The aim of our three-year study was to determine the influence of sewage sludge and a consortium of aerobic microorganisms (BAF) on the dynamics of the development and activity of proteolytic and ammonifying bacteria, soil plant analysis development (SPAD), and the ratio of the leaf assimilation area in willows. Four experimental variants were used in the study: control, soil with sewage sludge, soil with sewage sludge and the BAF inoculant, and soil with the BAF inoculant. The experiment was conducted on the Start willow cultivar. In each year of our research (2013-15), soil samples for microbiological and chemical analyses were collected three times – in spring, summer, and autumn (in May, July, and September).
The research involved the application of the Koch plate method to measure the count of proteolytic and ammonifying bacteria, and spectrophotometry to determine protease and urease activity. The green seeker index (SPAD) was measured by means of non-destructive methods with a SPAD index. The ratio between the willow leaf assimilation area and leaf area (LAI) was measured by means of the SunScan Canopy Analysis System. The content of chlorophyll a and b was analyzed by means of dimethyl sulfoxide extraction.
The highest count and activity of proteolytic bacteria was observed in the soil in the third year of the experiment. Urease exhibited the highest activity in the second year of the research. Sewage sludge stimulated the proliferation and activity of proteolytic bacteria, the content of chlorophyll a and b, the leaf assimilation area, and the green seeker index (SPAD).
None of the experimental variants stimulated the count of ammonifying bacteria. The research proved that BAF stimulated urease activity, which was positively correlated with the green seeker index (SPAD) and the ratio of the leaf assimilation area. The research also revealed positive dependence between the protease activity and the content of chlorophyll a and b. The protease and urease activity proved to be the most sensitive indicators of nitrogen transformation in soil, which is reflected by willow biomass formation.