Freeze-thaw cycles effects on soil physical properties under different degraded conditions in Northeast China

Freeze-thaw cycles alter soil properties markedly and cause a subsequent change in soil erosion, however previous studies about freeze-thaw cycles' influence on soil physical properties were restricted to simulating runoff and soil loss on cropping slopes in cold regions and failed to invoke responses of soils under different degraded conditions to freeze-thaw cycles. This study was designed to compare and quantify the responses of different degraded soils to freeze-thaw cycles in laboratory setting. The soil conditions were divided into five types: original profile, degraded profile, parent profile, deposited profile and compacted surface. Samples were collected from the black soil region in Northeast China and were frozen (-12 degrees C for 12h) and then thawed (8 degrees C for 12h) for certain times. Samples without freeze-thaw cycles were treated as control group. Porosity, aggregate mean weight diameter, saturated hydraulic conductivity and water retention curves were tested for control and experimental samples. Results showed that porosity and saturated hydraulic conductivity significantly increased (maximum for degraded profile), while mean weight diameter decreased (maximum for compacted surface) compared with control group. After 30 freeze-thaw cycles, remaining water contents increased in deposited and original profiles, while decreased in compacted surface. Generally, well-structured soils are more difficult to be broken by repeated FTCs. The first freeze-thaw cycle displayed evident influence on soil physical properties under original profile, and at least one threshold of cycle time (between 5 and 20) existed. These findings may help improve understanding the functional mechanism of freeze-thaw cycles on soil erosion processes.