Incorporation of carbon originating from CO₂ into different compounds of soil microbial biomass and soil organic matter†

†Revised version of a paper presented at the 23^rd Meeting of the German Association for Stable Isotope Research (GASIR) September 25–27, 2002,Warnemünde, Germany.

Abstract

In general, soils without the vegetation growing on them are regarded as sources of CO₂. However, there are indications that CO₂ is also fixed by soil microorganisms. Although this process is not significant from a quantitative point of view, it may change the isotopic composition of soil organic matter. Therefore, we conducted an incubation study with soil and ¹³C-labeled CO₂ to investigate this process. We found that the label was transferred from CO₂ into organic compounds in soil. At the end of a 61-day incubation period, 1.3 μmol C g⁻¹ soil, corresponding to ∼0.08% of the soil organic carbon, had been fixed. CO₂ may, therefore, be an additional source of soil organic carbon. Compound-specific analysis of amino sugars, amino acids, and fatty acids indicated that the label is incorporated into microbial, mainly bacterial, biomass. All groups of microorganisms were involved in the assimilation of CO₂, but the relatively high enrichment of mono-unsaturated and mid-chain branched fatty acids indicates that gram negative bacteria and actinomycetes may be slightly more important in this process than other groups of microorganisms.

†Revised version of a paper presented at the 23^rd Meeting of the German Association for Stable Isotope Research (GASIR) September 25–27, 2002,Warnemünde, Germany.

Keywords:

• Amino acids
• Amino sugars
• Carbon-13
• Carbon dioxide
• Fatty acids
• Microorganisms
• Soil organic matter

Acknowledgements

The study was financially supported by the Deutsche Forschungsgemeinschaft (Priority Program 1090). Matthias Gehre (Department of Analytical Chemistry, UFZ) supported us in the acquisition of the compound specific isotopic compositions, Ingrid Flügel (Department of Analytical Chemistry, UFZ) conducted the analysis of bulk δ¹³C of the soil samples.

Notes

†Revised version of a paper presented at the 23^rd Meeting of the German Association for Stable Isotope Research (GASIR) September 25–27, 2002,Warnemünde, Germany.