Expanding Understanding: Investigating the Information Value of Calorespirometric Ratio in Dynamic Processes of Soil Microbial Growth Using Calorimetry

The preservation of soil services and functions is sustained by the catalytic activity of microbial communities. These communities use energy and carbon – especially for microbial growth – from the transformation of organic matter (OM).  A fraction of this OM is assimilated as carbon source for growth (anabolism). Another fraction is oxidized and the resulting electrons are transferred to various terminal acceptors to derive the energy for growth (catabolism). The distribution of carbon and energy into anabolism and catabolism determines carbon use efficiency (CUE) and energy use efficiency (EUE). Accurate quantification of the relationships between carbon and energy fluxes relies on key parameters like the metabolic heat (Q_m),  calorespirometric ratio (CR), carbon dioxide evolution rate (CER), the apparent specific growth rate (μ_app), and the degree of anaerobicity (η_A).

However, determining these parameters faces challenges at technical (sample size and instrument sensitivity) and experimental (thermal disturbance, sample aeration) levels impacting the precise quantification of energy and carbon flux relationships. To address these challenges under controlled conditions, we examined microbial turnover processes in a model arable soil amended with a readily metabolizable substrate (glucose). We utilized three commercial isothermal microcalorimeters (IMC)  with volume-related thermal detection limits (LOD_V) ranging from 0.05 to 1 mW L^-1.

Comparison between three IMCs were conducted to figure out the influence of LOD_V on measuring accuracy. Calorimetric experiments (half ampoules were closed and half were aerated for 5 minutes on selected days) were compared to explore the effect of oxygen limitation and thermal perturbation on the calorimetric signal. CER was monitored by measuring the additional heat resulting from CO₂ absorption in NaOH solution used as CO₂ trap. The range of errors associated with calorimetrically derived μ_app, Q_m, and CR was determined experimentally and compared with the requirements for quantifying CUE and η_A from a theoretical perspective.

Significant differences in Q_m and µ_app were observed between IMCs which have the lowest and highest LOD_V. IMC with the lowest LOD_v provided the most accurate results. Opening ampoules for gas exchange did not significantly impact Q_m. However, regular ampoule opening during calorimetrically derived CER measurements led to notable measurement errors for CER due to strong thermal perturbation of the signal. If established models are used to calculate CUE and η_A from CR, unrealistically high values are obtained and the accuracy of CR do not fit to the requirements.

There are two ways to cope with this problem. On the one hand, new thermodynamic balance models need to be developed that dispense with the error-prone CR value. On the other hand, new calorespirometric methods must be developed to determine the CR value more reliably. Initial results for both approaches will be presented.