22 Feb 2021

22 Feb 2021

Review status: this preprint is currently under review for the journal SOIL.

Impact of freeze-thaw cycles on soil structure and soil hydraulic properties

Frederic Leuther and Steffen Schlüter Frederic Leuther and Steffen Schlüter
  • Department of Soil System Science, Helmholtz Centre for Environmental Research – UFZ GmbH, Halle, 06120, Germany

Abstract. The ploughing of soils in autumn drastically loosens the 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. Farmers benefit from the structural transformation by frost action in terms of better seedbed preparation and improved hydraulic connectivity. Previous studies have mainly focused on the effects of freezing and thawing on soil structure stability rather than on the impact on pore structure. From the pore perspective, it is still unclear (i) under which conditions frost action has a measurable effect on soil structure, (ii) what the impact on soil hydraulic properties is, and (iii) how many freeze-thaw cycles (FTCs) are necessary to induce soil structure changes.

The aim of this study was to analyse the cumulative effects of multiple FTC on soil structure and soil hydraulic properties for two different textures and two different initial structures. A silt clay with a substantial amount of swelling clay minerals and a silty loam with less swell/shrink dynamics were either kept intact in undisturbed soil cores taken from the topsoil from a grassland or repacked with soil clods taken from a ploughed field nearby. FTCs were simulated under controlled conditions and changes in pore 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 enhanced towards narrow macro-pores with subsamples scanned at 10 µm.

The impact of FTC on soil structure was dependent on the initial structure, soil texture, and the number of FTCs. Frost action induced a consolidation of repacked soil clods, resulting in a systematic reduction in pore sizes and macro-pore connectivity. In contrast, the macro-pore systems of the undisturbed soils were only slightly affected. Independent of the initial structure, a fragmentation of soil clods and macro-aggregates larger than 0.8 to 1.2 mm increased the connectivity of pores smaller than 0.5 to 0.8 mm. The fragmentation increased the unsaturated hydraulic conductivity of all treatments by a factor of 3 in a pF range of 2.0 to 2.5, while water retention was only slightly affected for the silt clay soil. Already 2 to 5 FTCs enforced a well-connected meso-pore system in all treatments, but it was steadily improved by further FTCs. This steady improvement in structural quality in terms of meso-pore connectivity is put at risk by milder winters in mid-latitudes due to global warming.

Frederic Leuther and Steffen Schlüter

Status: open (until 06 Apr 2021)

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Frederic Leuther and Steffen Schlüter

Frederic Leuther and Steffen Schlüter


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Short summary
Freezing and thawing cycles are an important agent of soil structural transformation during the winter season in the mid-latitudes. This study shoows that it promotes a well-connected pore system, fragments dense soil clods, and hence increased the unsaturated conductivity by a factor of 3. The results are of importance for predicting the structure formation and hydraulic properties of soils in prospect of milder winters due to climate change and thus for farmers' seedbed preparation in spring.