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 CO 2 , 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 12 C (unlabeled) and 13 C (labeled) glucose was used in four experiments. Comparisons with commercial isothermal microcalorimeters confirmed the suitability of CR-12 for soil systems, providing reliable heat, CO 2 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 13 C-labeled glucose was subjected to mass spectrometric analysis (m/z 44 for 12 C-CO 2 ; m/z 45 for 13 C-CO 2 ) 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. • Soil microorganisms drive energy flows, mineralizing organic matter and releasing CO 2 . • Conventional techniques have limitations like low sample throughput and small sample sizes. • A multi-channel macrocalorespirometer (CR-12) was developed for improved soil analysis. • CR-12 provides reliable heat and CO 2 data, as well as calorespirometric ratios in soils. • Combining calorespirometry and mass spectrometry reveals detailed soil carbon dynamics.