 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com 14 DOI: 10.1002/jpln.200700054 J. Plant Nutr. Soil Sci. 2007, 170, 14–26 Review Article Physical carbon-sequestration mechanisms under special consideration of soil wettability § Jörg Bachmann 1 *, Georg Guggenberger 2 , Thomas Baumgartl 3 , Ruth H. Ellerbrock 4 , Emilia Urbanek 5 , Marc-O. Goebel 1 , Klaus Kaiser 2 , Rainer Horn 6 , and Walter R. Fischer 1 1 Institute of Soil Science, Leibniz University Hannover, Herrenhäuser Str. 2, 30419 Hannover, Germany 2 Soil Biology and Ecology Group, Institute of Agricultural and Nutritional Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle/Saale, Germany 3 Department of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia 4 Institute of Soil Landscape Research, Leibniz-Centre for Agricultural Landscape Research (ZALF), 15374 Müncheberg, Germany 5 School of Environment and Society, University of Wales, Swansea, Singleton Park SA2 8PP, UK 6 Institute for Plant Nutrition and Soil Science, Christian Albrechts University zu Kiel, 24098 Kiel, Germany Abstract The protective impact of aggregation on microbial degradation through separation has been de- scribed frequently, especially for biotically formed aggregates. However, to date little information exists on the effects of organic-matter (OM) quantity and OM quality on physical protection, i.e., reduced degradability by microorganisms caused by physical factors. In the present paper, we hypothesize that soil wettability, which is significantly influenced by OM, may act as a key factor for OM stabilization as it controls the microbial accessibility for water, nutrients, and oxygen in three-phase systems like soil. Based on this hypothesis, the first objective is to evaluate new findings on the organization of organo-mineral complexes at the nanoscale as one of the pro- cesses creating water-repellent coatings on mineral surfaces. The second objective is to quan- tify the degree of alteration of coated surfaces with regard to water repellence. We introduce a recently developed trial that combines FTIR spectra with contact-angle data as the link between chemical composition of OM and the physical wetting behavior of soil particles. In addition to characterizing the wetting properties of OM coatings, we discuss the implications of water-repel- lent surfaces for different physical protection mechanisms of OM. For typical minerals, the OM loading on mineral surfaces is patchy, whereas OM forms nanoscaled micro-aggregates together with metal oxides and hydroxides and with layered clay minerals. Such small aggre- gates may efficiently stabilize OM against microbial decomposition. However, despite the patchy structure of OM coating, we observed a relation between the chemical composition of OM and wettability. A higher hydrophobicity of the OM appears to stabilize the organic C in soil, either caused by a specific reduced biodegradability of OM or indirectly caused by increased aggregate stability. In partly saturated nonaggregated soil, the specific distribution of the pore water appears to further affect the mineralization of OM as a function of wettability. We conclude that the wettability of OM, quantified by the contact angle, links the chemical structure of OM with a bundle of physical soil properties and that reduced wettability results in the stabilization of OM in soils. Key words: C sequestration / sorption / nanoscaled micro-aggregates / physical protection / wettability / contact angle / aggregate strength / soil respiration Accepted July 13, 2007 * Correspondence: Prof. Dr. J. Bachmann; e-mail: bachmann@ifbk.uni-hannover.de § Topical Issue Soils as a source and sink for CO 2 – Mechanisms and regulation of organic matter stabilisation in soils (editors: I. Kögel- Knabner and E. Matzner). Synthesis of the DFG Priority Program SPP 1090 (German Research Foundation—“Deutsche Forschungs- gemeinschaft”) 1 Introduction Distribution and accessibility of water, nutrients, and oxygen in soils are important controlling factors for organic-matter (OM) decomposition (Marschner and Kalbitz, 2003; Ekschmitt et al., 2008, this issue, pp. 27–35). With respect to microbial pro- cesses, water governs the convective-diffusive nutrient and enzyme transport and microbial mobility and is important for the aeration status of the soil. Another requirement for OM decomposition is the microbial accessibility of the organic