Freezing Cycle Effects On Water Stability of Soil Aggregates.

Although commonly considered a period dominated by freezing and thawing, over-winter conditions encompassing the period from the first freezing in the fall to final thawing in the spring may also include wetting and drying through both freezing induced water redistribution and freeze drying effects.  These processes may strongly influence soil aggregate stability at the end of the winter season.

Hand sieved 1 - 2 mm diameter aggregates from three soils of varying clay content (a sandy loam, a clay loam, and a clay) and water content (0.10 kg kg^-1, 0.20 kg kg^-1 or 0.30 kg kg^-1) were subjected to three treatments involving freezing and thawing.

The freeze-thaw treatment (FT) involved freezing at -15^oC for 24 hr followed by thawing at +15^oC for 24 hr.  The freeze treatment (F) omitted the thaw period by freezing at -15^oC for 24 hr followed immediately by aggregate stability determination from the frozen state.  The freeze-dry treatment (FD) involved freezing at -15^oC for 24 hr followed by thawing at +15^oC for 24 hr under vacuum to facilitate sublimation.  A control treatment (T) included soil aggregates that remained in the thawed state and were not frozen.  Post treatment aggregate stability determination was via wet aggregate stability (WAS) and dispersible clay (DC) liberated from the sample.

The order of severity of the treatments on aggregate stability was as follows: F > FT > T > FD.  Freezing was observed to result in pore water migration out of the aggregates to centers of freezing in the larger inter-aggregate pore spaces.  That the freeze treatment (F) resulted in the lowest post-treatment aggregate stability followed by freeze-thaw (FT) treatment suggests that incubation in the thawed state following freezing counteracts the destructive effects of freezing through the re-activation of intra-aggregate cementing materials during aggregate re-wetting.  The highest aggregate stabilities resulting from the freeze-dry treatment (FD), even higher than the control treatment (T), suggests desiccation  in concert with the prevention of re-wetting results in a hydrophobic state that effectively resists overall aggregate breakdown during stability testing.