“In situ” vermicomposting of biological sludges and impacts on soil quality

A laboratory experiment was carried out to study soil quality amelioration through “in situ” vermicomposting of biological sludges. The experiment dealt with the stabilization, through the action of worms (Eisenia fetida), of five mixtures containing aerobic and anaerobic biological sludges spread on the soil surface. The results showed that by increasing the percentage of anaerobic sludge in the mixtures, the number of worms which left the sludge and chose the soil as their habitat increased. The chemico-structural changes of the sludges left on the soil surface by worms were evaluated through the technique of pyrolysis-gas chromatography, which showed that the degrees of mineralization and humification of organic matter were dependent on the composition of the sludge mixtures. When the amount of aerobic sludge in the mixtures was higher than 50%, a stimulation of soil microbial metabolism occurred, as demonstrated by the index of metabolic potential (defined by dehydrogenase/water soluble carbon ratio). All treatments increased the percentage of soil total shrinkage area, mostly due to the formation of cracks of small–medium size (<1000 μm), which represent a favourable site for microbiological and biochemical processes in the soil. A positive statistical correlation between soil dehydrogenase activity, C and N substrates, and cracks of small–medium size was found.