. 14C actiÕity measurements

The 14C activity measurements of bulk soil samples were carried out using theconventional macro-technique of liquid scintillation as described by Becker-

Ž .Heidmann et al. 1988 for the whole soil samples. Particle size as well asdensity fractions contain small amounts of carbon. Therefore 14C activity

Žmeasurements were carried out by accelerated mass spectrometry Vogel et al.,


. Ž .1987 . The lignite content of the sample, M % can be obtained by correctingthe measured 14C activity for the elevated 14C activity in recent plant material, k,which occurs in mixture with lignite carbon, according to

14Ms 1y C activityrk =100. 1Ž .Ž .The 14C activity of recent plant material has increased with regards to the

standard, dating from 1950 due to the nuclear testing back in the 1960s. Thecorrection of the measured values was carried out assuming that the recently

Žformed humic material at the studied sites has an activity of 115 pmC Rumpel.et al., 2000 .

2.6. 13C CPMAS NMR analysis

The solid-state 13C NMR spectra were obtained on a Bruker MSL 100 NMRspectrometer at a frequency of 25.2 Hz. Cross-polarisation with magic angle

Ž . Ž .spinning CPMAS was applied Schaefer and Stejskal, 1976 at 4–5 kHz. The13C chemical shifts were referenced to tetramethylsilane. A contact time of 1 ms

Žwas used. Pulse delays between 250 and 350 ms were chosen Knicker and.Ludemann, 1995 . To remove paramagnetic compounds and to concentrate the¨

carbon content of the soil sample, NMR-analyses were carried out after treat-Ž . 13ment with 2% HF as described by Skjemstad et al. 1994 . The solid-state C

NMR spectra were integrated using the integration routine of the spectrometer.The chemical shift regions 0–45, 45–110, 110–160 and 160–220 ppm wereassigned to alkyl-C, OrN-alkyl-C, aromatic C and carboxyl-C, respectivelyŽ .Wilson, 1987 .

2.7. CuO oxidation

ŽThe amount of lignin was estimated by CuO oxidation Ertel and Hedges,.1984 . The phenolic CuO oxidation products were determined by gas-chro-

Ž .matography Kogel-Knabner, 1995 . It is not possible to quantify the amount of¨lignin in soil directly by any independent method. Therefore, the sum of vanillyl

Ž .V, syringyl S and cinnamyl C units VqSqC released by CuO oxidation wasadopted as an indicator of the amount of lignin present.

3. Results and discussion

3.1. Particle size fractionation

3.1.1. Carbon and nitrogenŽ .In the Ai horizon, the organic carbon OC content ranged from 25–238 grkg

in the particle size fractions and was considerably higher than in the Cv horizonŽ .Table 2 . The OC distribution in the particle size fractions of the Ai horizonwas bimodal with maxima in the 200–63 and 20–6.3 mm fractions. The highest

Ž .nitrogen N content was found in the -20 mm fractions of both horizons. The

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Table 2Chemical and physical parameters of particle size fractions of the Ai and Cv horizon

Sample Total Magnetic OC OC N N distribution CrN Lignitemass susceptibility distribution carbon

y8 3Ž . Ž . Ž . Ž . Ž . Ž . Ž .% 10 cm rkg grkg %C recovered grkg %N recovered % of total C

Ai horizon)630 mm 15 437 25 4 0.1 0 )250 11"0.6630–200 mm 24 231 51 12 0.3 4 )170 41"0.6200–63 mm 29 166 123 33 0.3 31 33 38"0.763–20 mm 11 303 94 9 0.5 6 45 55"0.620–6.3 mm 10 285 238 23 1.6 28 26 30"0.56.3–2 mm 6 436 208 11 1.9 16 23 37"1.9-2 mm 5 476 160 7 2.9 15 15 50"0.7DOC – – – 1 – – – –recovery 98 95 100

CÕ horizon)630 mm 6 0 17 3 0.2 2 85 –630–200 mm 30 0 6 5 0.1 4 60 –200–63 mm 34 0 7 7 0.1 5 70 –63–20 mm 10 14 24 7 0.5 7 48 –20–6.3 mm 7 15 107 22 2.3 22 47 –6.3–2 mm 5 13 155 24 3.7 26 42 –-2 mm 8 16 126 31 3.6 41 35 –DOC – – – 1 – –recovery 99 102 175

The unit of the magnetic susceptibility is 10y8 m3rkg.


