Character of transitions causing the physicochemical aging of a sapric histosol

Physicochemical aging of soil organic matter is assured by the dynamic character of weak interactions stabilizing its supramolecular structure. However, aging is difficult to monitor, due to low organic matter content in most soils and relatively large time constants. In order to overcome those problems, a model soil, sapric histosol, was exposed to the accelerated aging after a short heating event to 110 A degrees C, and its thermal characteristics were monitored over several months. Classical and temperature modulated differential scanning calorimetry, microcalorimetry and solid-state NMR were used to elucidate the character of involved transitions. The heating event caused separation of an initially broad transition into two processes; a melting, which showed almost no response on the previous heating and a step transition, which is associated with the disruption of water molecule bridges (WaMB) between molecular segments of organic matter. Both processes are preceded by a preparatory phase, starting at subambient temperatures, in which aliphatic domains probably recrystallize and water molecules condensate forming WaMB stabilizing the physical structure of sapric histosol. The aliphatic moieties showed a particular behavior reflected in higher imperfection in crystallite structure upon slow cooling, which was attributed to their interaction with surrounding porous and heterogeneous structures. The results show that soil organic matter aging, considered as a natural process driven by thermodynamic principles, is caused by successive development of WaMB. This is potentially accompanied by recrystallization of aliphatic structures and both processes lead to higher physicochemical stability of soil organic matter.