Comparative studies of grass compost lignin and the lignin component of compost humic substances

Abstract

The aim of this research was to compare the structural features of lignin and alkali-soluble humic substances isolated from grass compost at different stages of composting, and to estimate the applicability of basic analytical techniques used in lignin chemistry for characterising the lignin structures transformed during composting. Elemental, functional analyses, alkaline nitrobenzene oxidation, analytical ozonation and FT-IR techniques were applied. Compost Björkman lignins were used as reference samples. FT-IR of the compost Björkman lignins was typical for guaiacyl–syringyl lignins. The humic substances contained methoxyl groups and gave the same products as Björkman lignins by nitrobenzene oxidation and ozonation. The lignin component of humic substances, calculated on the basis of chemical markers, was lower than that for the Klason procedure and decreased in following order: methoxyl group>nitrobenzene oxidation products>ozonation products. Methoxyl groups are recommended for lignin content evaluation in compost because of their higher stability in the compost environment among the chemical markers under study. Our results showed that lignin macromolecules were already sufficiently modified at the early stage of composting to be dissolved in alkali. Modified lignin constituted a significant part of grass compost humic substances.

Keywords:

• grass compost
• lignin
• humic substances

Acknowledgements

This work was financially supported by Research Directorate General of European Commission, EU Science Technology Fellowship in Japan (contract number ICB-CT-2000-80002).