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Published in J. Environ. Qual. 33:628-636 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORT

Plant and Environment Interactions

Chemical Composition of Organic Matter in Extremely Acid, Lignite-Containing Lake Sediments Impacted by Fly Ash Contamination

A. Chabbi*,a,c and C. Rumpelb,c

a Brandenburg Technical University, Faculty of Environmental Science, Department of Soil Protection and Recultivation, P.O. Box 10 13 44, D-03013 Cottbus, Germany
b Lehrstuhl für Bodenkunde, Technische Universität München, D-85350 Freising-Weihenstephan, Germany
c CNRS, Laboratoire de Biogéochimie des Milieux Continentaux, Centre INRA Versailles-Grignon, Bâtiment EGER, Aile B, F-78820 Thiverval-Grignon, France

* Corresponding author (abad.chabbi{at}grignon.inra.fr).

Received for publication February 6, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In the Lusatian lignite mining district of eastern Germany, extremely acid lakes developed during ground water rising after exploitation of lignite in open-cast mines. The reasons of plant colonization (Juncus bulbosus L.) of some lakes exhibiting moderate pH values while others remain extremely acid and unvegetated are unknown. Alkalinity gain may be achieved by addition of alkaline materials and/or decomposition of organic matter. Our objective was to examine fly ash deposition and the resulting changes in organic matter composition in the uppermost 0 to 5 cm of the sediment sampled from vegetated and unvegetated lakes. Bulk soil and particle size fractions were analyzed for elemental composition, magnetic susceptibility, and chemical structure of the organic matter by 13C solid-state nuclear magnetic resonance (NMR) spectroscopy. The lignite content of the samples was estimated by 14C activity measurements. The pH values decreased with increasing depth and the changes in pH were found to be correlated with changes in magnetic susceptibility. Carbon and nitrogen contents were found to decrease with increasing depth. The C to N ratios are consistent with the (i) the presence of decomposing plant residues and/or microbial material such as algae in the upper 0 to 5 cm of the sediment and (ii) the dominance of lignite in the layers below this depth as confirmed by 14C activity measurements. The structural analyses of the particle size separates from the 0- to 5-cm depth were consistent with the presence of organic matter derived from plant material. This study confirms that fly ash is an important source of alkalinity in the upper 0 to 5 cm of the sediment that enhanced plant growth and led to enrichment of the sediment with organic matter derived from plant material.

Abbreviations: CPMAS, cross polarization magic angle spinning • NMR, nuclear magnetic resonance • OC, organic carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BEFORE 1990, open-cast lignite-mining operations in the Lusatian mining district (eastern Germany) had disastrous environmental effects and resulted in devastation of almost 1000 km2 of land (Hüttl, 1998). At these sites, phytotoxic acidic conditions are prevailing due to the deposition of pyrite-containing sediments originating from the Miocene. These sediments are extremely acid (pH 2.5–3) and can contain up to 50 g kg–1 carbon derived from lignite (Hüttl and Weber, 2001).

During the rehabilitation process, which took place after lignite exploitation, the acidity and the low nutrient status of the terrestrial sites were ameliorated by addition of alkaline lignite-derived ash and nitrogen, phosphorus, and potassium fertilizers before the establishment of forest stands (Katzur and Haubold-Rosar, 1996). Due to those measures, after 40 yr of afforestation, an evolution of the element budgets of the soils was noted (Schaaf et al., 1999). The composition of the organic matter as well as the accumulation and degree of humification of recent organic matter derived from plant material becomes similar to natural sites (Rumpel et al., 1999).

In addition to the terrestrial sites, 12 000 ha of acidic lakes are present (Lausitzer Braunkohle Aktiengesellschaft, personal communication, 1996). These lakes developed due to ground water rising in areas of mass deficiency. Because of the pyrite oxidation process taking place in the overburden material, the resulting lakes are extremely acid (pH 2.5–3) with high contents of dissolved Fe, Mn, and Al (Kalin and Geller, 1998; Geller et al., 1998). In contrast to the terrestrial sites, the lakes were not ameliorated and most of them, even after 100 years, show little change of the acidic conditions. The area covered by surface waters will be increased from 1 to 8% after the end of the mining activities in 2020. Therefore, there is a great need to develop rehabilitation strategies for those lakes.

