SOM genesis: microbial biomass as a significant source

• Published in: Biogeochemistry 2012-11, Vol.111 (1-3), p.41-55
• Main Authors: Miltner, Anja, Bombach, Petra, Schmidt-Brücken, Burkhard, Kästner, Matthias
• Format: Article
• Language: English
• Subjects: Acid soils; Agricultural soils; Animal and plant ecology; Animal, plant and microbial ecology; bacteria; Biochemistry; Biogeosciences; Biological and medical sciences; biomarkers; Biomass; carbon; Carbon dioxide; carbon nitrogen ratio; Cell walls; cells; degradation; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Ecosystems; Environmental Chemistry; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; fungi; Geochronology; greenhouse gas emissions; groundwater; hydrophobicity; Incubation; Isotope geochemistry. Geochronology; isotope labeling; isotopes; Life Sciences; Microbial biomass; microbial colonization; microbial growth; Organic matter; Organic soils; pollution control; scanning electron microscopy; soil; Soil biochemistry; Soil fertility; Soil management; Soil microorganisms; Soil organic matter; Soil pollution; soil stabilization; Soil water; Soils; spatial variation; Surficial geology; Synecology; SYNTHESIS AND EMERGING IDEAS; Terrestrial ecosystems
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-011-9658-z

Proper management of soil organic matter (SOM) is needed for maintaining soil fertility and for mitigation of the global increase in atmospheric CO₂ 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 ¹³C-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.