Description:    Proper management of soil organic matter (SOM) is needed for maintaining soil fertility and for mitigation of the global increase in atmospheric CO 2 concentrations and should be informed by knowledge about the sources, spatial organisation and stabilisation processes of SOM. Recently, microbial biomass residues (i.e. necromass) have been identified as a significant source of SOM. Here, we propose that cell wall envelopes of bacteria and fungi are stabilised in soil and contribute significantly to small-particulate SOM formation. This hypothesis is based on the mass balance of a soil incubation experiment with 13 C-labelled bacterial cells and on the visualisation of the microbial residues by means of scanning electron microscopy (SEM). At the end of a 224-day incubation, 50% of the biomass-derived C remained in the soil, mainly in the non-living part of SOM (40% of the added biomass C). SEM micrographs only rarely showed intact cells. Instead, organic patchy fragments of 200–500 nm size were abundant and these fragments were associated with all stages of cell envelope decay and fragmentation. Similar fragments, developed on initially clean and sterile in situ microcosms during exposure to groundwater, provide clear evidence for their formation during microbial growth and surface colonisation. Microbial cell envelope fragments thus contribute significantly to SOM formation. This origin and the related macromolecular architecture of SOM are consistent with most observations on SOM, including the abundance of microbial-derived biomarkers, the low C/N ratio, the water repellency and the stabilisation of biomolecules, which in theory should be easily degradable. Content Type Journal Article Category Synthesis and Emerging Ideas Pages 1-15 DOI 10.1007/s10533-011-9658-z Authors Anja Miltner, UFZ – Helmholtz-Centre for Environmental Research, Department of Environmental Biotechnology, Permoserstr. 15, 04318 Leipzig, Germany Petra Bombach, UFZ – Helmholtz-Centre for Environmental Research, Department of Isotope Biogeochemistry, Permoserstr. 15, 04318 Leipzig, Germany Burkhard Schmidt-Brücken, Institute of Material Science, Technische Universität Dresden, Hallwachsstr. 3, 01069 Dresden, Germany Matthias Kästner, UFZ – Helmholtz-Centre for Environmental Research, Department of Environmental Biotechnology, Permoserstr. 15, 04318 Leipzig, Germany Journal Biogeochemistry Online ISSN 1573-515X Print ISSN 0168-2563