A pioneer plant called bulbous rush is able to survive in the extreme conditions and to colonize the littoral areas of some of the lakes (Chabbi, 1999, 2003; Chabbi et al., 2001). Reasons for the abundance of bulbous rush plants at some sites while others remain unvegetated are not clear. A possible explanation for this may be the amelioration of the extremely acid conditions by the deposition of airborne alkaline fly ash of lignite-fired power plants or briquette factories, which had occurred over large areas as evidenced in forest ecosystems (Hoffmann and Heinsdorf, 1993; Klose et al., 2001). Fly ash was deposited in great amounts during the last 20 yr before reunification in 1990.

Biomass input into the acid mining lakes may aid in the establishment of nutrient cycles in the sediment by decomposition of the plant material and may serve as an electron donor for the microbially induced sulfate reduction, a process that generates alkalinity (Anderson and Schiff, 1987; Peiffer, 1994). Therefore, biomass turnover could be most important for the amelioration of the hostile conditions. In natural lakes, organic matter originating from plant biomass is mineralized as well as humified. Under anaerobic conditions, proteins and polysaccharides are subject to extensive decomposition whereas lignin and lipid material is selectively preserved (Hatcher and Clifford, 1997). The organic matter of anaerobic sediments was found to be a mixture of degraded plant material as well as microbial and/or algal remains (Goñi and Thomas, 2000). The chemical composition of the organic matter in acid lake sediments and the extent to which dead biomass derived from the pioneer plant is decomposed under extreme conditions is not known.

In this study, samples were taken from extremely acid lignite-containing mine sediment colonized by bulbous rush. The conceptual approach included the analyses of bulk soil samples as well as particle size fractions for elemental composition, magnetic susceptibility, and chemical structure by 13C cross polarization magic angle spinning (CPMAS) NMR spectroscopy. The objective of the study was to analyze (i) if fly ash deposition ameliorated the conditions for plant growth in the sediment of a vegetated lake compared with unvegetated lakes and (ii) the chemical composition of a sediment affected by fly ash deposition and addition of bulbous rush biomass.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site
The study was performed at an interconnected chain of strip mining lakes located in the Koyne–Plessa mining district (State of Brandenburg) in the eastern part of Germany (Fig. 1) . The substrate is composed of mainly tertiary sediments containing a lignite-bearing sequence of Miocene age (16 million years old), which was excavated during mining operations. The sandy overburden material had been relocated and deposited as a spoil bank. After the beginning of ground water flooding of the mining area in 1920 (Lausitzer Braunkohle Aktiengesellschaft, personal communication, 1996) several mining lakes developed. Whereas Lakes 76, 78, and 111 do not show any plant growth 80 yr after their establishment, Lake 109 is vegetated with the pioneer species bulbous rush with a plant cover of 80%. This lake is very shallow with maximum depth of 1 m. There is water level fluctuation particularly in summer but it never dries out.



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  		Fig. 1. Illustration of Lake 109 in the Koyne–Plessa mining district (modified from Küsel, 2003).

 
Sampling
On 16 Sept. 2000, three replicate samples were taken from the littoral area of Lake 109 at a water depth of approximately 10 to 15 cm with a sediment corer (8.5-cm diameter, 30-cm length). The samples were transported to the laboratory, where they were sectioned at 5-cm intervals with a thin metal plate. The pH and magnetic susceptibility measurements and total carbon and nitrogen analysis versus depth were performed for the three replicate sediment cores. For particle size fractionation and further chemical analysis of the organic matter of the upper and intermediate layers (0–5 and 5–10 cm), three replicate cores taken from the sediment of Lake 109 were pooled.

