characterisation of organic matter and carbon cycling in rehabilitated lignite-rich mine soils

CHARACTERISATION OF ORGANIC MATTER AND CARBONCYCLING IN REHABILITATED LIGNITE-RICH MINE SOILS

CORNELIA RUMPEL1 ∗ and INGRID KÖGEL-KNABNER2

1 CNRS, Laboratoire de Biogéochimie des Milieux Continentaux, Université Pierre et Marie Curie,F-75252 Paris, France; 2 Lehrstuhl für Bodenkunde, Technische Universität München, D-85350

Freising-Weihenstephan, Germany(∗ author for correspondence, e-mail: [email protected], fax: (33) 144 27 41 64)

(Received 25 July 2001; accepted 21 January 2002)

Abstract. Open-cast lignite mining in the Lusatian mining district resulted in rehabilitated minesoils containing up to four organic matter types: (1) recent plant litter, (2) lignite deposited by miningactivity, (3) carbonaceous ash particles deposited during amelioration of the lignite-containing parentsubstrate and (4) airborne carbonaceous particles deposited during contamination. The influence oflignite-derived carbon types on the organic matter development and their role in the soil carboncycle was unknown. This paper presents the findings obtained during a six year project concerningthe impact of lignite on soil organic matter composition and the biogeochemical functioning of theecosystem. The organic matter development after rehabilitation was followed in a chronosequenceof rehabilitated mine soils afforested in 1966, 1981 and 1987. A differentiation of the organic mattertypes and an evaluation of their role within the ecosystem was achieved by the use of 14C activitymeasurements, 13C CPMAS NMR spectroscopy and wet chemical analysis of plant litter compounds.The results showed that the amount and degree of decomposition of the recent organic matter de-rived from plant material of the 30 year old mine soil was similar to natural uncontaminated forestsoil which suggests complete rehabilitation of the ecosystem. The decomposition and humificationprocesses were not influenced by the presence of lignite. On the other hand it was shown that lignite,which was thought to be recalcitrant because of its chemical structure, was part of the carbon cycle inthese soils. This demonstrates the need to elucidate further the stabilisation mechanisms of organicmatter in soils.

Keywords: 13C CPMAS NMR spectroscopy, humus, lignite, microbial activity, mine soils, radio-carbon

1. Introduction

In many parts of the world, lignite is an important energy source. Lignite miningand processing accounts for many environmental problems, such as devastatedunfertile land prone to erosion, acid mine drainage and release of combustionproducts of lignite and lignite dust. An extreme example of environmental dis-turbance from lignite-mining is found in the former German Democratic Republic.Lignite was the principal energy source and 80% of the lignite used was suppliedfrom the Lusatian mining district (Hüttl, 1999). To meet the high demands forlignite, specific mining and rehabilitation practices were developed for open-castlignite mining operations in the area (Figure 1). The lignite containing sediment

Water, Air, and Soil Pollution 3: 153–166, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

154 CORNELIA RUMPEL AND INGRID KÖGEL-KNABNER

Figure 1. The Lusatian lignite mining district before 1990.

overlying the lignite seam was removed, dumped at spoil banks and left for rehabil-itation (Häge, 1996). The top soils were not re-used for reclamation and overburdenmaterial was the parent substrate for soil development. The spoil material was richin lignite-derived carbon (up to 50 g/kg Corg) from lignite coal fragments. Ad-ditionally lignite-termed carbon was disseminated by lignitic phytoclast particles,which were present in the fine particle size fractions of the mud and shale material.The spoil material also contained substantial amounts of pyrite. In former times,long-term acidification potential from pyrite oxidation was counterbalanced by theuse of alkaline ashes from industrial burning processes (Katzur and Haubold-Rosar,1996). After increasing the pH-values of the spoil material from 3 to about 6–7 byash addition, the sites were amended with NPK-fertilizer and planted with broad-leaved or coniferous trees. Additionally tremendous amounts of emissions werereleased by the lignite-mining industry and consequently, large areas have beensubject to airborne contamination with combustion products of lignite and lignitedust (Heinsdorf, 1993). Due to low decomposition rates, lignite carbon may remainin soil (Robertson and Morgan, 1995), and may have an impact on the quantity andcomposition of the soil organic matter (SOM) (Rumpel et al., 1998a, b; Schmidtet al., 1996).