CrN ratios ranged from 35 to 85 for the particle size fractions of the Cvhorizon, decreasing from coarse to fine fractions. The same pattern was ob-served for the Ai horizon. However, in comparison to the Cv horizon, lowerCrN values were obtained in the fractions -200 mm. In the clay fraction, theCrN ratio of 15 indicated higher contribution of material rich in N.

In natural soils, most of the OC and N is usually associated with the -6.3Žmm particle size fractions Christensen, 1985, 1992; Turchenek and Oades,

.1979 . This is in contrast to our results which suggest that in the lignite-richmine soil a large proportions of C is present in the sand fractions. Usually,recognisable plant debris, which is easily decomposable, contributes to the

Ž .coarse fractions Ladd et al., 1977; Amato and Ladd, 1980; Bonde et al., 1992 .Decomposition of this macro-organic matter leads to enrichment of nitrogen inthe fine particle size fractions. Our results indicate that this is also true for thelignite-rich mine soil. In the Ai horizon, CrN ratios, although being wider than

Ž .observed for natural soils e.g., Buyanovsky et al., 1994 suggest that wellhumified material is present in the fine particle size fractions. However, only18% of the carbon was found to be associated with the -6.3 mm fractions. Innatural soils, the -6.3 mm fractions usually account for more than 80% of all

Ž .carbon e.g. Christensen, 1992 . Thus, our results show that not much carbon ispresent in well humified form in comparison to natural soils.

3.1.2. Magnetic susceptibilityAsh input into soils can be followed by determination of the magnetic

susceptibility which increases in particular particle size fractions of the mineralŽ .soil after ash addition Rumpel et al., 1998a . To analyse the location of ash

particles originating from the ash-amelioration practice or airborne contamina-tion, the magnetic susceptibility of the particle size fractions was determinedŽ .Table 2 . In the Cv horizon, magnetic susceptibility showed low valuesŽ y8 3 . y8 3-20=10 m rkg , whereas in the Ai horizon values )100=10 m rkg

Ž y8 3 .were recorded. The highest values 437, 436 and 476=10 m rkg weremeasured in the )630, 6.3–2 and -2 mm fractions. These data indicate thatin the Ai horizon mineral ash particles are present in all particle size fractions.Ferrimagnetic particles of airborne combustion products may be present in thefine particle size fractions because these materials contributed mainly to the

Ž-6.3 mm fractions of a forest soil contaminated by those materials Rumpel et. Ž .al., 1998a . Rumpel et al. 1998a,b showed that combustion products contain

Ž .substantial amounts of OC up to 200 grkg Corg and it is thus possible thatorganic matter of lignite-derived ash may add to the carbon content of theparticle size fractions.

3.1.3. 14C actiÕityData from 14C analysis of lignite-containing mine soils provide information

Žabout the quantitative contribution of lignite related OC Rumpel et al., 1998b;


.2000 , and can provide information on the allocation of lignite in specificparticle size fractions. The coarse sand fraction was found to contain 89% recent

Ž .OC Table 2 that originated from plant residues. Lignite-derived carbon waspresent in considerable amounts in all other fractions. The highest lignite

Ž .contribution 50–55% was found in the 63–20 mm and the -2 mm fraction.In natural soils, particle size fractionation yields fractions which separate plant

Ž .debris from metabolic products Oades, 1988 . In lignite rich mine soils, besideplant debris other organic matter types are present and it is hardly possible toseparate organic matter compartments which are solely related to one organicmatter type.

By multiplying the OC content of the fractions with the %C assigned tolignite, the lignite carbon as well as the recent carbon content and distribution of

Ž .the particle size fractions of the Ai horizon was calculated Figs. 1 and 2 .Assuming that the carbon distribution of the Cv horizon represents the

lignite-containing parent material for soil development, its lignite content anddistribution should be the same as those calculated for the Ai horizon. The

Fig. 1. Content and distribution of lignite derived carbon in the particle size fractions of the Aihorizon.


Fig. 2. Content and distribution of OC formed during the decomposition of plant litter in theparticle size frations of the Ai horizon.

observed pattern of lignite content in the particle size fractions of the Ai horizonwas the same as in the Cv horizon, with the highest OC content in the fine

Ž .particle size fractions -6.3 mm . Lignite distribution, however, changedsomewhat with most lignite carbon being found in the 630–6.3 mm fraction ofthe Ai horizon. This may be an indication that beside lignite, carbonaceousparticles of lignite-derived ash are present.

