The background of non-crystalline materials and its role in soil organic matter turnover in subtropical volcanic soils

The high levels of soil organic matter (SOM), frequently found in soils with large content of noncrystalline minerals, are traditionally considered the result of strong organo-mineral interactions. Amorphous oxides are likely to provide active sites for SOM sequestration at the colloidal level. Considering that preservation of SOM is favoured by adsorption of humic colloids onto mineral surfaces, as well as by physical inaccessibility of particulate SOM fractions after encapsulation in the mineral matrix, some authors have recently correlated the complex fractal structure in allophane particles with the low availability of SOM to microbes and enzymes. Apart from the above extrinsic SOM stabilization factors, some studies have also considered the intrinsic recalcitrance of SOM derived from its condensed three-dimensional structures with large amounts of disorderedly arranged aromatic “building blocks”.