Soil samples must usually be stored for a time between collection and measurements of microbial metabolic properties. However, little is known about the influence of storage conditions on microbial metabolism when studied by calorespirometry. Calorespirometry measures the heat rate and the CO2 rate of microbial metabolism, where the ratio of heat and CO2 released, the calorespirometric ratio, informs about the nature of substrates being used by microorganisms. Application to soil microbiology is very recent, and little is known about the influence of the common soil preparation practices between collection and analysis on the calorespirometric measurements. For these reasons, the effect of storage at 4 °C on the microbial metabolism was determined by calorespirometry. Results show CO2 production rate decreases with storage time while the evolution of metabolic heat rate is more stable. The calorespirometric ratio increases with storage time in soil samples with organic matter characterized by lower carbohydrate contribution to the total carbon and higher aromaticity and is unaffected in soil samples with lower carbohydrates in the organic matter and higher aromaticity. Therefore, the calorespirometric ratio values may vary for the same soil sample, such that the soil organic matter properties, as well as the time stored at 4 °C, must be considered in interpreting calorespirometric data on soils.
Keywords: CO2 rate; calorespirometric ratio; heat rate; microbial metabolism; soil; storage.