Freezing Induced Desiccation Effects on Water Stability of Soil Aggregates.

The state of soil structural stability following the winter months in cold climates is often attributed to the effects of the volumetric expansion associated with ice formation during the freeze-thaw process.  Controversy exists however regarding the efficacy, and even the net effect of the freeze-thaw process.  Freezing induced desiccation may be an unrecognized yet important factor controlling soil structural stability.  The objective of the study was to establish the effects of the freezing induced desiccation process on soil aggregate stability.  Aggregates from soils of differing clay content (0.11 kg kg⁻¹ and 0.33 kg kg⁻¹) and initial water content (0.10 kg kg^-1, 0.20 kg kg^-1 or 0.30 kg kg^-1) were subjected to two freeze dry treatments (FD-C and FD-W).  The first freeze dry treatment (FD-C) included fast freezing the aggregates with liquid nitrogen and drying under vacuum.  The second freeze dry treatment (FD-W) involved more realistic field conditions where aggregates were frozen and allowed to slowly desiccate via sublimation at -15°C.  Results were compared with treatments of freeze only (F), freeze-thaw (FT) and oven drying (OD). Post-treatment aggregate stability determination was via wet aggregate stability (WAS) and dispersible clay (DC) with the treatment effects being compared to a not frozen control treatment (T).  Both freeze dry treatments resulted in increased aggregate stability with the FD-W treatment resulting in the greatest aggregate stability values and comparable to those of the OD treatment. The freeze only treatment (F) resulted in increased aggregate stability over initial values while the FT treatment resulted in decreased aggregate stability.  These data suggest the freezing induced desiccation process improves aggregate stability while the addition of a thaw component following freezing, with the attendant liquid water, is responsible for degradation of aggregate stability.  Clay content and initial water content are important factors governing the magnitude of this process.