Studies concerning the impact of the mining and rehabilitation techniques on thebiogeochemical functioning of those ecosystems as well as on the role of lignitecarbon in the soil carbon cycle are scarce, mainly because of the lack of techniquesfor the differentiation of the organic matter types present in these soils. During asix years project, the composition of the organic matter in these soils was studiedwith a variety of complementary techniques and the biogeochemical functioningof the ecosystem as well as the role of the different carbon types in the soil carbon

CARBON CYCLING IN MINE SOILS 155

cycle were elucidated. The objective of this paper is to give an overview of thesefindings.

2. Differentiation of carbon types present in rehabilitated lignite-richmine soils

To study the occurrence of the different types of organic matter present in thelignite-rich mine soils of the Lusatian mining district, samples were taken fromthe litter layer (Oh and Of horizon), the upper few centimetres of the mineral soiland the parent substrate in 1 m depth of a soil chronosequence under pine forestsof different age (planted in 1966, 1981 and 1987, Table I). A soil chronosequenceis characterized by similar conditions for soil development (i.e. geology, slope,climate, vegetation), the only difference being the time of soil development (Jenny,1980). Additionally a red oak stand planted in 1962 next to the oldest chrono-sequence site was sampled (Table I). All sites had been ash ameliorated as well asNPK fertilized before planting. Bulk soil samples as well as particle size fractionswere analysed for elemental composition, magnetic susceptibility, 14C activity andthe chemical structure of the organic matter.

2.1. ELEMENTAL COMPOSITION AND MAGNETIC SUSCEPTIBILITY

In the rehabilitated mine soils of the Lusatian mining district the contributionof lignite carbon led to carbon contents of the mineral soil between 20 and 177g/kg d.w. (Table I). In the Ai horizon, as well as the forest floor, high magneticsusceptibility values were also recorded. High magnetic susceptibility of the Ofand Oh horizons of Lusatian mine soils indicate that fly ash particles and airbornelignite dust accumulated on the soil surface (Stryszcz, 1993; Rumpel et al., 1998b).Magnetic particles in lignite ash were formed during the combustion process inlignite-fired power plants by the oxidation of pyrite to magnetite (Locke et al.,1986). Therefore, magnetic susceptibility measurements can be used as an indica-tion of addition of fossil fuel combustion products to soils (Beckwith et al., 1986).The carbon content of lignite combustion products such as fly ash can be as high as230 g/kg (Rumpel et al., 1998a, b) indicating that ash additions during ameliorationas well as contamination must be regarded as additional carbon sources in thesesoils.

The results obtained from elemental analysis and magnetic susceptibility meas-urements suggested that up to four organic matter types may be present in re-habilitated lignite-rich mine soils: (1) lignite as deposited with the overburdenmaterial, (2) carbonaceous ash particles derived from amelioration, (3) carbon-aceous particles from airborne input of lignite combustion products and/or lignitedust and (4) recent organic matter derived from plant material (Rumpel et al.,1998a).


TAB

LE

I

Che

mic

alan

dph

ysic

alch

arac

teri

stic

sof

soil

sam

ples

from

are

pres

enta

tive

soil

profi

leat

each

site

Site

Hor

izon

Sam

plin

gpH

Mag

netic

Org

anic

NC

/NL

igni

teal

kylC

+ar

omat

icC

(yea

rof

plan

tatio

n)de

pth

susc

eptib

ility

carb

onO

Cco

ntri

butio

nO

-alk

ylC

+ca

rbox

ylC

(age

in19

98)

(cm

)(H

2O

)(1

0−8

m3/k

g)(g

/kg)

(g/k

g)(%

ofto

talC

)

Scot

spi

neL

2-1

4.3

1945

39.

