Over-winter freezing and thawing of soils in the northern US states causes movement of soil water and solutes in the soil profile. We conducted field and laboratory experiments using soil columns repacked into PVC plastic cylinders 0.13 m inside diameter and 1.2 m long. Each column was packed with topsoil from a Webster silty clay loam to bulk densities of either 1.0 or 1.2 Mg/m3. Potassium bromide tracers were placed in the 0.05 to 0.15 m soil layers in some of the columns. Twenty-four columns were buried vertically in a field near Ankeny, IA during each of the 1993-94 and 1994-95 winters and sampled in groups of four at roughly two week intervals. Twelve columns were placed in a special freezing chamber where approximately the top 0.3 m of soil in each column was frozen during a 48 hour period. Variables in the experiments were water content, soil bulk density, and the presence or absence of earthworms. All of the field columns and eight of the laboratory columns were sectioned into 0.05 m layers and analyzed for water content, bulk density, electrical conductivity, and bromide content. The remaining four laboratory columns were used for leaching break-through determinations prior to sectioning into 0.05 m layers. Changes in soil water content and bulk density at the freezing interface were distinct. Water moved upwards towards the freezing zone and carried some solutes along. However, solutes were excluded from the forming ice lenses leaving briny films in the pores around them. Pressures from freezing ice lenses caused abrupt changes in localized soil bulk density either by forcing the soil matrix apart of by compressing the soil matrix. Electrical conductivity and bromide analyses verified the movement of solutes during the freeze/thaw periods. After thawing, many of the soil physical properties returned towards their original condition, but not completely. In many cases, a net movement of water and solutes was observed. Random variations among soil columns was great and not completely explained by treatment differences. The leaching experiments suggested the formation of random macropore cracks due to the freezing action which resulted in large variations of water outflow. The movement of water and solutes during the winter has important implications for water quality and soil quality. This research adds to knowledge needed for the development of effective, overwinter, management practices.
Jerry K. Radke USDA, ARS, National Soil Tilth Laboratory 2150 Pammel Drive Ames, Iowa 50011 Phone: (515) 294-0213 Fax: (515) 294-8125 E-mail: jkradke@iastate.edu
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