Gabriele Ellen Schaumann

• Glassy, rubbery and crystalline phases are representants of supramolecular structures which strongly differ in order, density and other characteristics. In this contribution, the amorphous nature of soil organic matter (SOM) is reviewed with respect to the glassy/rubbery model, glass transition mechanisms, interactions of soil organic matter with water and physical aging. Glass transition behavior and physical aging are inherent properties of amorphous solids, and numerous spectroscopic investigations give insights into different domain mobilities of humic substances (HS). The correlation between sorption nonlinearity and glassiness of polymers and HS supports a relation between sorption and amorphicity in Aldrich humic acid. Further evidence is still required for the transfer to soil HS and SOM. Sorption and differential scanning calorimetry (DSC) data suggest a correlation between aromaticity and glassiness in HS, and the available data do currently not allow to decide unambiguously between specific sorption and hole filling as explanation. This needs to be verified in future research. Although parts of the investigations have up to now only been conducted with humic substances, the collectivity of available data give strong support for the glassy/rubbery conception of SOM. They clearly indicate that amorphous characteristics can not be excluded in soil organic matter. This is further supported by the observation of different types of glass transition behavior in humous whole soil samples. In addition to classical glass transitions in water-free soil samples, water surprisingly acts in an antagonistic way as short-term plasticizer and long-term antiplasticizer in a second, non-classical transition type. Latter is closely connected with physicochemical interactions with water and suggests water bridges between structural elements of SOM (HBCL-model). The gradual increase of Tg* in SOM indicates physicochemical aging processes, which are not restricted to polymers. They may be responsible for contaminant aging, changes in surface properties and increased soil compaction in agricultural soils.