Three replicate samples were taken from the littoral area of unvegetated Lakes 76, 78, and 111 at a water depth of approximately 10 to 15 cm with a sediment corer (8.5-cm diameter, 30-cm length) and sectioned at 5-cm intervals. Analysis for pH, magnetic susceptibility, and carbon and nitrogen were performed using the upper and intermediate layers (0–5 and 5–10 cm). For comparison, three replicate sediment samples from natural lake sediment of an adjacent biosphere reserve, which showed extensive bulbous rush vegetation, were taken, sectioned, and analyzed as described above.

The bulbous rush biomass was quantified at the two vegetated lakes (Lake 109 and biosphere reserve) in triplicate using a metal square of 25 x 25 cm (0.625 m2). The harvested material was dried at 60°C until constant weight was reached and weighed.

Sample Pretreatment, pH, and Elemental Analysis
Roots and visible plant remains were mechanically removed from the sediment samples where possible. The samples were freeze-dried and the >2-mm fraction was removed by dry sieving. For chemical analyses an aliquot was ground. The pH (H2O) values were measured with a glass electrode in the supernatant of a 1:2.5 w/v mixture of sediment and water. Magnetic susceptibility of the bulk sediments and various fractions was measured with a FMA 5000 ferromagnetic analyzer (Forgenta, Berlin, Germany) . For calibration and conversion of the measured values to the SI unit 10–8 m3 kg–1 a Kappa-Bridge KY-2 (Advance Geoscience Instruments, Brno, Czech Republic) was used. Carbon and nitrogen concentrations were recorded with a Vario (Hanau, Germany) EL-CNS analyzer.

Particle Size Fractionation
Particle-size fractionation was performed after the >2-mm fraction had been removed by dry sieving. Complete dispersion of the aggregates was achieved by ultrasonic dispersion (Christensen, 1992). The amount of ultrasonic energy necessary for complete dispersion of the aggregates was calibrated following the procedure suggested by Schmidt et al. (1999). The calorimetrically determined energy input was 650 J mL–1 to obtain complete dispersion. The amount of energy needed for complete dispersion is considerably higher than that needed for the dispersion of natural soils due to the high iron oxide content of the samples.

To prevent disruption and redistribution of particulate organic matter, 30 g of soil in 150 mL of water were pre-dispersed using a light ultrasonic treatment (5 min at 100 W corresponding to 60 J mL–1). Thereafter, the sand fractions (coarse, 2000–630 µm; medium, 630–200 µm; fine, 200–63 µm) were recovered by wet sieving (Amelung and Zech, 1999). The clay fraction (<2 µm) and the three silt fractions (coarse, 63–20 µm; medium, 20–6.3 µm; fine, 6.3–2 µm) were recovered after complete dispersion of the <63-µm fractions with a high-energy input (20 min at 300 W corresponding to 650 J mL–1) and sedimentation. The soil suspensions containing the sediment fractions were filtered using polysulfone membrane filters (<0.45 µm). The solid material remaining on the filter paper was recovered, freeze-dried, and stored in brown-glass bottles. The aqueous phase recovered was analyzed for its dissolved organic carbon (DOC) content with a total organic carbon (TOC) analyzer (TOC-5000; Shimadzu, Kyoto, Japan). The DOC data were recorded for mass balance calculations and will not be discussed further in the text. The variation coefficient of the carbon content of the fractions obtained for duplicate fractionation was 5%.

Scanning Electron Microscopy
Selected particle size fractions were examined with a Zeiss (Stuttgart, Germany) scanning electron microscope (SEM) (DMS 962) at 20 kV with a working distance of 25 mm. The samples were mounted on stoops and sputter-coated with gold.

Carbon-14 Activity Measurements
Solid samples needed to be converted to CO2 to be analyzed for their 14C activity. Carbon dioxide was obtained from the sediment samples by ignition at 900°C. The CO2 was reduced to graphite, which was analyzed by accelerator mass spectrometry (AMS) (Nadeau et al., 1998). The {delta}13C ratio of the samples was recorded at the same time in the AMS spectrometer. The 14C activity was corrected for isotopic effects according to Stuiver and Polach (1977). The measurements were performed in the Leibniz Laboratory at the University of Kiel. The AMS measurements are reproducible at 0.3% modern carbon.

