Influence of the sample history and the moisture status on the thermal behavior of soil organic matter

Recent studies indicate that glassiness represents a characteristic feature of soil organic matter (SOM). It is however unknown, to which extent the transitions detected in humic substances and whole soil samples correspond to common models of synthetic polymers providing the theoretical basis for explaining their glass transition characteristics. Physical aging associated with structural relaxation of amorphous substances below their glass temperature is one fundamental basis for the glass transition behavior of synthetic polymers. According to the results of this study, aging processes also occur in SOM. In whole soil samples, this process can be observed by the shift of glass transition-like step transitions to higher temperatures within the time scale of years. Not only the structural relaxation of the macromolecular organic substances, but also interactions with water molecules, which may exhibit both plasticizing and antiplasticizing properties, influence the aging process of SOM. Especially under moistening or drying conditions, a differentiation between the effects of water and of alterations of the SOM structure in the course of time on the rigidity of the macromolecular network is difficult.