Freeze-drying processes and wind erodibility of a clay loam soil in southern Alberta

Freeze-drying has been implicated as a factor causing soil aggregate breakd own on the Canadian Prairies and northern Great Plains. Aggregates of a Dar k Brown Chernozemic clay loam soil sampled in October 1993 and January and April 1994 were subjected to repeated cycles of wetting (to 0.1, 0.2 and 0. 3 kg kg(-1) water contents) freezing, and freeze-drying under laboratory co nditions. The October 1993 samples showed less disruption when initially ex posed to freeze-drying cycles compared to samples taken in January and Apri l 1994. Using regression analysis, we predicted that 31 freeze-dry cycles w ere required for the 0.1 kg kg(-1) water content aggregates to reach 60% er odible fraction (EF, % aggregates <0.86 mm);, 9 cycles for the 0.2 kg kg(-1 ) aggregates and 2 for 0.3 kg kg(-1) aggregates. In a field study, conducte d over the 1994-1995 winter on a similar clay loam soil, we estimated the n umber of freeze-drying cycles using large vapour pressure (VPL) and small v apour pressure (VPS) gradients between the soil surface (which had a mean w inter water content of similar to 0.1 kg kg(-1)) and the atmosphere. With s olar energy adjustments, we predicted that the number of freeze-dry cycles required for the soil to reach 60% EF was 60 for VPL and 37 for VPS conditi ons. The latter number was similar to the 31 cycles predicted in the labora tory study of aggregates at 0.1 water content. Our results demonstrate that freeze-drying is an important overwinter process in the breakdown of soil aggregates and hence wind erosion risk in the Canadian prairie region.