Carbon content and distribution of recently formed organic matter in the Aihorizon remained the same as found in the bulk soil, showing maxima in the

Ž .200–63 and 20–6.3 mm fractions Fig. 2 . A similar bimodal OC and NŽ .distribution was observed by Jocteur Monrozier et al. 1991 for a Vertisol and

an Alfisol. From these results, it was concluded that organic matter in the )50mm fraction is mainly residues from vegetation. In the Ai horizon examinedhere, the first maximum in the 200–63 mm fraction indicated that organic matterin this young soil was present as partly degraded plant material. The secondmaximum of the distribution of recently formed OC in the 20–6.3 mm fractionmay be explained on the basis of selective preservation of recalcitrant carbon

Ž .structures of plant litter Baldock et al., 1992 . The content and distribution of


carbon derived from plant litter and carbon derived from lignite in the coarseparticle size fractions show that organic matter in the young mine soil studiedconsist of carbonaceous particles from lignite-derived combustion products aswell as partly degraded plant material.

3.1.4. 13C CPMAS NMR spectroscopy13C CPMAS NMR spectra of the bulk soil are presented in Fig. 3. The

spectrum of the L horizon is dominated by signals in the O-alkyl carbonŽ .45–110 ppm region, which are characteristic for plant material. The spectrumof the Cv horizon consists mainly of aliphatic and aromatic carbon speciesŽ . 130–45 and 110–160 ppm , characteristic for lignite. The C CPMAS NMRspectrum of the bulk Ai horizon indicates that the organic matter consists of amixture of carbon species related to lignite and carbon species formed during the

Ž .decomposition of plant material Rumpel et al., 1998b .In this study, spectra of the particle size fractions of the Ai horizon were

Ž .obtained Fig. 4 to study for the first time the chemical composition of organic

Fig. 3. 13C CPMAS NMR spectra of the bulk soil of the horizons from the lignite-rich mine soilŽ .data from Rumpel et al., 1998b .


Fig. 4. 13C CPMAS NMR spectra of particle size fractions of the Ai horizon from a lignite-richmine soil.

matter in particle size fractions of mine soils rich in lignite carbon. Spectra ofthe 2000–630 mm fraction show carbon species characteristic for plant material,

Ž . Ž .consisting of lignin 56, 119, 130, 150 and polysaccharides 72 and 100 ppmŽ .Wilson, 1987; Kogel-Knabner, 1993 . Aromatic and aliphatic carbon species of¨lignite material are present in all other particle size fractions. In these fractions,

Žsignals of lignin and polysaccharides also contribute. In the clay fraction -2.mm , higher contribution of O-alkyl and alkyl C can be observed in the

spectrum. The aromatic fraction of this spectrum is dominated by a peak at 130ppm representing unsubstituted or C substituted aromatic rings. Signals of


lignin-derived phenolic carbon can hardly be found. This could be an indicationthat the recent organic matter of this fraction consists of microbial components

Ž .like they were found in the clay fraction of natural soils Baldock et al., 1992 .In the 630–6.3 mm fractions, the aromatic carbon content is high. It isdominated by a sharp peak at 130 ppm. Lignite derived airborne contaminationwas found to increase the content of aromatic carbon species in these fractionsŽ .Rumpel et al., 1998a . For the lignite-containing mine soils, we suggest that thesharp signal at 130 ppm may be assigned to the aromatic structure of carbona-ceous particles of lignite-derived ash. The high aromatic content of the fineparticle size fractions may be due to signals of recalcitrant material of decom-posing plant litter, consisting mainly of aromatic structures of lignin. These

Ž .substances tend to accumulate in the silt fraction Baldock et al., 1992 and mayexplain the aromatic C content of the spectrum of the 20–6.3 mm fraction where

Ž .less carbon was found to be derived from lignite Table 2 .In the 13C CPMAS NMR spectra of lignite-containing mine soils, signals of

plant litter are not dominant. This is in contrast to natural soils where 13CŽCPMAS NMR spectra give evidence of transformation of plant litter Guggen-

.berger et al., 1995 . Our results indicate that lignite carbon contributes to thechemical composition of the organic matter in all particle size fractions.Transformation of recently formed organic matter as particle size decreases ismasked by the contribution of lignite-derived carbon species.

