Isolation by isoelectric focusing of humic-urease complexes from earthworm ( Eisenia fetida )-processed sewage sludges

Abstract

Vermicomposting is an eco-biotechnological process that transforms energy-rich and complex organic substances into a stabilized humus-like product. In a laboratory experiment, Eisenia fetida (Sav.) earthworms were employed to process putrescible sewage sludges into a high-value biofertilizer, very rich in urease activity and humic-urease complexes (stabilized extracellular enzymes). Extracellular humic-urease complexes were extracted by a single 24-h extraction at 37 °C using neutral pyrophosphate (0.1 M); then, the extracts were dialysed and characterized by means of an analytical isoelectric focusing technique. This technique gave a multiplicity of humic bands enzymatically active, with isoelectric points ranging from 4.8 to 5.6. The results demonstrated that, after an 18-week incubation period, sewage sludge had undergone a biochemical evolution, which caused a doubling of absolute urease activity and a six-fold increase in specific activity (activity with reference to the humic C fraction). The biochemical evolution of the vermicompost was evaluated also from the sharp decrease in pyrophosphate-extractable C and water-soluble C. Stabilization of organic C during vermicomposting and the activity of humic-urease complexes expressed at low pH values are of extreme importance when organic wastes are used in acid soils for biochemical restoration purposes.