Composition of organic matter in sandy relict and cultivated heathlands as examined by pyrolysis-field ionization MS Steven Sleutel Æ Peter Leinweber Æ Shamim Ara Begum Æ Mohammed Abdul Kader Æ Patrick Van Oostveldt Æ Stefaan De Neve Received: 17 September 2007 / Accepted: 10 June 2008 / Published online: 24 June 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Unusually high SOC levels have been reported for sandy cropland soils in North-Western Europe. A potential link with their general heathland land-use history was investigated by comparing two soil pairs of relict heathland and cultivated former heathland in the Belgian sandy region. A sequential chemical fractionation yielded similar sizes in corresponding SOM fractions between the heathland and cropland soils (i.e. NaOCl resistant: 12.3–15.0 g C kg -1 and NaO- Cl + HF resistant: 2.6–5.3 g C kg -1 ). Higher amounts of clay sized N in the cropland plots can be attributed to N additions from mineral fertilizers and animal manure. Temperature resolved Pyrolysis Field Ionization Mass Spectroscopy analysis showed that the composition of both relict heathland and cultivated soils was surpris- ingly similar, in spite of over 60 years of intense cropland management. The mass spectra of SOM in both heathland-cropland soil pairs investigated was dominated by signals from lipids, alkylaromatics and sterols. The accumulation of this SOM rich in aliphatics was logically linked to the high input of lipids, long- chain aliphatics and sterols from heathland vegetation and the low soil pH and microbial activity. Based on the relatively high OC surface loadings of HF-extractable OM (13–44 mg C m -2 Fe and 1.2–2.3 mg C m -2 clay), direct organo-mineral bonds between OM and Fe- oxides or clay minerals seem to be only partly involved as a stabilization mechanism in these soils. The distinct bimodal shape of the thermograms indicates that OM- crosslinking could furthermore contribute substantially to SOM stabilization in these soils. This study therefore corroborates the previously proposed view that lipids may be bound in networks of alkylaromatics, the structural building blocks of OM macromolecules. We hypothesize that such binding is able to explain the measured retention of these OM components, even under several decades of cropland management. Keywords Soil Organic Matter  Heathland  Analytical Pyrolysis  Chemical fractionation  Land-use Abbreviations CLSM Confocal laser scanning microscopy MOC and MN Mineral protected organic C and N OC Organic carbon Py-FIMS Pyrolysis Field Ionization Mass Spectroscopy ROC and RN Recalcitrant organic C and N SOM Soil organic matter S. Sleutel (&)  S. A. Begum  M. A. Kader  S. De Neve Department of Soil Management and Soil Care, Ghent University, Coupure Links 653, 9000 Gent, Belgium e-mail: steven.sleutel@UGent.be P. Leinweber Institute of Land Use, University of Rostock, Justus von Liebig Weg 6, 18059 Rostock, Germany P. Van Oostveldt Department of Molecular Biotechnology, Ghent University, Coupure Links 653, 9000 Gent, Belgium 123 Biogeochemistry (2008) 89:253–271 DOI 10.1007/s10533-008-9217-4