Isothermal Macrocalorespirometry – Novel Instrument Design to Analysis Microbial Metabolism in Soil Systems

Isothermal microcalorespirometry is a non-destructive technique widely used to study terrestrial activity in ecosystems by measuring the heat and the carbon dioxide (CO₂) released by metabolic reactions of soil organisms. Therefore, microbial communities naturally present in the soil play a key role in the C and N cycle thereby releasing heat and CO₂ which are quantitatively related to the matter fluxes via the law of Hess. In order to measure both quantities simultaneously, current methods follow mainly a purely calorimetric approach (absorbent method or GC analysis) ^[1]. In the absorbent method, CO₂ is measured indirectly via the heat released during the absorption reaction in a NaOH-solution (CO₂-trap), which is placed in the sample vessel together with the soil sample. This approach presents a few disadvantages, e.g. indirect CO₂ measurement, small sample size, low sample throughput, low CO₂ partial pressure and oxygen limitation. 

To overcome the drawbacks of the current calorespirometric approach, a newly designed isothermal macrocalorespirometer (IMCR) was developed by combining a classic respirometer and the proven concept of isothermal microcalorimetry. The IMCR is composed of 10 mobile channels placed in a thermally isolated box, water-thermostated at 20°C. Each channel is composed of a heat sink and a heat sensor directly in contact with the sample vessel (calorimetric unit), plus a vessel with a KOH-solution (CO₂-trap) in which a pair of electrodes is immersed (respirometric unit) connected to the channel’s lid. The spatial separation between the two units, the use of electrodes and the size of the channel, make it possible to overcome the disadvantages of the absorbent method (NaOH-solution) mentioned above. The new approach has been successfully tested with glucose-induced microbial metabolic activity in soil samples, allowing the quantification of the calorespirometric ratio . Additionally, TGA-DSC-MS and GC-MS analysis will be performed, necessary to close balances of mass and energy fluxes.

This newly designed IMCR will be applied in the wider frame of calorimetric environmental soil studies, aiming at understanding the carbon dynamics in soil, the latter being known as the biggest carbon pool among the natural matrix. New knowledge in this area support potential solutions for climate change, intimately connected to the global carbon fluxes.

^[1] Wadsö L., A method for time-resolved calorespirometry of terrestrial samples, Methods 76 (2015) 20–26