Types and chemical composition of organic matter in reforested lignite-rich mine soils

In the post-mining landscapes of Lusatia, forest soils develop from extremely acid, lignite-rich open cast mine spoils. The sites have been ameliorated with ash from lignite-fired power stations prior to afforestation. During stand development, incorporation of plant-derived organic matter leads to an intimate mixture with the substrate-derived lignite in the first centimetres of the soil (Ai horizon). The objective of the study was to characterise and to compare the composition of organic matter of mine soils under forest which contain substantial amounts of lignite. Therefore, the forest floor and the mineral soil (Ai and Cv horizon) under a 20-year-old pine and a 36-year-old red oak site were analysed for elemental composition, magnetic susceptibility, chemical structure by cross-polarisation magic angle spinning (CPMAS) ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy and lignite content by radiocarbon dating. The ¹³C CPMAS NMR spectra of the forest floor and Ai horizon reveal signals at 56, 72, 105, 119, 130 and 150 ppm, indicating the presence of carbohydrates and lignin originating from plant material. Additionally, structures characteristic for lignite material (aromatic and aliphatic carbon) could be observed in the Oh and Ai horizons. Using radiocarbon dating, 25 to 83% of the total carbon in these horizons can be assigned to lignite. Lignite carbon may also indicate carbonaceous particles derived from amelioration ash as well as from lignite-derived airborne contamination, which are possible carbon sources of the forest floor and the surface soil.¹³C NMR and radiocarbon dating show that the subsoil (Cv horizon) is dominated by carbon derived from lignite. From these results it is concluded that mine soils, rich in lignite contain up to four organic matter types, namely lignite inherent to the parent substrate, organic matter derived from decomposition of plant residues, carbonaceous particles in amelioration ash and carbonaceous particles from airborne lignite-derived contamination. ¹⁵N NMR spectroscopy revealed that most of the nitrogen of these soils is of recent biogenic origin.