Lignite was formed during the Miocene period 16 million years ago and therefore does not show 14C activity (Rumpel et al., 2000b). The relative 14C activity, which in the case of a mixture of lignite and recent organic matter derived from plant material equals the recent carbon contribution, is referenced to a standard defined for 1950, assuming that recent plant material has a 14C activity of 100% modern carbon. However, for the calculation of the lignite content it must be taken into account that in reality this is not the case. The lignite content (X, in %) of the sample can be estimated from the measured 14C activity, Ameasured, and the 14C activity of recent organic matter derived from plant material (Arecent):

[1]

The lignite content of the samples was estimated with a reasonable degree of accuracy (±10%) by using 115% modern carbon for recent organic matter (Rumpel et al., 2003).

Carbon-13 Cross-Polarization Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy
Solid-state 13C NMR spectra were obtained with a Bruker (Billerica, MA) DSX-200 NMR spectrometer. Cross polarization with magic angle spinning (Schaefer and Stejskal, 1976) was applied using a spinning speed of 6.8 kHz. The 13C chemical shifts were referenced to tetramethylsilane and calibrated with glycine. A contact time of 1 ms was used. A ramped 1H-pulse was used during contact time to circumvent spin modulation of Hartmann–Hahn conditions. Pulse delays between 200 and 2000 ms were chosen. To remove paramagnetic compounds present in the sediment samples, NMR analyses were performed after treatment with 10% hydrofluoric acid (HF) according to Schmidt et al. (1997). Solid-state 13C NMR spectra were recorded as free induction decay (FID) and integrated using the integration routine of the spectrometer. The chemical shift regions 0 to 45, 45 to 110, 110 to 160, and 160 to 220 ppm were assigned to alkyl C, O-alkyl C, aryl C, and carboxylic C, respectively. The intensity of the aryl C was corrected for the spinning side bands by adding their intensities to the aryl signal. The variation of integration data of signals due to the treatment of a well-resolved FID (fourier transformation, phasing and baseline correction) is <5% (Knicker, 1993).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Physical and Chemical Parameters of the Sediment of Vegetated Lake 109
Bulk Sediment
At Lake 109, the pH values range from 5.3 in the upper layer (0–5 cm) to 4.5 in the intermediate layer (5–10 cm) of the sediment. Beneath this depth, pH drops to very low values (<3) due to the chemical weathering of pyrite (Table 1). The magnetic susceptibility in the upper and intermediate part of the sediment is 44 and 29 x 10–8 m3 kg–1 and decreases with increasing depth (Table 1). The decrease of magnetic susceptibility versus depth coincides with a decrease in pH (r2 = 0.98, significant at the 0.05 probability level). At Lake 109, biomass values of 431 g dry wt. m–2 were observed (Table 2). Comparable amounts of biomass are growing at an adjacent natural site (biosphere reserve) at similar pH values. This is in sharp contrast to the unvegetated areas of Mining Lakes 76, 78, and 111, which show very low pH values and no magnetic susceptibility in both layers (0- to 5- and 5- to 10-cm depths) (Table 2).


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  		Table 1. Carbon and nitrogen content of bulbous rush vegetation and lignite, pH, magnetic susceptibility, carbon, nitrogen, and lignite contribution in the sediment core versus depth of Lake 109.{dagger}

 

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  		Table 2. Productions of bulbous rush biomass in different lakes and pH, carbon, nitrogen content, and magnetic susceptibility of the upper and intermediate layers (0–5 and 5–10 cm) of vegetated and unvegetated lakes.{dagger}

