Energetic Return on Investment Determines Overall Soil Microbial Activity
54 Pages Posted: 3 Jan 2022
Microbial communities are a critical component of the soil carbon (C) cycle as they are responsible for the decomposition of both organic inputs from plants and of soil organic C. However, there is still no consensus about how to explicitly represent their role in terrestrial C cycling. The objective of the study was to determine how the energetic properties of organic matter affect the metabolic activity of the resident microbial communities in soils. This was achieved by cross-amending six soils with organic matter extracted from the same six soils and measuring heat dissipated due to the increase in microbial metabolic activity. The energetic properties of the organic matter were used to estimate a potential energetic return on investment that microbial communities could obtain from the transformation of the organic matter. Specifically, the energetic return on investment (ROI) was calculated as the ratio between the total net energy available (ΔE) and the weighted average standard state Gibbs energies of oxidation half reactions of organic C (ΔG°Cox). ΔE was measured as the heat of combustion using bomb calorimetry. ΔG°Cox was estimated using the average nominal oxidation state of C (NOSC) of the molecular species in the organic matter. We show that the potential energetic return on investment was positively related to the overall metabolic activity of microbial communities. However, the observed temporal differences in metabolism across soils indicate that bacterial communities do not exploit the potential energetic return on investment in the same ways. Overall, our results suggest that microbial communities preferentially use organic matter with a high potential energetic return on investment.
Keywords: Microbial Reaction Energetics, Community Composition, Ultra High Resolution Mass Spectrometry, Calorimetry, Organic Carbon
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