Quantifying the contribution of lignin to humic acid structures during composting

Abstract

The transformation behavior of different types of lignin structure was investigated during composing with particular interest of revealing the contribution rate of lignin functional groups as well as their interaction to humic acids (HA). Rice straws, tree branches, and pine needles were selected as composting materials, because they predominantly contain G-S-H, G-S, and G-type lignin. Results showed that microorganisms were the driving force behind the conversion of lignin into HA. During composting, appeared different levels of cleavage, tearing, and fragmentation on the superficial lignin structures. In depth analysis of lignin transformation, lignin primarily consisted of aromatic groups, CO bonds, alkyl groups, hydroxyl groups and linkage between monomeric units (β-O-4, β-5 and β-β) in the early stage of composting. As lignin degraded, the CO bonds were gradually exposed. But due to their unstable nature, the CO bonds were prone to undergo changes with the cleavage of the aromatic rings at positions 2, 5, and 6 during composting. CO bonds could be converted into aromatic rings, alkyl groups, hydroxyl groups, and CO groups in HA. Through multiple linear regression analysis, it could be concluded that the CO bonds in G-type and S-type monomeric units contributed significantly to the aromatic rings, accounting for 30.4 % and 48.4 %, respectively. The main mechanism might be related to the cleavage of CO bonds and subsequent atomic rearrangement after the release of carbon atoms. This in-depth analysis filled the gap in the mechanism of HA formation during composting.