 
In terrestrial systems, magnetic susceptibility has been used to monitor ash-amelioration activities or as an indicator of the deposition of airborne lignite-derived combustion products because ash from lignite-fired power stations shows strong magnetic properties (Rumpel et al., 1998a, 1998b; Klose et al., 2001). Fly ash obtained its strong magnetic properties during the burning of pyrite-containing lignite in power plants (Locke and Bertine, 1986). Lignite-derived ash is alkaline and has been used in the past to ameliorate the pH of acid overburden sediments. Minimal contents of CaCO3 were noted for the ashes, CaO being their major buffering substance (Katzur and Haubold-Rosar, 1996). It thus appears that alkaline fly ash affected the sediments of Lake 109 and induced an increase in the pH particularly in the upper and intermediate layer of the sediment cores, which correspond to much of the rooting zone of bulbous rush. The high biomass values recorded at this lake (431 g dry wt. m–2), compared with the other extremely acid mining lakes where no magnetic susceptibility was detected (Table 2), suggests that fly ash input may have stimulated the growth of bulbous rush.

The organic carbon content (OC) in the sediment core ranges between 140 and 63 g kg–1 and the nitrogen content (N) between 5 and 1.5 g kg–1 (Table 1). The OC and N concentrations are very high in the 0- to 5-cm sediment interval. The OC and N contents, however, markedly decrease with depth at 15 to 20 cm and then increase slightly again in the deepest layer (20–30 cm). The C to N ratios vary between 26 and 59. The upper 0 to 5 cm of the sediment cores exhibit much lower C to N ratios than were observed in the deeper sediment layers. The bulbous rush vegetation has a higher C to N ratio than was observed for the upper 0 to 5 cm of the sediment (Table 1). Decrease of the C to N ratio usually indicates increasing decomposition of the plant material (Hedges and Weliky, 1989; Dick and Osunkoya, 2000). In upper 0 to 5 cm of the mining sediment, the lower C to N ratio compared with the bulbous rush vegetation is consistent with its degradation occurring in this zone. Another explanation could be the contribution of different source material such as microorganisms including plankton and algae, which have generally very low C to N values. It is interesting to note that in the uppermost 0 to 5 cm of unvegetated lakes, the C to N ratio is between 47 and 53 (Table 2) and thus similar to the values recorded at depth in the sediment of vegetated Lake 109 (Table 1). The C to N ratios do not show a clear variation with depth (10- to 30-cm depth) despite the marked variation of OC and N versus depth. Goñi and Hedges (1995) observed the same patterns in marine coastal sediments and suggested that the organic matter remaining in the sediments and disappearing with depth both have similar C to N ratios. Fresh Lusatian lignite sampled in the seam has a C to N ratio of 60 (Table 1). Weathered lignite may show C to N ratios between 40 and more than 50. Given the C to N ratios measured at the 10- to 30-cm depth (Table 1), the major organic carbon source in these layers may be lignite-derived. This claim is supported by 14C activity measurements, which were used to estimate the lignite contribution (Rumpel et al., 2003). These data show that 91% of the organic carbon in the 5- to 10-cm layer of the sediment core is lignite derived (Table 1). In the 0- to 5-cm section, the lignite contribution is lower, indicating that recent carbon input most probably derived from plant litter of bulbous rush is occurring at this depth. Carbon and nitrogen data are in agreement with Schmidt et al. (1996) and Rumpel et al. (1998b)(2000a) who observed that elevated C contents and C to N ratios of more than 40 were the main characteristics of mine soils rich in lignite-derived carbon or of soils contaminated with brown coal dust. The influence of carbonaceous particles of fly ash usually exhibiting a C to N ratio of more than 100 could not be detected in the bulk samples. To analyze the influence of fly ash and to follow the fate of the bulbous rush biomass, the samples of the 0- to 5- and 5- to 10-cm depth of the sediment were subjected to a particle size fractionation.

