The authors investigated the impact of different sewage sludge doses on the developmental dynamics of soil microorganisms as well as on soil respiration activity. The following soil combinations were employed: control (soil + NPK), 2 t d.m. of sewage sludge·ha^-1·year^-1 + NPK, 4 t d.m. sewage sludge·ha^-1·year^-1 + NPK and 8 t d.m. sewage sludge·ha^-1·year^-1 + NPK.
The experiments were carried out in field conditions. During the first year of analyses, winter rye was used as the test plant, while in the second year rye was replaced by potatoes as test plants. Soil samples for analyses were collected during the consecutive plant developmental stages and, using the plate method, the total numbers of bacteria, fungi as well as pathogenic bacteria from the Salmonella, Clostridium perfringens and Escherichia coli genera were determined. In addition, using the absorption method, the amount of CO₂ released from the soil was also ascertained.
On the basis of the obtained results it was found that the introduction into the soil of organic matter in the form of sewage sludge failed to cause any statistically significant changes in the total counts of bacteria and fungi. Moreover, pathogenic bacteria were found in the examined soils (Salmonella sp., Clostridium perfringens and Escherichia coli) and they persisted in the soils even after 66 (Salmonella sp.) and 94 days (Clostridium perfringens and Escherichia coli) of analyses.
The analysis of the CO₂ released on the control plot and on other soil treatments showed that the fertilization of soils with sewage sludge resulted in a slight increase of their respiration activities in relation to the control.