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We studied the chemical and stable carbon isotope composition of soil organic matter at a mine site, rehabilitated for soil development 40 years ago. The site contains two types of organic matter: fresh plant material and lignite inherent to the parent substrate. A transect was sampled, consisting of mine soil rehabilitated with forest, partially submerged sediment and submerged sediment. Our objective was to assess whether the stable carbon isotope signature is related to the chemical composition of the organic matter. We analysed the chemical composition using 13 C cross polarization, magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy and lignin parameters determined after CuO oxidation. The organic matter was most enriched in 13 C in the bulk soil of the forest plot and partially submerged plot. The stable isotopic signature (δ 13 C) was related to the chemical composition of the sample as seen with 13 C CPMAS NMR spectroscopy. The ratio (alkyl + aromatic)/(O-alky + carbonyl C) was strongly correlated with the δ 13 C value regardless of the site conditions (aerobic soil or submerged sediment). Such a correlation was also observed between δ 13 C and lignin content (V + S + C, where V = vanillyl, S = syringyl, C = cinnamyl oxidation products), as well as parameters indicating lignin diagenetic state [(Ac/Al)V, S/V and C/V, where Ac = acids, Al = aldehydes]. Taking δ 13 C as an indicator for lignite contribution, we explain the relationships by an increasing influence of lignite contribution not only on bulk chemical composition but also on lignin parameters when the organic matter shows 13 C enrichment.

The mine soils and sediments in the Lusatian open-cast lignite mining district (Germany) contain substantial amounts of lignite in addition to recent organic matter derived from plant litter. Knowledge of organic matter (OM) types and their transformation in mine soils and sediments is essential for the evaluation of ecosystem functioning in this region. The aim of the study was to use the isotopic signature of OM to: (1) determine lignite C contribution and (2) elucidate the contribution of OM derived from plant litter to three physical fractions in order to deduce its degree of degradation in three contrasting environments. We used stable and radioactive C isotopes to quantify the contribution of lignite C and to characterize humification processes occurring under contrasting vegetation and redox conditions. The upper mineral soil/sediment, the litter layer and the vegetation of three plots were sampled along a transect covering a forest soil, a partially submerged sediment at the land–water interface characterized by fluctuating water levels, and a constantly submerged sediment. The soil/sediment samples were fractionated into a macro (>250 lm), a humus (material separated by flotation) and a sand (mainly mineral material) fraction to follow the transformation processes of plant and lignite. Radiocarbon analysis of the humus fraction showed a lignite C contribution ranging from 20% to 80% of total organic carbon (OC). The 14 C activity was correlated with the d 13 C value (r 2 = 0.95). Even if not precise, in this case the correlation was used to get an idea about the lignite contribution in coal mining-impacted freshwater sediment and soil. The 13 C data showed that lignite C contribution increases with depth in every fraction of the forest and partially submerged plots. At the submerged plot, the humus and macro fraction were almost free of lignite C, suggesting high amounts of fresh plant material at this plot. This was confirmed by the highest contribution (up to 40%) of the macro fraction, which is mainly composed of fresh plant material, to the total OC content in this plot. In the first 5 cm of all plots, most OC was found in the humus fraction. Combined elemental and isotopic analysis of the three physical fractions indicated that high amounts of humified OM were present in the constantly submerged sediment, whereas most intensive degradation of OM was occurring in the partially submerged sediment at the land–water interface. Here, d 13 C values show that lignite C contributes to