All chemical, physical, and biological processes are accompanied by a net flow of heat (either heat production or heat consumption).

Microcalorimetry is a promising tool to fulfil the increasing need for more sensitive ecotoxicity test methods. Microcalorimetry enables detection of very small heat flows. For example the TAM (Thermal Activity Monitor by LKB Bromma) measures +/- 50 nW heat-flow (0.5∙10^-6 °C temperature difference). The response of a testorganism to adverse effects is accompanied by increased or decreased heat production. It means that heat production or changes in the heat production can be a sensitive endpoint in ecotoxicology compared to the traditional ones (i.e. cell counts, number of survived organisms) (Gruiz et al, 2010), because test organisms react to the toxic effects by decreasing (inhibition, lethality) or in some cases, increasing (defence) heat flow.

Well known bacterial (Azotobacter agile), animal (Tetrahymena pyriformis, Panagrellus revividus, Folsomia candida) and plant test organisms (Sinapis alba) were used by Gruiz et al, 2010 to test the toxicity of an artificially contaminated forest soil. The heat response of the test organisms to forest soil contaminated with metals (Cu, Hg and Zn) and spiked with organic pollutants (transformer oil, diesel oil, phenanthrene, cypermetrine, BDNPA and PCP) was measured, evaluated and interpreted. The activity of the soil microflora itself could be also measured by this method, but in this case the heat flow of the uncontaminated forest soil was measured. To receive the best signals the characteristics of the used test organisms (cell concentration, number of animals and plants, age) and the test conditions (temperature, moisture-content, etc.) were optimized.

Before the measurement the sterile soil samples were weighed into 3–5 ml (in some cases 2 ml) sterile glass ampoules and after adequate preparation (i.e. wetting, nutrient addition) the test organisms were introduced into the system. The ampoules were then tightly closed and the temperature change was monitored in the samples kept for 24-72 hours at constant temperature (25 °C) in the microcalorimeter. The power-time curve of the samples was obtained real time during the measurements. The maximum heat flow, the maximum time elapsed, the slope of the curve and the area under the curve etc. could be read from the power-time curve to predict soil toxicity.

Gruiz et al, 2010 concluded that microcalorimetry is a more sensitive method compared to the conventional test methods in case of all three test organisms (Azomonas agilis, Sinapis alba and Folsomia candida).

Feigl et al, 2014 conducted microcalorimetric assessment of a red-mud and red mud soil mixture (originated from the Ajka red mud flood area) and of red mud containing soil substitutes to determine their microbial activity and ecotoxicity. Feigl et al, 2014 concluded that 5% red mud added to waste soil (removed from Budapest metro construction) or 10% red mud added to acidic sandy soil can be tolerated by the test organisms (Sinapis alba and Tetrahymena pyrifomris) and increased the microbial activity of both soil types.