Airborne contamination of immature soil (Lusatian mining district) by lignite-derived materials: Its detection and contribution to the soil organic matter budget

Large areas in eastern Germany have been subjected to substantial airborne contamination by fly ash, soot and lignite dust. The objective of the study was to detect the input of lignite-derived airborne contamination into forest soils and to examine the chemical and structural characteristics of the soil organic matter, consisting of natural humic material and lignite- derived carbon in reforested immature mine soils. The mine soil developed on sandy overburden material that was excavated in open-cast lignite mines and had been relocated and deposited at a spoil bank. Samples were taken from the forest floor (L, Oh), the humic surface horizon (Ai), and the parent substrate (Cv) of an immature mine soil under a 25-year-old red oak (Quercus rubra), situated close to a briquette factory. The conceptual approach includes analyses of bulk soil as well as particle-size fractions for C and N contents, magnetic susceptibility, radiocarbon age and chemical structure by using ¹³C CPMAS NMR spectroscopy. High magnetic susceptibility of the Oh and Ai horizon is the result of airborne contamination by lignite-derived ash. Fly ash contamination consisting of ferrimagnetic minerals contributes mainly to the <20 μm fractions. In the Oh and Ai horizon, 44% and 46% of the C was found to be of anthropogenic origin. Structural information indicates that lignite-derived dust and/or soot are present in the coarse particle size fractions (6.3-200 μm). Anthropogenic C increased the C content as well as the contribution of alkyl and aromatic C species in the organic matter.