Impact of dairy sewage sludge on enzymatic activity and inorganic nitrogen concentrations in the soils

Sewage sludge from the treatment wastewater contains nutrients but also contaminants, such as metals, pathogens and organic chemicals. On the contrary, dairy sewage sludges (DSS) usually do not contain these contaminants. We have studied the effect of adding the different doses of DSS (0, 30, 60, 80, 120, 200, 300 and 600 Mg ha(-1)) on activities dehydrogenase, protease and urease and content of exchangeable NH4+, NO3-, NO2- in a brown soil and a grey-brown podzolic soil in a 240 days experiment. Generally, after the application of DSS enzyme activities initially increased and this increase depended on the dairy sewage sludge added to soil. Afterwards, these values decreased, but generally they remained higher than those of the unamended soils. The decrease in NH4+ concentration coincided with a subsequent increase in the NO3- level. The incorporation of DSS can have a positive effect on the microbial functioning of both grey-brown podzolic and brown soil but the effects should also be verified under field conditions.