Biochemical origin and refractory properties of humic acid extracted from maize plants: The contribution of lignin

The soil organic carbon (SOC) pool is the largest terrestrial reservoir of carbon and plant residues play an important role in its maintenance. Up to 70-80% of SOC in arable soil is composed of humic substances (HS). In these soils post-harvested residues, left in arable soil after harvesting the crops, are the basic source of humus. Previous research indicated that maize plants residue contain a humic acid (HA) fraction possessing recalcitrant compounds that contributed to soil-HA fraction. This study presents updated results obtained using Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) to provide an indication of the contribution of the lignin to the soil HA. Results obtained indicated that the HAs from maize plants were mainly composed of lignin-derived moieties that were likely derived from the partial hydrolysis of p-coumaric and ferulic acid that are linked to lignin, polysaccharides or other biopolymers of the cell wall. Lignin composing the HAs derived from plants and incubated in soil was substantially preserved. Nevertheless the modification of the syringyl/guaiacyl ratio and the oxidation of the side-chains of lignin, suggested a turnover of lignin-derived molecules in soil-HA fraction. This fact indicated an involvement of the alkali insoluble fraction of maize plant residue (humin) in the soil-HA formation, up-dating our previous knowledge.