Particle Size Fractions
The particle size distributions (% mass) of the upper and intermediate depth (Table 3) reveal contrasting patterns. Clay- and silt-sized particles dominate in the upper 0 to 5 cm (92%), whereas sand-sized particles are dominant in the intermediate 5 to 10 cm (63%). The magnetic susceptibility ranges between 17 and 51 x 10–8 m3 kg–1 in the particle size fractions of the upper layer. Highest values are recorded in the <63-µm fractions. In the intermediate depth, the magnetic susceptibility is similar in all particle size fractions. A possible explanation for the high amount of silt in the 0- to 5-cm layer of the sediment may be the input of fly ash with a high magnetic susceptibility. This assumption is in accordance with results by Sims et al. (1995), who found that the input of fly ash to sandy material increased the total mass of the silt fractions.


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  		Table 3. Chemical and physical parameters of particle size fractions of the sediments profile.

 
To analyze the effect of fly ash particles with a high magnetic susceptibility on the particle size distribution of the 0- to 5-cm sediment layer, the mass difference of the particle size fractions in the two layers was calculated as follows:

[2]
The difference between the masses of the particle size fractions separated from the upper and the intermediate layer correlate with the magnetic susceptibility (r2 = 0.85, significant at the 0.001 probability level; Table 3). A relationship between the magnetic susceptibility and the difference in mass recovery in both sediment layers suggests that airborne input of lignite combustion products led to the changes in mass distribution of the particle size separates in the upper layer of the sediment. This indicates that high amounts of airborne lignite-derived combustion products were deposited in the area. The input of these materials was confirmed by scanning electron microscopy, showing large amounts of spherules, which are indicators of fly ash contamination (Rose, 1996). These particles can be observed in the coarse silt fraction (Fig. 2) .



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  		Fig. 2. Scanning electron microscope photograph of the 63- to 20-µm fraction of the upper 0 to 5 cm. Spherules of inorganic fly ash are clearly visible on the photomicrograph.

 
High amounts of OC and N were found in the clay and silt fractions. The fractions of <20 µm contribute 79 to 90% of the OC and 94 to 91% of the N in the upper and intermediate layers with only minor variations between both depths. The strong association of the OC and N with clay and silt fractions is usually observed in natural soils and taken as an indication for the presence of carbon with a high degree of decomposition (Christensen, 1985, 1992). In lignite-containing soils, lignite is contributing to the OC in the fine particle size fractions (Rumpel et al., 2000a). This may also be the case for the sediment and is reflected by high C to N ratio of the intermediate layer, which has a lignite carbon contribution of 91%. The C to N ratios of the particle size fractions in the upper sediment layer are substantially lower than those of bulbous rush plants. This is consistent with decomposition of the original plant material and/or the presence of algae. As plant debris is usually found in the coarse particle size fractions of soils and sediments (Baldock et al., 1992), C to N ratios between 21 and 35 in the >200-µm fractions of the upper sediment layer may be explained by the presence of bulbous rush debris (see below). Decomposition of this macro-organic matter leads to enrichment of carbon and nitrogen in fine fractions and the more narrow C to N ratios recorded in the clay fraction of the 0- to 5-cm layer (<2 µm, Table 3). This pattern was also found by previous studies on size fractions of soils summarized by Ladd et al. (1977) and Bonde et al. (1992). Additionally, in lake sediments, microorganisms such as algae may be present in the fine particle size fractions. The higher OC and N in the 200- to 6.3-µm fractions together with C to N ratios of >43 suggest that these fractions were subject to contamination with carbon particles resulting from airborne lignite combustion products (Schmidt et al., 2000).

