The ultimate fraction – Chemical characterization of humins from forest vs grassland soils

Humin, defined as the residual fraction of soil organic matter remaining after alkaline extraction, has been recognized as the predominant and most stable form of organic matter in mineral soils for centuries. Consequently, it is imperative to reconsider this at the modern instrumental level, as the current evidence about its composition is primarily influenced by the analytical methods applied. Here, humin was recovered from the 50 % in forest soils. Obtained humins were then investigated by 13C-NMR, Pyrolysis-GC-MS, THM-GC-MS, acid hydrolysis followed by monosaccharide detection and quantification, and thermal analyses (DTG and DSC). The studied humins consisted of a broad mixture of C types, including alkyl C (∼34 %) and O-alkyl C (∼30 %), while they were remarkably poor in carboxyl C, which may explain their insolubility in alkali solutions. Overall, humins from forest soils seemed more microbially transformed than those from grassland soils. In particular, carbohydrates were mainly of microbial origin, lipids preserved the features of their original precursors (suberin, epicuticular waxes), whereas the degree of preservation of lignins was unclear. Carbon recalcitrance, measured by acid hydrolysis, was higher in forest humins (40 %) than in grassland humins (34 %). At the same time, humins from forests showed a lower thermal stability than those from grasslands, but were characterized by a higher energy density. Contrary to views that propose a humin composition predominantly aromatic, alkyl or poly-alkyl in nature, our study showed that humins are highly chemodiverse, containing a wide range of organic compounds, none of them being predominant. Their degree of microbial reworking varied with vegetation type and, thus, with corresponding plant inputs; on a larger scale, it probably varies with climate and parent material, a hypothesis to verify in future research.