Soil organic matter characteristics as indicator of Chernozem genesis in the Baltic Sea region Andre Acksel a , Wulf Amelung b , Peter Kühn c , Ernst Gehrt d , Tom Regier e , Peter Leinweber a, ⁎ a Soil Science, University of Rostock, Justus-von-Liebig-Weg 6, 18051 Rostock, Germany b Institute of Crop Science and Resource Conservation (INRES), Division Soil Science, University of Bonn, Nussallee 13, 53115 Bonn, Germany c Department of Geosciences, Research Area Geography, Laboratory of Soil Science and Geoecology, Eberhard Karls Universität Tübingen, Rümelinstr. 19-23, 72070 Tübingen, Germany d State Authority for Mining, Energy and Geology of Lower Saxony (LBEG), Stilleweg 2, 30655 Hannover, Germany e Canadian Light Source Inc., Saskatoon, Saskatchewan S7N 5A8, Canada abstract article info Article history: Received 21 September 2015 Received in revised form 1 April 2016 Accepted 4 April 2016 Available online 11 April 2016 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) spectrosco- py 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 C org ) exceeded those in the Ap/Ah-horizons (6.7 to 12% of C org ) 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; r 2 = 0.539**). In summa- ry, 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. © 2016 Elsevier B.V. All rights reserved. Keywords: Anthropo-pedogenesis BPCA Soil color SOM Fire Mass spectrometry XANES 1. Introduction Soils in some areas in the Baltic Sea region have unusually deep mollic horizons that are diagnostic for Chernozems (chroma ≤ 3.5, value ≤ 3.5; thickness N 40 cm; WRB, 2014). In the German soil classifi- cation these mollic horizons, likewise diagnostic for Chernozems (thickness N 40 cm), are designed as Axh-horizons (“x” for biogenically mixed) (Boden, 2005). These deep black horizons are nowadays below the Ap- (ploughed) and Ah- (nondisturbed humic topsoil) horizons, which are affected by the recent organic matter cycling. Understanding the genesis of the Axh-horizons and their specific properties is the clue to explain the soil genesis. These black soils lay outside of the typical dis- tribution areas of Chernozems in central Europe loess landscapes (Böttcher, 1959; von Bülow, 1938; Diemann, 1973). The soil organic matter (SOM) composition resembles typical Chernozems worldwide more than other major soil units (Thiele-Bruhn et al., 2014) though parent material, climatic conditions and the potential natural vegetation in the Baltic Sea area are atypical for Chernozem landscapes (Albrecht and Kühn, 2011). Up to now it is still assumed that the Chernozems in Germany are relic soils, which have been developed during the Late Glacial and early Holocene (Altermann et al., 2005; Eckmeier et al., 2007). Numer- ous further theories and explanations for the genesis of Chernozems and their preservation exist, but none is generally accepted (Eckmeier et al., 2007; Gehrt, 2000). It is discussed that the clearing of forests by slash and burn and subsequent development with burning of agricul- tural vegetation residues, soil tillage and amendments with combustion and other organic residues were important factors in the development of mollic topsoils which are characteristic for Chernozems (Eckmeier et al., 2007; Gerlach et al., 2006). Various authors showed that charcoal and other combustion residues, also termed “black carbon” (BC) are characteristic components of the SOM in Chernozems (Glaser and Amelung, 2003; Rodionov et al., 2006; Geoderma Regional 7 (2016) 187–200 ⁎ Corresponding author. E-mail address: peter.leinweber@uni-rostock.de (P. Leinweber). http://dx.doi.org/10.1016/j.geodrs.2016.04.001 2352-0094/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geoderma Regional journal homepage: www.elsevier.com/locate/geodrs