Soil organic matter characteristics as indicator of Chernozem genesis in the Baltic Sea region

•Comprehensive organic matter characterization of Chernozems from the Baltic Sea region using a multi methods-approach•XANES, Py-FIMS and BPCAS pointed to the impact of fire and/or charred residues to the mollic horizons•Pyrogenic organic matter resulted from human activity rather than from natural fires, indicating an anthropo-pedogenesis

The genesis of Chernozems in the Baltic Sea region is still insufficiently understood and the composition of its organic matter may be a key for a better pedogenetic interpretation of the mollic, biogenically mixed (Axh) horizons. Therefore, we investigated the composition of soil organic matter in the Ap/Ah- and Axh-horizons from seven soil profiles at the islands of Poel, Fehmarn (Germany), and Sjalland (Denmark). In a multi-methods approach we carried out synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy at the carbon (C) and nitrogen (N) K-edges, pyrolysis-field ionisation-mass spectrometry (Py-FIMS) and soil color measurements as non-specific, bulk SOM characterization methods, and quantified the benzene polycarboxylic acids (BPCAs) as specific markers for black carbon (BC). The XANES- and Py-FIMS-spectra in good agreement revealed relative enrichments of aromatic C and heterocyclic N compounds in the lower Axh- in comparison to the corresponding upper Ap/Ah-horizons. In almost all horizon pairs, the BC portions in the Axh- (7.6 to 18% of Corg) exceeded those in the Ap/Ah-horizons (6.7 to 12% of Corg) and were similar to the BC portions in typical Phaeozems and Chernozems from European loess areas. The ratios of penta- to hexacarboxylic benzoic acids (B5CA/B6CA) of 0.9 to 2.2, mean 1.2, indicated contributions of combustion residues from domestic and grassland fires. This pyrogenic organic matter and its transformation products governed the soil color, which was closely correlated to the BC portions (L*-value = − 0.880 x + 52.13; r2 = 0.539**). In summary, the enrichments of stabilized aromatic and heterocyclic compounds in the Axh-horizons derived from C- and N-XANES, Py-FIMS and the BC-determination are interpreted by an input of pyrogenic matter into the Baltic Chernozems, indicating an anthropo-pedogenesis in the period between Bronze age and the Early Middle Ages.