Dernopodzolic soils are usually affected by long-term freezing and various agricultural treatments. In the present researches, changes in the water retention properties and swelling-shrinkage of samples of a sandy loam dernopodzolic soil were studied after 17-week freezing and 13-week microbial removal of soil organic matter. The disturbed moist samples taken from a plough horizon were frozen at -10oC or incubated at 65oC. Water retention of soil samples in a range of matric potentials of -5 to -300 kPa was determined by a pressure plate apparatus. Studies of the swelling and shrinkage processes during three repeated cycles were carried out using a Vasiljev's device.
Content of soil organic carbon measured by a wet combustion method declined from 21,8 to 20,7 and 19,4 g kg-1 after freezing and incubation of soil samples, respectively. Soil thermophilic microflora was used as a tool for a rapid removal of organic matter without an essential damage of mineral solid phase of soils. A transformation of soil organic matter induced by thermophilic microorganisms caused a decrease in water retention of soil samples. Results of our earlier researches showed close relationships between water retention and content of humic acids and humins in the light-textured dernopodzolic soils (Balashov, 1995). Freezing of soil samples as compared with their incubation resulted in greater decreasing the swelling only during the first cycle of swelling process at an initial bulk density of 1,30 Mg m-3. Long-term freezing and incubation of soil samples also contributed to a shrinkage process. Although there were no significant differences among the relationships between bulk densities and moisture contents in the untreated and treated samples of soil at an initial bulk density of 1,10 Mg m-3 during the first cycle of shrinkage process. If initial values of bulk density of soil samples increased from 1,10 to 1,30 Mg m-3, a greater increase in differences between these relationships was observed, especially for the frozen soil samples which had final values of bulk density of more than 1,50 Mg m-3 after the third cycle of shrinkage process. Our further studies will be devoted to effects of freezing of soil on stability of organic matter (humus) and its role for a soil physical status.
Dr. Eugene V.Balashov Agrophysical Research Institute 14 Grazhdansky prospect St. Petersburg 195220 Russia Telephone: (812) 535-5215 E-mail: ivl@agrophys.spb.su