150

n.d.

n.d.

1987

Of

1-0

5.9

8023

48.

229

42.2

0.79

11ye

ars

Ai

0-2

5.6

8410

22.

541

84.0

1.20

Cv

100

2.5

1436

0.7

5210

0.0

1.60

Scot

spi

neL

3-2

4.7

1750

77.

964

n.d.

n.d.

1981

Of

2-0

5.0

5732

011

.528

29.8

0.84

17ye

ars

Ai1

0-1

5.0

4378

2.4

3368

.90.

84

Ai2

1-3

5.8

3146

1.1

4282

.61.

38

Cv

100

3.0

2020

0.5

40n.

d.n.

d.

Scot

spi

neL

6-5

4.5

1551

49.

455

n.d.

n.d.

1966

Of

5-2

5.0

6936

115

.528

n.d.

n.d.

32ye

ars

Oh

2-0

5.6

161

184

5.4

34n.

d.n.

d.

Ai

0-5

5.5

204

177

4.9

3364

.00.

90

Cv

100

3.3

1436

0.9

4199

.51.

66

Red

oak

L3-

24.

615

462

6.8

68n.

d.n.

d.

1962

Oh

2-0

6.7

158

224

11.1

2021

.40.

96

36ye

ars

Ai

0-5

6.9

274

110

5.5

2047

.00.

96

Cv

100

3.2

1437

0.4

9396

.21.

38

n.d.

=no

tdet

erm

ined

.


Figure 2. Recent carbon accumulation (g C/m2) in the rehabilitated mine soils of different age and anatural forest soil (data from Rumpel et al., 1999).

2.2. 14C ACTIVITY MEASUREMENTS

The quantification of the lignite carbon contribution in the lignite-rich mine soilswas possible due to the measurement of the 14C activity. Lignite was formed duringthe Miocene period 16 million years ago and therefore does not show 14C activity(Rumpel et al., 2000a). The relative 14C activity, which in the case of a mixture oflignite and recent organic matter derived from plant material, equals the recent car-bon contribution, is referenced to a standard defined for 1950, assuming that recentplant material has a 14C activity of 100 percent modern carbon (pMC). However,for the calculation of the lignite content it must be taken into account that in realitythis is not the case. Plant material, after correction for isotopic fractionation, hasthe same 14C activity as the atmospheric CO2 which the plants acquired duringphotosynthesis and incorporated in their tissue. Measurements of the 14C activ-ity of the atmosphere have shown that the 14CO2 concentration has varied since1950 due to atmospheric nuclear weapon testing (Manning et al., 1990; Nydaland Lovseth, 1996; Levin and Kromer, 1997). Thus the 14C activity referenced tothe international standard defined for 1950 with a 14C activity of 100 pMC maygive misleading results. This problem can be overcome by correcting the measuredvalue for the elevated 14C concentration in recent plant material as can be easilydeducted from the isotopic mass balance:

X = [1 − (14C activity/14C activity of recent material)] × 100 (1)

The 14C activity of the recent endmember of the mixture must be estimated locallyfor exact quantification. On the study sites, the 14C activity of the recent plantmaterial can be in the range of 110 to 120 percent modern carbon (pMC) (11 year


TABLE II

Assignment of carbon species to the signals from 13C CPMAS NMRspectra (reference substance: Tetramethylsilane = 0) after Knicker et al.(1993)

Chemical shift (ppm) Assignment

220-160 carboxyl/carbonyl-C

160-110 aryl-C

160-140 aromatic C with O or N substitution

140-120 alkyl substituted or unsubstituted aromatic C

120-110 protonated aromatic C

110-45 O-alkyl C

110-90 acetal- or ketal C

90-60 carbohydrate-C, C-OH groups

60-45 methoxyl groups, α-amide C

45-0 alkyl-C

45-25 CH2 groups

25-0 Terminal CH3 groups

old site) and 110 to 180 pMC (36 year old site) (Levin and Kromer, 1997). Forthe Lusatian mine soils it was shown that the lignite content of the samples can beestimated with a reasonable degree of accuracy (± 10%) by using 115 pMC forrecent plant material (Rumpel et al., 2001a).

