Freezing and thawing affect the pore-space structure in agricultural soils with implications for soil hydraulic properties and water flow. Previous studies have focused on the upper few centimeters of the tilled topsoil, where most freeze-thaw (FT) cycles occur, even though deeper soil layers are also subject to freezing and thawing in cold climates. Thus, little is known about how freezing and thawing affect untilled soil layers, which often show high bulk densities that restrict vertical water movement. Furthermore, it remains unclear how shifts in FT patterns with climate change may change the pore-space structure and water flow through these soil layers. Here we investigated the effects of freezing and thawing on X-ray imaged pore-space characteristics, water retention and near-saturated hydraulic conductivity () in untilled soil directly below plough depth. Intact cores were sampled at two sites in central Sweden under the same long-term reduced tillage management. The two soils, a silt loam and a silty clay loam, were subjected to three FT scenarios in a laboratory environment intended to represent FT patterns that are considered likely under current and future winter conditions for this region. The latter scenario was characterised by more FT cycles and a lower freezing temperature. Freezing and thawing increased in the near-saturated range in both soils, which we attribute to observed small (<0.01 mm mm) increases in the volume of pores of diameters close to the X-ray resolution limit. Concomitant increases in pore network connectivity and critical pore diameter, especially in the denser silty clay loam soil, probably contributed to this increase in . The water retention data suggested that changes in pore-space characteristics below X-ray resolution also occurred in both soils. Furthermore, our results indicate that both soils may show higher drainage rates due to shifts in FT patterns in the future, although longer-term changes in pore-space structure with an increasing number of FT cycles would mostly be limited to soils with relatively high clay contents. These soils are often more compacted below plough depth and, thus, benefits from improvements in soil structure such as improved root growth and plant water supply are also expected to be larger.

中文翻译：

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冻融对压实土孔隙空间和水力特性的影响以及气候变化的潜在后果

冻结和融化会影响农业土壤的孔隙结构，从而影响土壤水力特性和水流。以前的研究主要集中在耕作表土的上部几厘米处，大多数冻融（FT）循环都发生在此处，尽管更深的土壤层在寒冷气候下也会遭受冻融。因此，人们对冻结和融化如何影响未耕种的土壤层知之甚少，这些土壤层通常表现出限制水垂直运动的高堆积密度。此外，目前还不清楚 FT 模式随气候变化的变化如何改变这些土壤层的孔隙空间结构和水流。在这里，我们研究了冻融对耕深正下方未耕土壤的 X 射线成像孔隙空间特征、保水性和近饱和导水率 () 的影响。在瑞典中部的两个地点在相同的长期减少耕作管理下采集了完整的核心样本。这两种土壤（粉质壤土和粉质粘壤土）在实验室环境中进行了三种 FT 情景，旨在代表该地区当前和未来冬季条件下可能出现的 FT 模式。后一种情况的特点是更多的 FT 循环和更低的冷冻温度。两种土壤的冻融都​​在接近饱和的范围内增加，我们将其归因于观察到直径接近 X 射线分辨率极限的孔隙体积的小幅增加（<0.01 mm mm）。孔隙网络连通性和临界孔径的伴随增加，特别是在较致密的粉质粘壤土中，可能导致了这种增加。保水数据表明，两种土壤中也发生了低于 X 射线分辨率的孔隙空间特征变化。此外，我们的结果表明，由于未来 FT 模式的变化，两种土壤可能会表现出更高的排水率，尽管随着 FT 循环次数的增加，孔隙空间结构的长期变化主要限于粘土含量相对较高的土壤。内容。这些土壤通常在犁耕深度以下更加压实，因此，改善土壤结构（例如改善根系生长和植物供水）带来的好处也预计会更大。