"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 dea lt with the stabilization, through the action of worms (Eisenia fetida), of five mixtures containing aerobic and anaerobic biological sludges spread o n the soil surface. The results showed that by increasing the percentage of anaerobic sludge in the mixtures, the number of worms which left the sludg e and chose the soil as their habitat increased. The chemico-structural cha nges of the sludges left on the soil surface by worms were evaluated throug h the technique of pyrolysis-gas chromatography, which showed that the degr ees of mineralization and humification of organic matter were dependent on the composition of the sludge mixtures. When the amount of aerobic sludge i n the mixtures was higher than 50%, a stimulation of soil microbial metabol ism 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 cra cks of small-medium size (< 1000 mu m), which represent a favourable site f or microbiological and biochemical processes in the soil. A positive statis tical correlation between soil dehydrogenase activity, C and N substrates, and cracks of small-medium size was found. (C) 2000 Elsevier Science Ltd. A ll rights reserved.