Humin: Its Composition and Importance in Soil Organic Matter

Humin is the insoluble component of soil organic matter (SOM) that remains after extraction of the other components of SOM that are soluble in aqueous base. Humin usually makes up a substantial component of SOM, but its lack of solubility and intractable nature have made it difficult to study. To put this contemporary review of humin into perspective, a summary of the historical development of SOM studies and the terminology used to describe the various components of SOM is presented. Two major recent advances, namely, sophisticated nuclear magnetic resonance (NMR) techniques and the introduction of a solvent system (acidified dimethylsulfoxide) capable of dissolving humin, have facilitated studies of its composition. The NMR techniques used are briefly explained and the results obtained from their application are presented. Based on this information, the possible contributions to humin of various plant/soil components are assessed. The major components of the humin are predominantly aliphatic hydrocarbon functionalities, especially those found in lipids, waxes, cuticular materials, cutin/cutan, and suberin/suberan, which are relatively minor components of plants. There is also evidence for small amounts of carbohydrate (possibly highly ordered cellulose), peptide, and peptidoglycans; but there is little evidence for lignin-derived structures. The composition of humin differs considerably from the base-soluble components of the SOM. All components of SOM eventually decompose but at different rates, but the accumulation of humin constituents relative to their levels in plants indicates that they are relatively resistant to decomposition. The environmental consequences of humin composition and the opportunities presented are discussed and future possibilities outlined.