Use of microbial parameters to assess treatment methods of municipal sewage sludge applied to grey forest soils of Tatarstan

The benefits and limitation of sewage sludge application as an organic fertilizer need to be investigated for various procedures of sludge treatment. For this purpose, three types of municipal sewage sludge of the city of Kazan (Tatarstan, Russia), i.e., anaerobically digested, composted and untreated, were added to field plots of a grey forest soil (Haplic Greyzem) and planted to spring barley (Hordeum distichum L.). Soil microbial biomass carbon, respiration and N2-fixing activity as indicators of soil fertility as well as plant yield and the metal content in soils and plants were measured to determine the most suitable type of sludge as organo-mineral fertilizer. As shown, sludge additions resulted in significant increase of metal contents in the soil, although the maximum metal concentrations found remained lower than the current critical limits established for Russia. The application of composted sludge to soils was followed by the increase in microbial biomass (about 1.9-4.4-fold), basal respiration (about 2.3-6.3-fold) and in N2-fixing activity (about 2.1-35-fold) in comparison with the parameters found for control soil without any sludge addition. The application of anaerobically digested sludge had no significant effects on microbial biomass and activity. In the case of untreated sludge application, a significant decrease in N2-fixing activity was noted. Beneficial effects on microbial biomass and activity were greater in plots that had received composted sludge. In the absence of any detrimental effect on crop quality, this study lends support to using this type of sludge as the organo-mineral fertilizer for grey forest soil of Tatarstan. © 2001 Elsevier Science B.V. All rights reserved