Evidence of lignin residues in humic acids isolated from vermicomposts

Abstract

Three vermicomposts derived from animal manures exposed to the action of the earthworms Eisenia foetida or Lumbricus rubellus were investigated to determine the nature of the humic acid fraction in relation to the potential fertility of the vermicomposts. The isolated humic acid fractions were analysed by pyrolysis-gas chromatography-mass spectrometry. The greater part of the humic acids consisted of lignin and/or lignin residues as revealed by the very prominent and characteristic methoxy and dimethoxy-phenols pyrolysis products. The chromatograms of the pyrolysates are similar to those of grass lignin. This points to an incomplete degradation or selective preservation of grass lignin during earthworm composting. The results further indicate that neither the quality nor the quantity of the humic acids has changed considerably by vermicomposting.