3.1.5. Lignin signatureTo elucidate the contribution of carbon from lignin to the particle size

fractions the lignin component was characterized by CuO oxidation. The highestyields of phenolic CuO oxidation products Vanillyl, Syringyl, and Cinnamyl

Ž .units VqSqC were found in the 630–63 and 6.3–2 mm fractions of theŽ .lignite-rich mine soil Table 3 . The amount of VqSqC in size separates

corresponded well with the amount of VqSqC determined for whole soil. In

Table 3Yield of CuO oxidation products in the particle size fractions of the Ai horizon

Ž .Sample V qSqC grkg C Standard error

)630 mm 19.5 0.8630–200 mm 24.1 2.4200–63 mm 25.7 2.563–20 mm 10.8 1.820–6.3 mm 14.1 0.56.3–2 mm 22.1 0.4-2 mm 10.9 2.1sum of separates 20.5whole soil 20.7 3.5


natural soils the highest amount of VqSqC was also found in sand sizeŽ .fractions Guggenberger et al., 1994 . The data obtained after CuO oxidation

confirm data obtained by 13C CPMAS NMR spectroscopy which indicate thatlignin from recent plant litter can be found in the 2000–63 and 20–6.3 mmfractions. The 63–20 mm and the -2 mm fractions contain little lignin whichcannot account for the aromaticity found in the 13C CPMAS NMR spectra ofthese fractions.

3.1.6. Density fractionsThe highest OC content was recorded in the light fraction of the Ai horizon

Ž .Table 4 . The CrN ratio was lower than the CrN ratio of the lignite-containingparent substrate, indicating that the organic matter of this fraction is derivedfrom decomposing plant material. Lowest OC content with a narrow CrN ratiowas found in the heavy fraction )2 grcm3. In this fraction, clay-associated

Ž .organic material of microbial origin occurs Golchin et al., 1994 . Presumably,this material has a high degree of humification. The contribution of organicmatter with a high degree of humification to lignite-rich mine soils seems to below, as only 3% of the carbon from the Ai horizon was found in the heavyfraction. This result is in contrast to natural soils where most of the carbon was

3 Ž .found in the )2.0 grcm fraction Golchin et al., 1995 . The contribution oflignite carbon to the total OC content was highest in the heavy fraction where

Ž .lignite accounted for more than half of the OC present Table 4 . However, thehighest amount of lignite was found in the light fraction where the lignitecontribution to the OC was lowest. This shows that plant material and ligniteaccumulated in the same density fractions. The recent organic matter still has alow degree of humification and the formation of stable organo-mineral com-plexes has not yet occurred.

The 13C CPMAS NMR spectra of all density fractions show signals character-Ž . Ž .istic for polysaccharides and lignin 56, 72, 105, 119, 150 ppm Fig. 5 .

Additional signals characteristic for lignite in the alkyl and aromatic regionsŽ .0–45 and 110–160 ppm can be observed. This indicates that in all threedensity fractions contribution from plant litter as well as from lignite can be

Table 4Element and lignite content of the density fractions

Ž .Density Mass OC OC distribution N grkg N distribution CrN Lignite3Ž . Ž . Ž . Ž . Ž .grcm % grkg %C recovered %N recovered content

Ž .% total C

-1.6 20 358 70 15.1 42 24 32"0.31.6–2.0 13 218 27 5.9 10 36 45"0.5)2.0 67 5 3 0.4 48 13 54"0.4recovery 100 94 172


Fig. 5. 13C CPMAS NMR spectra of density fractions of the Ai horizon from a lignite-rich minesoil.

expected. Thus density fractionation does not yield fractions which consist ofonly one organic matter type.

4. Conclusion

Particle size and density fractions of the Ai horizon of the lignite-rich minesoil studied consist of a mixture of organic matter types related to lignite andrecent organic matter derived from plant material, except for the 2000–630 mmfraction, which was found to contain large amounts of carbon derived from plantlitter. Aromatic and aliphatic carbon species, which are characteristic for lignitecontribute to all other fractions. Determination of the lignite distribution of theparticle size fractions gave evidence of additional lignite related organic mattertypes, i.e. carbonaceous particles of lignite-derived ash or airborne lignite-de-rived contamination. Carbon analysis and determination of the lignite content inthe fractions showed that the carbonaceous particles are present in the 630–6.3mm fractions. This result is supported by magnetic susceptibility measurements,which indicated that ash particles are present in every particle size fraction ofthe Ai horizon. Analysis of the content and distribution of recent OC in particlesize and density fractions showed that recent organic matter consists mainly ofpartly degraded plant material.


Acknowledgements

The Deutsche Forschungsgemeinschaft is acknowledged for financial support.Ž . 14We are grateful to P. Becker-Heidmann University of Hamburg for the C

Ž .activity measurements. We thank H.-D. Ludemann University of Regensburg¨for providing the NMR facilities. H. Stanjek is acknowledged for help with thecalibration of the magnetic susceptibility.

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Composition and distribution of organic matter in physical fractions of a rehabilitated mine soil...

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