Is glassiness a common characteristic of soil organic matter?

Until now, glass transitions were detected in isolated humic and fulvic acids as well as in distinct soil samples with usually high Corg contents. The results of this study indicate that glassiness has to be considered a common characteristic of soil organic matter (SOM). However, two types of glassiness were observed in various soil types. Additionally to a typical glass transition with low intensity, a slowly reversing glasstransition-like step transition with significantly higher intensity was detected in 52 out of 102 tested soil samples. The intensity of this transition type is correlated to the organic matter content of the samples. The transition behavior additionally depends on characteristics of the locations and changes within soil profiles. Relations to particulate organic matter (POM), mineral-associated organic matter (MOM), and the thermostable fraction of the soil samples were not significant. A surprising result of the study is that the step transition temperatures of all analyzed air-dried soil samples range between 51 and 67 degrees C in closed systems, pointing to a superordinate mechanism which controls the matrix rigidity of the organic molecules. This may be represented by the formation of hydrogen bond based cross-links between water molecules and SOM suggested in a previous study. Thus, glassiness in SOM may be caused by physical and physicochemical mechanisms.