Wednesday, 9 November 2005 - 3:55 PM
271-7

The Amorphous Nature of Soil Organic Matter.

Gabriele Schaumann, Technische Universität Berlin, KF 3 - Strasse des 17. Juni 135, Berlin, D-10623, Germany

Glassy, rubbery and crystalline phases are representants of supramolecular structures which strongly differ in order, density and other characteristics. The amorphous nature and the existence of glassy and rubbery regions in soil organic matter is made responsible for numerous sorption irregularities. In this contribution, the amorphous nature of soil organic matter will be discussed with respect to the glassy/rubbery model, glass transition mechanisms, interaction of soil organic matter with water, physical aging and its evidence from literature data. Aging is not restricted to polymers and provides an explanational basis for understanding and prediciton of the development of soil organic matter. Glass transition behavior is an inherent property of amorphous solids. Two mechanisms of glass transitions have been distinguished in humous soil samples. One of them is closely related with water binding and may be explained by the formation and disruption of crosslinking water bridges between individual strands. Due to its slowly reversing character, this transition does not fully represent a classical glass transition; like classical glass transitions, it is, however, connected with a decrease in matrix rigidity. Water in this transition surprisingly acts in an antagonistic way as short-term plasticzer and long-term antiplasticizer in soil organic matter. The second transition can be detected in water-free systems only and indicates classical glass transition behavior. Aging is an inherent property of glassy amorphous solids and is closely connected with the glass transition mechanism. The transition temperature of soil organic matter gradually increases with time, indicating physical or physicochemical aging processes. The gradual increase is observed only if the external conditions (temperature, humidity) are kept constant, and water is most probably involved in the aging of soil organic matter. It is hypothesized to be responsible for contaminant aging, changes in surface properties as well as in increased soil compression in agricultural soils.

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