The determination of the lignite content and hence the accumulation of recentcarbon derived from plant litter in the bulk samples and physical fractions showedthat (i) lignite carbon can be found in the mineral soil as well as in the forest floor,(ii) it is present in every particle size and density fraction of these soils (Rumpelet al., 2000b), and (iii) the contribution of recent carbon increases with soil age.After 30 years the amount of carbon accumulated in the first few centimetres iscomparable to natural soils (Figure 2, Rumpel et al., 1999).

2.3. CHEMICAL STRUCTURE

The impact of the four organic matter types present in the lignite-rich mine soilson the composition of SOM was studied by 13C cross polarisation magic anglespinning (CPMAS) nuclear magnetic resonance spectroscopy (NMR), a techniqueused in organic matter studies for elucidating the bulk chemical composition. Afterintegration of the 13C NMR spectra the relative contribution of carbon to the four


Figure 3. 13C CPMAS NMR spectra and lignite content of the SOM precursors and the organicmatter mixture present in rehabilitated lignite-rich mine soils.

chemical shift regions assigned to alkyl-C, O-alkyl C, aromatic C and carboxylicC (Table II) can be determined. The 13C CPMAS NMR spectrum of the lignitepresent in the <2 mm fraction of the parent substrate is characterised by signalsindicating a high contribution of aromatic and aliphatic carbon species (0–45 ppmand 110–160 ppm) (Figure 3). Only a small signal can be observed in the O-alkylregion. The spectrum is characteristic of lignite (Meiler and Meusinger, 1991).The 13C CPMAS NMR spectrum of lignite-derived ash only shows a signal in thearomatic region (110–160 ppm) peaking at around 130 ppm, indicating a highlycondensed aromatic carbon structure. Compared to lignite and lignite-derived ash,modern soil organic matter contains large amounts of polysaccharide, protein, andlignin (Kögel-Knabner, 1993). Indicative of those structures are the O-alkyl region50–110 ppm with the peak at 72 ppm and a shoulder at 105 ppm, most prob-ably assigned to polysaccharide structures (Figure 3). Signals at 119, 130, 150


Figure 4. Relationship between data from 13C CPMAS NMR spectroscopy and radiocarbon dating(from Rumpel et al., 2000b).

and 56 ppm may originate from lignin and represent protonated, C-substituted,O-substituted aromatic C and methoxyl C. The peak at 192 ppm can be assigned tocarbonyl groups. In the spectrum of the Ai horizon, a decrease in signal intensityof polysaccharide and lignin carbon can be observed compared to the spectrum ofplant litter (Figure 3). A higher amount of aromatic and aliphatic carbon speciesderived from lignite carbon contributes to this spectrum.

The ratio of signal intensities [A = (alkyl C + aromatic C) / (O-alkyl C +carboxylic C)] was proposed as an indicator for lignite contribution to soils(Schmidt et al., 1996), despite the fact that the analysis of organic matter derivedfrom coal with the CPMAS technique could be biased due to underestimation ofaromatic structures (Snape et al., 1989). However, the CPMAS technique may beappropriate for the structural analysis of lignite carbon because lignite representsan early stage of coalification and does not contain polycyclic aromatics (Hatcher,1988). A decrease in the ratio A in the Ai horizons with age indicated a highercontribution of O-alkyl and carboxylic carbon species, characteristic for recentplant material. These values were in agreement with data obtained by 14C activitymeasurements. For all soil horizons (Table I) as well as particle size fractions (datanot shown), a linear relationship between the ratio A and the lignite contribution asdetermined by 14C activity measurements was established, indicating that a higher


contribution of lignite carbon to soil increases alkyl and aromatic carbon species inthe 13C CPMAS NMR spectra (Figure 4, Rumpel et al., 2000a).

