The impact of freeze-thaw-cycles on soil structure and soil hydraulic properties

The ploughing of soils drastically alters soil structure and at the same time reduces its stability against external stresses. A fragmentation of these artificially produced soil clods during winter time is often observed in areas with air temperatures fluctuating around the freezing point. In this study, the cumulative effects of multiple freeze-thaw cycles (FTCs) on soil structure and soil hydraulic properties were analyzed for two different soil textures, a silty clay loam with a substantial amount of swelling clay minerals and a silty loam with less swell/shrink dynamics. The soil material was brought into two different initial states: (i) undisturbed soil cores taken from the topsoil from a grassland, and (ii) cylinders repacked with soil clods taken from a ploughed field nearby. FTCs were simulated under controlled conditions in the lab, changes in soil structure ≥48 µm were regularly recorded using X-ray µCT. After 19 FTCs, the impact on hydraulic properties were measured and the resolution of structural characteristics were increased to 10 µm by subsampling.

The effect of FTC on soil structure was found to be dependent on the initial structure, soil texture and number of FTCs. Freezing and thawing induced a consolidation of the repacked soil clods taken from both field sites, resulting in a systematic reduction in pore sizes and macro-pore connectivity. The macro-pore system of the undisturbed samples was only slightly affected. Fragmentation of soil elements larger than 0.8 to 1.2 mm increased the connectivity of pores smaller than 0.5 to 0.8 mm. Frost action increased the unsaturated hydraulic conductivity of all treatments, while the water retention was only slightly affected. This leads to the conclusion that multiple FTCs enforces a well-connected meso-pore system at the expense of a fragile macro-pore system. A change in soil structure that benefits farmers but could be reduced in the face of milder winters due to global warming.