Energetic Controls on Biodegradation of Organic Matter from Diverse Environmental Sources.

Organic matter (OM) decomposition is a central process in the global carbon cycle, regulating whether OM accumulates as a long-term sink or is mineralized into greenhouse gases. Although multiple explanations have been proposed to what controls OM degradation, the energetic and thermodynamic properties of OM have received less attention, particularly in cross-system studies. We incubated a wide range of OM from soils, sediments, and plant material, simultaneously measuring CO2 production and O2 consumption to quantify decomposition rates and the respiratory quotient (RQ, i.e., CO2/O2 molar ratio) of the degrading communities. We also analyzed various OM properties, including energy content and intrinsic thermodynamic stability (ΔGf, Gibbs free energy of OM formation), to assess how these factors explain the variability in decomposition rates and the metabolic degrading capacity across ecosystems. Our results showed that ΔGf and specific energy content (both expressed in kJ g C-1) can substantially influence the degradation rates across environmental samples, and that OM-mineral interactions can further constrain decomposition. The RQ ranged from 0.34 in low-organic deposits to 1.36 in fresh plant material, showing similar trends to degradation rates and indicating differences in substrate use and metabolic capacity. This is the first cross-system study linking microbial degradation to the energetic signature of OM, revealing how intrinsic energetic properties can explain the variability in carbon persistence across a wide range of natural OM sources.