Composition and distribution of organic matter in physical fractions of a rehabilitated mine soil rich in lignite-derived carbon

After open-cast mining operations in the eastern states of Germany, extremely acid overburden material with a high lignite content (up to 50 g OC/kg soil) is relocated and deposited at a spoil bank. For rehabilitation, these substrates are amended with alkaline ash from lignite-fired power stations and reforested with coniferous or broad-leaved trees. The aim of this study was to identify the allocation of lignite and recently formed humified compounds in soil compartments as separated by physical fractionation procedures. Samples were taken from the soil surface (0–5 cm, Ai 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 analysed for elemental composition, magnetic susceptibility, chemical structure by solid-state 13C CPMAS NMR spectroscopy and lignite content by 14C activity measurements. The contribution of lignin to the particle size fractions was estimated using CuO oxidation. In the bulk soil as well as in the physical fractions, carbon species related to lignite and carbon species related to plant litter occur in mixture. The 2000–630 μm fraction was found to be dominated by plant litter compounds whereas the 630–6.3 μm fractions consist mainly of aromatic and aliphatic carbon, most probably indicative of lignite and carbonaceous particles which were added to the Ai horizon during amelioration and airborne contamination with lignite combustion products. In the <2.0 μm fraction, CuO oxidation indicated that aromatic C compounds are derived from lignite rather than from recent plant lignin compounds. Physical fractionation with subsequent characterisation of the organic matter present in the fractions revealed that recently formed soil organic matter (SOM) consists mainly of partly altered plant residues rather than humic substances.