Soil Microbial Metabolism: Insights from Heat, CO2 emission and Isotope Analysis Using a Novel Macrocalorespirometer

Soil, as the largest terrestrial carbon sink, plays a pivotal role in the global carbon cycle. Soil microorganisms are fundamental to all biochemical processes in soil, ensuring its fertility and supporting a balanced ecosystem. Through their metabolic activities, these microorganisms drive energy and matter flows, mineralizing organic matter and releasing heat and CO₂, which can be measured via calorespirometry. A key limitation of conventional calorimeters lies in their inability to combine high sample throughput with sufficiently large sample sizes while avoiding oxygen limitation during measurement. In order to overcome these weaknesses, we have developed a multi-channel macrocalorespirometer (CR-12) for soil analysis. To demonstrate its application, agricultural soil (Dikopshof, Luvisol) amended with ¹²C (unlabeled) and ¹³C (labeled) glucose was used in four experiments. Comparisons with commercial isothermal microcalorimeters confirmed the suitability of CR-12 for soil systems, providing reliable heat, CO₂ measurements and calorespirometric ratios that align with known ranges for the aerobic turnover of carbohydrates. To further investigate the incorporation of carbon into the soil organic matter (SOM), a time series of soil samples amended with ¹³C-labeled glucose was subjected to mass spectrometric analysis (m/z 44 for ¹²C-CO₂; m/z 45 for ¹³C-CO₂) using thermogravimetry-differential scanning calorimetry-quadrupole mass spectrometry (TG-DSC-QMS). The integration of calorespirometric and mass spectrometric data demonstrated that combining these complementary techniques provides more detailed information on the fate of microbial carbon and energy turnover within SOM.