Carbon-13 Cross-Polarization Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy
The 13C CPMAS NMR spectra of bulbous rush root and shoot are characterized by a high contribution of signal intensity in the O-alkyl region (45–110 ppm) of the spectrum (Fig. 3) . The dominant peak at 72 ppm probably represents C2, C3, and C5 carbon atoms of polysaccharides. The signal at 105 ppm is tentatively assigned to the anomeric C1 carbon. Those signals are indicative of cellulose and hemicellulose material, which are present in fresh plant material. Moreover in this region the peak at 55 ppm is a combination of two signals: methoxyl carbon of lignin and {alpha}-C of proteins (Skjemstad et al., 1983). Signals at 119, 130, and 150 ppm may originate from lignin compounds and represent protonated, C-substituted, and O-substituted aromatic C. In the aryl region (110–160 ppm) of the spectra, little signal intensity is found. The signals in the aliphatic region (0–45 ppm) are characteristic for chain C of amino acids, paraffins, or fatty acids. In the carboxyl region of the spectra (160–220 ppm), which is characteristic for carboxyl C of esters, amides, and free carboxylic groups, little signal intensity is found. The spectra of the root and shoot from bulbous rush are similar to those of a land plant (Hiracium pilosella L.) reported by Knicker et al. (2000). The high contribution of O-alkyl carbon may indicate that shoot as well as root biomass of bulbous rush are readily decomposable. This claim is further supported by the high content of nitrogen in the bulbous rush vegetation (Table 1), which has been shown in several studies to accelerate decomposition (Valiela et al., 1984; Enríquez et al., 1993; Dick and Osunkoya, 2000). Signals from aryl and alkyl carbon species dominate the 13C CPMAS NMR spectra of lignite (Fig. 3). Aryl and alkyl carbon species may behave conservatively in terms of decomposition under anaerobic conditions (Hatcher and Clifford, 1997). These signals were found to be characteristic for lignite present in the lignite-containing overburden material of the Lusatian mining district (Rumpel et al., 1998b). Lignite-derived ash shows a main signal at 130 ppm being characteristic for C-substituted aromatic carbon (Fig. 3).



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  		Fig. 3. Carbon-13 cross polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra of bulbous rush shoot and root and spectra of lignite fraction and lignite-derived ash material.

 
Quantitative information on the distribution of carbon species in the organic matter of a sample may be obtained by integration of the peak areas (Knicker and Nanny, 1995). The reliability of such quantification was questioned, with the main problem being the presence of paramagnetic iron, which leads to selective shortening of the relaxation times of some carbon species and thus underestimation of such compounds (Kinchesh et al., 1995). By treatment with 10% hydrofluoric acid (HF), paramagnetic compounds are dissolved and although some carbon is lost by the treatment, the spectra may be ameliorated significantly (Schmidt et al., 1997). In a CPMAS 13C NMR experiment of organic matter, the possibility to observe signals of specific chemical groups varies, depending on the amount of H atoms associated with the 13C atoms, because if these two atoms are not closely associated, the 13C atoms are not detected. Especially condensed aromatic compounds may not be "seen" in a CPMAS experiment by the NMR technique (Snape et al., 1989). Therefore, for samples with significant amounts of carbon with a highly aromatic structure, CPMAS 13C NMR may not give quantitative data (Skjemstad et al., 1997). Aromaticity of coal samples increases with increasing degree of coalification (Stach et al., 1982). Lignite, as found in the soils and sediments of the Lusatian mining district, is one of the earliest stages of coalification and thus aromaticity is low. The underestimation of the aromatic signal is around 10 to 15% (unpublished data). The variation of the contact time in an NMR experiment may further influence the results, especially the distribution of cabonyl and aromatic carbon versus aliphatic carbon. A contact time of 1 ms was reported to yield quantitative spectra for soil organic matter (Fründ and Lüdemann, 1989). Hatcher (1988) employed contact times of 1 ms for CPMAS of lignite samples and suggested that the spectra are quantitative. Based on these evidences, we consider the spectra of the lignite-containing sediments to yield quantitative data with a reasonable degree of accuracy.

