Abstract
Abstract. Microorganisms regulate the carbon (C) cycle in soil, controlling the utilization and recycling of organic substances. To reveal the contribution of particular microbial groups to C utilization and turnover within the microbial cells, the fate of 13C-labelled glucose was studied under field conditions. Glucose-derived 13C was traced in cytosol, amino sugars and phospholipid fatty acid (PLFA) pools at intervals of 3, 10 and 50 days after glucose addition into the soil. 13C enrichment in PLFAs ( ∼ 1.5 % of PLFA C at day 3) was an order of magnitude greater than in cytosol, showing the importance of cell membranes for initial C utilization. The 13C enrichment in amino sugars of living microorganisms at day 3 accounted for 0.57 % of total C pool; as a result, we infer that the replacement of C in cell wall components is 3 times slower than that of cell membranes. The C turnover time in the cytosol (150 days) was 3 times longer than in PLFAs (47 days). Consequently, even though the cytosol pool has the fastest processing rates compared to other cellular compartments, intensive recycling of components here leads to a long C turnover time. Both PLFA and amino-sugar profiles indicated that bacteria dominated in glucose utilization. 13C enrichment decreased with time for bacterial cell membrane components, but it remained constant or even increased for filamentous microorganisms. 13C enrichment of muramic acid was the 3.5 times greater than for galactosamine, showing a more rapid turnover of bacterial cell wall components compared to fungal. Thus, bacteria utilize a greater proportion of low-molecular-weight organic substances, whereas filamentous microorganisms are responsible for further C transformations. Thus, tracing 13C in cellular compounds with contrasting turnover rates elucidated the role of microbial groups and their cellular compartments in C utilization and recycling in soil. The results also reflect that microbial C turnover is not restricted to the death or growth of new cells. Indeed, even within living cells, highly polymeric cell compounds are constantly replaced and renewed. This is especially important for assessing C fluxes in soil and the contribution of C from microbial residues to soil organic matter.
Highlights
Over the last decade, numerous studies have demonstrated the role of soil microorganisms in regulating the fate and transformation of organic compounds
The 13C enrichment in amino sugars of living microorganisms at day 3 accounted for 0.57 % of total C pool; as a result, we infer that the replacement of C in cell wall components is 3 times slower than that of cell membranes
The recovery of glucose-derived 13C in the cytosol pool decreased over time, with the largest decline from day 3 to day 10, and remained constant for the following month (Fig. 1)
Summary
Numerous studies have demonstrated the role of soil microorganisms in regulating the fate and transformation of organic compounds. Soil microorganisms produce exoenzymes to carry out the primary degradation of plant as well as microbial polymers to monomers. Further transformations of monomers take place within the microbial cells. Monomeric substances are taken up by the living microorganisms and are partly mineralized to CO2, while. A. Gunina et al.: Turnover of microbial groups and cell components in soil part is assimilated into cell polymers and incorporated into soil organic matter (SOM) after cell death (Kindler et al, 2006). Understanding the fate of substances originated from plants and microbial residues into living biomass is crucial for estimating the recycling of carbon (C) in soil and its stabilization as SOM
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