The contribution of alkali soluble (humic acid-like) and unhydrolyzed-alkali soluble (core-humic acid-like) fractions extracted from maize plant to the formation of soil humic acid

Alkali soluble (humic acid-like material) (HA-like) (yield of 132 g kg dm−1) and the unhydrolized-alkali soluble (core-humic acid-like material) (core-HA-like) (yield of 33.4 g kg dm−1) fractions were extracted from maize plants and characterized by C and N determinations, DRIFT, and 1H and 13C-NMR spectroscopy. Fresh plants were subsequently incubated for 6 months in an artificial mineral soil, and the HA-like and core-HA-like trends were monitored quantitatively (C fraction content) and qualitatively (spectroscopic approach) in order to study their contribution to the formation of soil humic acid. During incubation the HAC-like partially degraded (loss of 320 g kg HAC−1) and partially formed new fulvic-like acids (160 g kg HAC−1). On the contrary, the stable fraction of HAC, the core-HAC-like, was maintained (loss of 153 kg core-HAC−1), representing, after incubation, 846 g kg−1 of the initial core-HAC-like content. The core-HA-like fraction is composed of lignin residues, polysaccharides, lipids and proteins, probably structured into a well-defined network, i.e. the plant cell wall.