These results show that both methods yield corresponding results for a wholerange of soil samples. 14C activity measurements give quantitative informationabout the lignite-derived carbon in soil. With 13C CPMAS NMR spectroscopychemical structures derived from plant material can clearly be distinguished fromcarbon species derived from lignite.

3. Organic matter development and decomposition

14C activity measurements indicate that at the oldest site, comparable amounts ofcarbon accumulated in the first few centimetres of the lignite-containing mine soilas in natural unpolluted forest soils taken as a reference (Figure 2, Rumpel et al.,1999). 13C CPMAS NMR spectra of the organic matter in the Ai horizons of thesoil profiles were dominated by aromatic and alkyl carbon species characteristicfor lignite, but indicated as well an increasing contribution of carbon species fromdecomposing plant litter with soil age (Rumpel et al., 1999). However, reclam-ation success of lignite mine sites depends on the establishment of functioningbiochemical cycles (Tate, 1985). For the establishment of the carbon cycle, theformation of soil humus is particularly important (Leiros et al., 1993). To obtain acharacterisation of the degree of humification which can give information on thebiogeochemical functioning of the ecosystem, the soil samples were analysed forthe content of polysaccharides, proteins, lignin and lipids by wet chemical meth-ods. After normalisation of the amount of litter compounds to the recent carboncontent, the carbon identified by plant litter compound analysis decreased withincreasing depth and increasing age of the soils. After 32 years the values arecomparable to those of natural forest soils (Table III). These observations wereconfirmed by increasing degree of lignin alteration with stand age and soil depth(Rumpel et al., 1999). The data of wet chemical analysis complemented data ob-tained by 14C activity measurements and 13C CPMAS NMR spectroscopy and ledto the conclusion that 32 years after reforestation the degree of humification ofthe soil organic matter is in the same range as those of natural sites (e.g. Beyeret al., 1993; Wachendorf et al., 1996), showing that the decomposition and hum-ification processes of organic matter derived from plant litter and therefore thebiogeochemical function of the ecosystem is not influenced by the presence oflignite in rehabilitated mine soils.


TABLE III

Content of plant litter compounds in the soil profiles under thechronosequence of pine (data from Rumpel et al., 1999)

Sum of polysaccharides, lignin, proteins and lipids

11 years 17 years 32 years

g litter compound C/kg recent C

L 822 L 751 L 680

Of 495 Of 739 Of 581

Ai 628 Ai 404 Ai 318

4. Carbon cycling in rehabilitated lignite-rich mine soils

After obtaining these results we investigated the potential for lignite decompos-ition in rehabilitated mine soils thereby contributing to the soil carbon cycle. Toaccomplish this objective, the lignite-containing parent substrate, and the upper 20cm of mine soils planted in 1987 and 1962 (age 11 and 36 in 1998) were sampledand incubated for 16 months at 20 ◦C and 50% of the maximum water-holdingcapacity. The carbon mineralisation was monitored during the incubation period.The soil-respired CO2 as well as the microbial biomass were recovered after 6, 12and 16 months and analysed for their 14C activity. Additionally the 14C activityof humic acids extracted from the soils before incubation was determined. By 14Cactivity measurements the contribution of lignite carbon to the various fractionswas analysed, thereby assessing its role in the soil carbon cycle.

