Organic matter in constructed soils from a coal mining area in southern Brazil

In southern Brazil, the landscape restoration after the exhaustion of open cast coal mines involves the filling of mine cavities with both pedogenic and geological material. The objective of this work was to determine the content and chemical composition of the organic matter of two constructed soils (24 years and 2 years) in a coal mining area in southern Brazil. An undisturbed Acrisol and geological material from three sites were also sampled. Samples were analyzed for carbon and nitrogen contents by dry combustion and for chemical composition of the organic matter by 13C NMR CPMAS and FTIR spectroscopies. Prior to the spectroscopic analyses, the samples were treated with 10% (m/m) hydrofluoric acid solution, which lead to a carbon enrichment in the sample of 2–46 times. The three coal samples were mainly composed of aromatic C (46–63%) and alkyl C (10–28%), and differed largely in the carbon content (18–312 g kg−1). The C/N ratio of 27 and the proportions of O-alkyl C (26%) and aromatic C (29%) found in the native soil (18 g C kg−1 soil) suggest a coal contamination of the native site. The proportions of O/N-alkyl C (15–17%), alkyl C (22–23%) and aromatic C (39–41%) observed in the organic matter of the A1 horizon of the two constructed soils were intermediate to those of the coal samples and the native soil. In the younger constructed soil the proportion of O/N-alkyl diminished and that of aromatic C increased with depth, whereas in the older constructed soil this trend was less evident. Our results show that, during the 24 years after site reconstruction, the input of vegetation residues diluted the proportion of recalcitrant organic matter. The aromaticity index calculated from the FTIR data (I1620/I2920) correlated positively with the aryl C/alkyl C ratio, obtained from the NMR data, evidencing the applicability of the FTIR index for geological and coal contaminated samples.