Techniques for the differentiation of carbon types present in lignite-rich mine soils
Introduction
Lignite mining in the Lusatian mining district, in the eastern part of the Federal Republic of Germany is carried out in open-cast mines. After excavation and relocation, the sandy overburden is deposited at spoil banks. The overburden material contains lignite carbon of up to 5% w/w. After afforestation with deciduous or coniferous trees, plant litter accumulates and is incorporated into the mineral soil. Because of the dark colour of the parent substrate, humified materials formed during the decomposition of plant litter cannot be distinguished by macromorphological observation from lignite. Differentiation of lignite and recent carbon is necessary to quantify the accumulation of recent carbon during soil genesis and to assess the degree of humification in these soils (Rumpel et al., 1999).
While fresh plant material is mainly composed of carbohydrates, protein, lignin and lipids, lignite contains a considerable portion of aliphatic and aromatic structures, which were formed during coalification. 13C CP/MAS NMR spectroscopy was used to study changes occurring during humification (Preston, 1996) as well as coalification (34, 2). It was also shown that structural analysis by 13C CP/MAS NMR can provide an indication of lignite carbon contribution to soils (25, 14, 21, 22, 23). However, overlapping signals in the spectra do not allow for quantitative estimates of the lignite contribution to the spectra. Therefore, additional techniques are necessary in order to distinguish the two carbon types. Lignite is composed of carbon which was deposited several million years ago and consists only of stable carbon isotopes. Therefore, radiocarbon measurements can be used to quantify the lignite carbon contribution to the organic matter mixture of lignite-rich mine soils (Rumpel et al., 1998a). Another promising approach to differentiate carbon types which developed under different conditions is high-energy UV photo-oxidation. This technique was applied by 8, 30 to detect charred organic matter in soils.
In the present study, solid-state 13C NMR spectroscopy was applied to bulk soil samples and physical fractions of the surface soil horizon containing humified plant material (Ai horizon) of lignite-rich mine soils to elucidate structural differences of the carbon types. Additionally the samples were analyzed for radiocarbon activity to quantify the lignite content. The results were correlated with structural characteristics as revealed by 13C CP/MAS NMR spectroscopy. Selected samples were subjected to high-energy UV photo-oxidation.
Section snippets
Sampling
Prior to soil sampling an investigation of rehabilitated mine sites was carried out. A grid sampling and inventory of soil chemical parameters showed a great variability (21, 22). For soil sampling, soil profiles were obtained from lignite containing mine soils where the chemical parameters were found in the medium range of all samples taken in the grid. The soil profiles are located under a chronosequence of pine stands (Black pine, 11 years old, Scots pine, 17 and 32 years old) and a red oak
Radiocarbon measurements of the soil organic matter
Measurements of the radiocarbon activity of the sampled soils show that in the bulk mineral soil 47 to 100% of all carbon is derived from lignite (Table 1, Table 2). Lignite carbon contributions of less than 100% as is observed in the Ai horizons indicate that the carbon derived from lignite and from plant litter are mixed in this horizon (Rumpel et al., 1998a). In the Ai horizons of the pine chronosequence, the contribution of recent carbon derived from plant litter increases with stand age
Conclusion
It was shown that 14C activity measurements and 13C NMR spectroscopy can be used to differentiate lignite carbon and recent carbon in soils containing mixtures of both carbon types. Both methods yield corresponding results for a whole range of soil samples. 14C activity measurements give quantitative information about the lignite-derived carbon in soil. Using 13C CP/MAS NMR spectroscopy chemical structures derived from plant material can clearly be distinguished from carbon species derived from
Acknowledgements
The authors would like to thank the Deutsche Forschungsgemeinschaft for financial support. J. Taylor and S. Grocke, CSIRO, Adelaide, Australia are acknowledged for their help with the photo-oxidation work. We thank Dr. P. Clarke who recorded the 13C NMR spectra of photo-oxidised fractions. Dr. P. Becker-Heidmann, University of Hamburg, is acknowledged for the radiocarbon dating.
Associate Editor—C.E. Snape
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