14C activity measurements of the CO2 evolved indicated that lignite was miner-alised in the lignite-containing parent substrate as well as in the 11 and 36 years oldmine soil. The average decay rate of lignite was 0.025 and 0.007 g lignite-C kg−1

C yr−1 (Rumpel and Kögel-Knabner, 2001). Radiocarbon analysis of the microbialbiomass showed that lignite is used as a carbon source by micro-organisms inlignite-containing mine soils even at later stages of soil development (Figure 5,Rumpel et al., 2001b). Our data indicate that lignite present in soils is decom-posed and that microbial resynthesis may be an important process affecting thedecomposition of lignite. Microbial decomposition of pure lignite was observed inlaboratory experiments (e.g. Willmann and Fakoussa, 1997; Waschkies and Hüttl,1999). It was shown that the microbial degradation of lignite is carried out byunspecific extracellular attacks similar to the degradation of lignin including ox-idative enzymes, hydrolytic enzymes, alkaline metabolites and natural chelators(Fakoussa and Hofrichter, 1999). Lignite decomposition was assumed to occur asa co-metabolic process and that like lignin it is not used as a carbon source bymicroorganisms. However, Lusatian lignite is at a very early stage of coalification.


Figure 5. 14C activity of the atmosphere, plant litter, soil organic matter, humic acids and microbialbiomass sampled on the oldest rehabilitated mine site.

The NMR spectra show a small contribution of O-alkyl C (Figure 3), indicatingthe presence of polysaccharide material, which could be incorporated into themicrobial biomass.

The high potential of micro-organisms to degrade coals seems to be in contrastto observations under natural conditions which indicated the lignite decompositionis not occurring (Fritsche et al., 1998). To analyse if decomposition of lignitecarbon present in soil is occurring under field conditions, the 14C activity wasmeasured in the humic acid fraction of field samples (Figure 5). The data showedthat lignite carbon was part of the humic acid fraction, indicating that lignite in thesoil is oxidised during biodegradation under field conditions (Rumpel and Kögel-Knabner, 2001). The proportion of lignite extracted as humic acid increased withsoil development. Mineralisation of lignite under field conditions was also con-firmed by the radiocarbon analysis of pine needles as well as the ground vegetationof the 32 year old site (Figure 5). The 14C activity of pine needles sampled fromthe oldest site were consistent with the 14C activity of the atmosphere. The groundvegetation showed a significantly lower pMC value, probably due to a recyclingof CO2 evolved from the soil due to lignite decomposition (Rumpel et al., 2001a).Thus, lignite in soil can be mineralised as well as humified and must be consideredin the soil carbon cycle.


5. Conclusion

The composition of SOM in rehabilitated mine soils containing lignite as well asrecent carbon derived from plant litter could be elucidated by the combinationof 14C activity measurements, 13C CPMAS NMR spectroscopy and wet chemicalanalysis of plant litter compounds. These methods yielded complementary results.Quantification of lignite contribution to the bulk soil as well as various fractionswas achieved by 14C activity measurements. The structure of lignite carbon wasshown to be highly aromatic compared to recent organic matter derived from plantlitter. By the combination of 14C activity measurements and wet chemical analysisit was possible to show that lignite does not have an adverse effect on the decom-position of organic matter in those soils. A laboratory incubation complementedby field analysis indicated that lignite may even be mineralised as well as humifiedand that it was incorporated into the soil microbial biomass. Therefore lignite mustbe considered in the carbon cycle of lignite-rich mine soils. These results showthat organic matter considered to be refractory in soils because of its chemicalcomposition may be decomposed and serve as a carbon source for micro-organismsunder certain conditions. The results of our study indicate the importance of furtherelucidation of the stabilisation processes of organic matter in soils.

Acknowledgements

The Deutsche Forschungsgemeinschaft is acknowledged for financial support. Thestudy was carried out at the Brandenburg University of Technology (Institute ofSoil Protection and Recultivation) under the framework of the Center of Excel-lence ‘Minesite Recultivation’. The authors would like to thank the director of theInstitute of Soil Protection and Recultivation Prof. Dr. R.F. Hüttl as well as thestaff of the analytical laboratory of this institute, in particular G. Franke, H. Köllerand R. Müller, who supported this study with scientific and analytical expertise.Additionally the authors thank A. Mariotti for comments on the manuscript.

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