Evaluating microbial activity in composts using microcalorimetry

Microcalorimetric isothermal monitoring was carried out for compost and compost-containing growing medium, to provide fingerprints of compost microbial activity at different conditions. Microbial activity is a key property of composts used in agricultural practice. Two aspects are addressed in this study: (1) microcalorimetric evaluation of compost response to glucose as a model stimulating agent; (2) examination of the impact of compost pre-drying and re-wetting on its biological activity.

Addition of glucose solution to the compost involved a strong increase in microbial activity, which was associated with a significant heat evolution without a lag period. In certain cases, this heat evolution was of complicated shape thus manifesting the heterogeneity of microbial populations in composts. Relation between cumulative energy evolved and the amount of added glucose was found to be helpful in distinguishing between aerobic and anaerobic regime of compost microbial activity. As distinct from non-dried compost or growing mixture, glucose addition to samples pre-dried at 65 °C resulted in delayed heat response with initial exponential-like heat evolution. This delay in heat evolution suggests that biological activity was significantly suppressed upon compost drying. Such a temperature-induced inactivation process might also result in dominance of a relatively homogeneous microbial population which survived the heating, thus involving a smooth, exponential-like initial step of the heat evolution. Noteworthy, addition of water (without glucose) to pre-dried compost results in an outburst of activity as compared with non-dried compost. This heat evolution outburst was also characterized by a lag period and an initial exponential-like phase. The heat evolution obtained with pre-dried compost upon water addition was not related to the compost wetting energy but rather reflected the formation of new carbon source due to the changes in compost organic matter upon heating or to the dead biomass of those microbial populations that did not resist the compost heating/drying.