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.

中文翻译：

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冻融循环对土壤结构和土壤水力特性的影响

摘要。秋季的土壤耕作会极大地放松土壤结构，同时降低其抵抗外界压力的稳定性。在冬季，在气温在冰点附近波动的地区，通常会观察到这些人造土壤土块的碎片。通过更好的苗床准备和改善的水力连通性，霜冻作用使农民受益于结构转变。先前的研究主要集中在冻融对土壤结构稳定性的影响上，而不是对孔隙结构的影响上。从孔隙的角度来看，目前还不清楚（i）在什么条件下霜冻作用对土壤结构有可测量的影响，（ii）对土壤水力性质的影响是什么，（iii）需要多少个冻融循环才能引起土壤结构的变化。这项研究的目的是分析多种FTC对两种不同质地和两种不同初始结构的土壤结构和土壤水力特性的累积影响。将含有大量溶胀粘土矿物的粉砂粘土和具有较少膨胀/收缩动力学的粉质壤土完整地保留在从草地表层土壤中取出的未扰动土壤芯中，或将其从附近耕地中取出的土块重新包装。在受控条件下模拟FTC，并使用X射线µCT定期记录孔结构≥48 µm的变化。在19个FTC之后，测量了对水硬性的影响，并通过在10 µm扫描的子样品，对狭窄的大孔增强了结构特性的分辨率。FTC对土壤结构的影响取决于初始结构，土壤质地和FTC的数量。霜冻作用导致重新堆积的土壤土块固结，从而导致孔径和大孔连通性的系统减小。相反，未扰动土壤的大孔系统仅受到轻微影响。与初始结构无关，大于0.8至1.2 mm的土壤块和大型聚集体的碎片增加了小于0.5至0.8 mm的孔的连通性。在2.0至2.5的pF范围内，碎片化将所有处理的不饱和水力传导率提高了3倍，而对于淤泥黏土，保水率仅受到轻微影响。在所有处理中，已经有2至5个FTC强制执行了一个连接良好的中孔系统，但进一步的FTC稳步改善了它。由于全球变暖，中纬度冬季较温和，使中孔连通性方面的结构质量不断提高受到威胁。