The spectra of the bulk 0- to 5- and 5- to 10-cm sediment samples exhibit similar overall characteristics (Fig. 4) . They tend to reflect the high lignite contribution (78 and 91% of total organic carbon) in the sediment as observed by 14C activity measurements. The presence of lignite in soils of the Lusatian mining district increases the contribution of aliphatic and aromatic carbon species of the soil organic matter as indicated by the ratio (alkyl C + aryl C) to (O-alkyl C + carboxyl C) (Rumpel et al., 2000b). When recent organic matter is in an advanced stage of decomposition, microbial products of an aliphatic nature are formed giving rise to the alkyl carbon peak in 13C CPMAS NMR spectra (Baldock et al., 1992; Golchin et al., 1996). Additionally, in sediment samples of this region, signals of algaenans, a stable carbon form derived from algae, may contribute. Studies conducted by Nixdorf et al. (2001) and Lessmann et al. (2000) showed that Chlamydomonas spp. and Ochromonas spp. are the dominant algal species in acidic mining lakes in the Lusatian lignite mining district. It is likely that these two species may be found in our specific lake because of their high plasticity with regards to the pH of the lakes (Huber-Pestalozzi, 1983). Thus, in the alkyl region, the origin of the signals (recent organic matter, algaenan, or lignite) may no longer be distinguished and decomposition of the bulbous rush plant material cannot readily be observed.



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  		Fig. 4. Carbon-13 cross polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra of bulk soil and particle size fractions of acid lake sediments colonized by bulbous rush. Total mass, carbon content, and distribution of the size fractions are listed in Table 3.

 
Quantitative analyses of the 13C CPMAS NMR spectra of the particle size fractions further reflect the strong lignite contribution to the total soil organic carbon. The ratio (alkyl C + aryl C) to (O-alkyl C + carboxyl C) in all size fractions of the intermediate depth (5–10 cm) falls within the range found for lignite (Table 4). In the upper layer (0- to 5-cm depth) only the clay fraction showed a (alkyl C + aryl C) to (O-alkyl C + carboxyl C) ratio intermediate between that of bulbous rush and lignite. The highest contribution of aromatic carbon was recorded in the 200- to 63-, 63- to 20-, and 20- to 6.3-µm fractions of the 0- to 5-cm sediment layer (Table 4). Aryl carbon peaking at 130 ppm (Fig. 4) could be derived from carbonaceous particles present in lignite-derived combustion products (e.g., fly ash), which consist exclusively of aromatic material. These materials were shown to accumulate preferentially in the fine sand and coarse and medium silt fractions (Rumpel et al., 1998b; Schmidt et al., 2000). This is in contrast to the inorganic fly ash particles, which may accumulate preferentially in the silt and clay fraction (see above; Table 3; Rumpel et al., 1998a).


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  		Table 4. Percentage of carbon species obtained from the integration of the 13C cross polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra of bulbous rush plant and bulk sediment, and particle size fractions from the sediment profile.{dagger}

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The chemical composition of extremely acid open-cast mining sediments was characterized by elemental analyses, magnetic susceptibility measurements, and 13C CPMAS NMR spectroscopy of bulk soil samples and particle size fractions. Fly ash input into the upper sediment layer (0–5 cm) as indicated by high magnetic susceptibility leads to pH increase and changes in the particle size distribution favoring the establishment of the pioneer plant bulbous rush. Plant litter input as detected by 14C activity measurements was found to account for 22% of the organic carbon present in the upper 0 to 5 cm of the sediment. A decrease of the C to N ratio in 0- to 5-cm depth compared with biomass of bulbous rush and from sand- to clay-sized particle size fractions in this layer as well as data on the chemical composition of the organic matter as shown by 13C CPMAS NMR spectroscopy are consistent with decomposition of the plant material in this environment.


   ACKNOWLEDGMENTS
 
The study was carried out under the framework of the Alkalinity Research Project financed by the German Ministry for Education, Research, Science and Technology Ministry (BMBF, FKZ 0339746). We acknowledge the helpful and valuable comments of Dr. H. Knicker. We also thank Professor P.M. Grootes (Leibniz Labor für Altersbestimmung und Isotopenforschung, Universität Kiel) for the 14C activity measurements. We are grateful to Mrs. H. Köller, R. Müller, E. Müller, and G. Franke for technical assistance (LS Labor, BTU-Cottbus) and Dr. W. Wiehe (ZAL, BTU-Cottbus) for providing the scanning electron microscope analyses. The valuable comments of the three reviewers and editors, which lead to considerable improvement of the manuscript, are greatly appreciated.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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JEQ 2004 33: 413-418. [Full Text]  




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