Linking mass balances and thermodynamic energy balances in simplified model systems with artificial soils

Soils represent the largest terrestrial carbon (C) sink and understanding its dynamics is crucial. The metabolic degradation and stabilization of soil organic matter (SOM) follow the rules of thermodynamics. In the catabolic reaction, SOM is oxidized to CO₂ and the part of the energy delivered by this reaction is used in anabolism, during which biomass formation and, thereby, energy and C conservation take place. C and energy fluxes are thus linked and contribute to the C transformation and stabilization in natural soil systems. These processes are among others largely influenced by environmental conditions (e.g. temperature, soil moisture, C/N ratio).

Due to the complexity and heterogeneity of soil, thermodynamic models and experimental approaches to study the linkage of C and energy fluxes in soil systems are rare and still in their infancy. To establish it, we use calorimetric and carbon mass balancing methods to study both C and energy fluxes in artificial soil systems in incubation experiments over 64 days with cellulose and over 16 days with glucose as substrates. This simplified system allows reliable measurement and interpretation of energy input, accumulation and output and their interaction with SOM turnover processes. Carbon and Energy Use Efficiency (CUE and EUE) are studied under varying environmental conditions. The heat production rate and the reaction enthalpy of metabolism in artificial soil systems are monitored with isothermal microcalorimeters. C mass balances consist of mineralization (measured using gas chromatography coupled with thermal conductivity detector), changes in total carbon (quantified by elemental analysis - isotope ratio mass spectrometry), and carbohydrates (recorded via a phenol sulphuric acid assay). In addition, biomass and necromass contents are quantified by phospholipid fatty acid and amino sugar analysis.

EUE will be calculated from calorimetric data and further we will build an energy balance model. Furthermore, evolution of carbon input and output measurements will be further utilized for carbon balance model. Calorimetric and respirometric data provide the calorespirometric (CR) ratio of the soil system, which is closely related CUE (Chakrawal et al., 2020; Hansen et al., 2004). Experimentally determined CUE will be compared to that derived theoretically from CR ratio through calorimetric data and biomass yield modelling (Brock et al., 2017). Preliminary results on the linkage between carbon and energy balance in soil systems will be presented.

Brock, A. L., Kästner, M., & Trapp, S. (2017). Microbial growth yield estimates from thermodynamics and its importance for degradation of pesticides and formation of biogenic non-extractable residues. SAR and QSAR in Environmental Research, 28 (8), 629–650. https://doi.org/10.1080/1062936X.2017.1365762

Chakrawal, A., Herrmann, A. M., Šantrůčková, H., & Manzoni, S. (2020). Quantifying microbial metabolism in soils using calorespirometry — A bioenergetics perspective. Soil Biology and Biochemistry, 148 (May), 107945. https://doi.org/10.1016/j.soilbio.2020.107945

Hansen, L. D., MacFarlane, C., McKinnon, N., Smith, B. N., & Criddle, R. S. (2004). Use of calorespirometric ratios, heat per CO₂ and heat per O_2, to quantify metabolic paths and energetics of growing cells. Thermochimica Acta, 422 (1–2), 55–61. https://doi.org/10.1016/J.TCA.2004.05.033