The objective of the study was to investigate the effect of various doses of municipal-industrial sewage sludge (1, 2.5, 5, 10, and 20%) on selected biochemical parameters of a podzolic soil under willow culture in the second year of the experiment. The effect of the sediment was observable both in the surface horizon and in deeper layers of the soil, though only in the case of some of the tests (cellulose mineralization, nitrification) it was somewhat more pronounced in the Ap horizon. The recorded changes usually intensified with increasing doses of the treatment. The study showed a stimulating effect in the soil of all applied doses of sludge on respiration, cellulose mineralization rate, nitrification and proteolytic activity, and of the higher doses - on dehydrogenases activity. In the case of ammonification, on the other hand, an inhibiting effect of the sludge was observed in almost all of the treatment objects. The observed effect of the sludge on most of the analysed biochemical parameters was relatively consistent in both the surface horizon (0-20 cm) and in the deeper layers of the soil (20-40 cm). Only the processes of cellulose mineralization (in treatment objects with higher doses of sludge) and those of nitrification were subject to stronger stimulation in the Ap horizon. Positive correlations were recorded between almost all of the analysed biochemical tests. An exception was ammonification which showed no correlation with cellulose mineralization and dehydrogenases activity, and negative correlations with the remaining parameters.