Tracking the Fate of Microbially Sequestered Carbon Dioxide in Soil Organic Matter

The microbial contribution to soil organic matter (SOM) has recently been shown to be much larger than previously thought and thus its role in carbon sequestration may also be underestimated. In this study we employ <sup>13</sup>C (<sup>13</sup>CO<sub>2</sub>) to assess the potential CO<sub>2</sub> sequestration capacity of soil chemoautotrophic bacteria and combine nuclear magnetic resonance (NMR) with stable isotope probing (SIP), techniques that independently make use of the isotopic enrichment of soil microbial biomass. In this way molecular information generated from NMR is linked with identification of microbes responsible for carbon capture. A mathematical model is developed to determine real-time CO<sub>2</sub> flux so that net sequestration can be calculated. Twenty-eight groups of bacteria showing close homologies with existing species were identified. Surprisingly, <i>Ralstonia eutropha</i> was the dominant group. Through NMR we observed the formation of lipids, carbohydrates, and proteins produced directly from CO<sub>2</sub> utilized by microbial biomass. The component of SOM directly associated with CO<sub>2</sub> capture was calculated at 2.86 mg C (89.21 mg kg<sup>–1</sup>) after 48 h. This approach can differentiate between SOM derived through microbial uptake of CO<sub>2</sub> and other SOM constituents and represents a first step in tracking the fate and dynamics of microbial biomass in soil.