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Submitted to: Soil Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/6/1998 Publication Date: N/A Citation: N/A Interpretive Summary: Many people have the misconception that all physical, chemical, and biological processes in the soil cease when the field freezes. Even though the processes are slowed, considerable activity does take place during the winter season. Soil water moves upward to the frost layer carrying with it some of the dissolved nutrients. Large forces caused by freezing ice crystals can compact some soil aggregates and crush others. Freezing fractures may provide preferential flow paths for water and nutrients during the spring thaw. These processes need to be better understood to design improved soil management systems. We packed open-ended plastic cylinders (13 cm inside diameter by 120 cm long) with a silty clay loam soil and buried them vertically in the field to be exposed to winter freeze/thaw conditions. Some of these soil columns had potassium bromide added near the soil surface to serve as a tracer. Periodically throughout the winter we removed some of the soil columns and sectioned them into 5-cm layers. Each layer was analyzed for water content, soil density, and potassium bromide concentration. Soil structure (the arrangement of soil aggregates and pore spaces) was less uniform after freezing and thawing. In some cases, this resulted in the unexpected movement of water and the potassium bromide tracer. The upward movement of water and nutrients during the freezing portion of the cycle is a desirable process. Leaching of nutrients beyond the rooting zone is not desirable. Our research provides important insights for developing management strategies that will keep water and nutrients in the rooting zone thus maximizing nutrient efficiency and minimizing pollution. This benefits the public as well as the farmer. Technical Abstract: Soil water and soluble nutrients move in the soil profile overwinter due to freezing and thawing. Understanding the freezing and thawing processes and their effects on water movement and soil structure is necessary to develop improved management strategies. Experiments were conducted to measure the effects of freeze/thaw on soil water and solute movement in a Webster silty clay loam (fine-loamy, mixed mesic Typic Haplaqualls). PVC cylinders (0.13 m inside diameter and 1.2 m long) were packed with topsoil. Potassium bromide tracer was placed in the top 0.05 to 0.15 m soil layers in some of the columns. The soil columns were buried vertically in the field and exposed to the winter freeze/thaw conditions. Field columns were extracted throughout the winter and sectioned into 0.05 m layers. Each layer was analyzed for water content, bulk density, and electrical conductivity. Water moved upwards to the freezing zone carrying along some solutes. Electrical conductivity values verified the movement of solutes during the freeze/thaw periods. Bulk density changed abruptly due to expansion and compression of the soil matrix during freeze/thaw periods. Physical properties of thawed soil retained some of the changes caused by freezing and remained more variable compared to the properties of unfrozen soil. This additional variability increases the complexity of predicting overwinter changes in the soil profile and the difficulty of developing management practices for maximizing nutrient efficiency while minimizing pollution. |
