SOM genesis: microbial biomass as a significant source
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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.
KeywordsSoil organic matter Humic compounds C turnover Microbial biomass Bacterial cell walls Scanning electron microscopy
This study was financially supported by the Helmholtz Centre for Environmental Research UFZ, by the German Research Council (DFG, Kä 887/1 and Mi 598/2) and by the European Commission (ModelPROBE, contract number 213161). We acknowledge long and fruitful discussions about this topic with Reimo Kindler (TU Berlin), Christian Schurig (UFZ) and Gabi Schaumann (University of Koblenz-Landau) who also gave helpful comments on earlier versions of this manuscript. Jörg Ackermann (Nano Technology Systems Division, Carl Zeiss NTS GmbH, Carl-Zeiss-Str. 56, 73447 Oberkochen) provided the EDX data and further helpful comments. The Martin Luther University of Halle granted access to their long-term agricultural experiment “Ewiger Roggenbau” for large-scale soil sampling. We thank two anonymous reviewers for valuable comments, which improved this manuscript significantly.
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